fuel pressure regulators


Staff member
Volume vs. Pressure

FUEL Volume and FUEL pressure are two completely different things when you talk about fuel,supplied to an engine, but they must work together to give you the proper air/fuel mixture delivery. Both carburetors and EFI systems require the proper fuel volume to make the desired horsepower, but the pressure differences between the two are quite different. A carburetor setup requires very low fuel pressure—somewhere between 5 and 6 psi,pressures higher than 6 psi can force open the flow control valve on some carbs,
be aware that you,ll need to correctly set the carbs floats to regulate the fuel level in most carbs and the needle & seat valve

your carburetor inlet fuel pressure should be a consistent 5 psi-5.5 psi in most cases, once you get over 6 psi you'll find the carburetor float bowls tend to flood and under 3 psi the carburetor can run lean under hard acceleration.
keep in mind the internal port size is not all that large on the cheap holley fuel pressure regulators , so they work ok on near stock applications but NOT on anything close to a race application.

EFI works at higher pressures (a 1985-1991 TPI corvette as an example generally operates in the 38psi-42 psi fuel pressure range)

fuel tank...to .fuel filter....to .-fuel pump....to feed into fuel rail& injectors. then out too....fuel pressure regulator...then too..return line to fuel tank

shop carefully, fuel line size, fuel pressure range,
consistency of maintaining both fuel flow and pressure, component quality, and durability as well as cost varies wildly




the vacuum line to the plenum drops under full throttle,
so the diaphragm and check ball,compensate, to maintain max flow rate to the injectors,
the diaphragm and check ball in the regulator become, or are just a tiny bit harder for the fuel pressure too force open,without the plenum vacuum above the the diaphragm and check ball
thus maintaining fuel rail volume and pressure before any of the fuel can exit the return line







it might help if you think, fuel pump provides fuel flow volume,
fuel pressure is a measure of resistance to fuel flow,
the fuel pressure regulator acts as a valve that opens if the pressure exceeds about 42 psi (YES some versions like yours are manually adjustable) but the reason its there is to maintain a consistent 39 psi-42 psi to do that you allow the fuel pump to stack or pressurize fuel in the fuel rail and in theory the fuel pressure regulator only allows excess fuel volume to flow back to the fuel tank through the return line if that fuel pressure exceeds the 42 psi, the pump is designed to provide a bit more than the required 42 psi too insure fresh cool fuel is cooling the fuel pump while maintaining that consistent fuel volume at the fuel pressure, when you tromp on the throttle the flow of fuel drastically increases, the regulator momentarily restricts out going fuel to maintain the fuel rail pressure and volume, until the flow demand drops enough, for excess volume and the resulting increased pressure that results to open the regulator again,


that controls fuel flow into the carb can get crud trapped between the needle and seat causing the fuel bowl to constantly flood, so check that carefully, if thats your issue!
while most EFI setups, require 40 psi too as much as 60 psi or more on the high side limit. This means the fuel pump you select needs are entirely different, for each system and regulating pressure is handled by different components as well. youll need too select a Regulator manufacturer who makes regulators specifically for each application.
A return-style fuel system keeps the flow of fuel through the entire system at a constant,rate, keeping the fuel cooler. Constant circulation of cooler fuel will also result in less tendency to get into conditions favoring vapor lock , than a dead-head–style system, as fuel sits in the lines and carburetor absorbing heat from the engine compartment.
Ive consistently had far better results with the bye-pass style regulators ,for several reasons, the non-bye-pass regulators allow more heat to build up in the fuel and tend to be slightly more prone to get the fuel temp up into detonation range, fuel thats cool absorbs some combustion chamber heat lower burn temps very slightly, you NEED a fuel pressure regulator with a BYE-PASS CIRCUIT from the regulator at the carburetors location back to the tank,If the pump is capable of more than 7PSI you should run a regulator regardless of the engine, simply because most carburetors needle and seat valves can not control flow over 7 psi and many can,t handle even 6 psi
and yes, use of a small handy and accurate fuel pressure gauge, mounted on the fuel injector fuel rail (0-100 psi for FUEL INJECTION)

or carb inlet port (0-15 psi for carburetors)is almost mandatory, they generally cost under $30


The engine does not care how much pressure you push to the carburetor, but the have the carburetor will have a needle & seat valve that does, because it must maintain a near constant float level in the fuel bowls to have a consistent fuel/air ratio.
All the engine cares about is that the carburetor feeds it a controlled fuel air mix ratio between about 12.7:1-15:1 & you need to maintain enough pressure & volume to keep the carburetor fuel bowls full under all conditions, WITH OUT FLOODING or fuel starvation.
dead head regulators will frequently NOT maintain a constant pressure , it tends to surge and fall, and running a electric pump without using fuel that flows constantly thru it will USUALLY cause the pump to heat up and LOOSE some efficiency, in fact they commonly loose efficiency or fail if heated up, trying to pump the same fuel constantly, the flow thru a bye-pass regulator with its separate fuel return line to the tank allows the fuel to circulate and pull heat from the pump,the better quality, return style regulators are vastly more effective
WHEN TESTING<be sure the fuel pressure gauge reads correctly by comparing it to a second test gauge, these fuel pressure gauges are frequently defective



The minimum fuel line size (from the pump to the regulator) is dependent on the horsepower output of the engine (and/or Nitrous system) regardless of the size of the pump. fuel pressure regulators are normally placed as close to the fuel rail or carburetor feed inlet port as possible to maintain most of the fuel feed lines under higher pressure to reduce vapor-lock, high g -force fuel starvation and to reduce the chance of air entering the carburetor ,or injectors and keep a constant flow of cool fuel reaching the injector's or carburetor, because theres a constant flow of new fresh fuel being forced up to and some bye-passed thru the system, and fed to the engine
keep in mind TPI fuel pressure regulators maintain about 40-45 psi while carburetors require about 4.5-6.5 psi MAX, but in most cases the fuel pressure at the carb inlet port NEEDS to read 4.5-5 psi MAX and you NEED to set the float levels per the carburetor manufacturers instructions, having the pressure at 6.5 psi will in many cases cause the needle & seat valve to constantly leak excess fuel, into the carburetor while the engine runs
if your running a return style fuel pressure regulator it depends on the instructions that come with it and the number of ports,its usually mounted AFTER the two carb inlet fuel feeds
the pump feeds the fuel log, the fuel log feeds both carb inlets and the regulator mounted on the far end of the fuel log from the fuel feed bleeds off excess pressure to the return line to the the tank.the problem with all decent quality fuel line supply component parts is they cost more money, than the cheap low quality parts and generally take up more room and take a bit more time and thought to install correctly
I can easily see where the diagram may be confusing ,, and while that cheap holley return style fuel pressure regulator works

the picture is correct on many of the Holley fuel pressure regulators
(these several versions)
the cheaper fuel pressure regulator feeds from the base

you need a fuel pressure gauge on the carb inlet port,to accurately set fuel pressure
and then you need to rev the engine and hold it at 3500 rpm for a few second's
then drop the rpms and watch the pressure gauge



at maintaining fuel pressure on the inlet port and can be used its far from the better choice
this is the type of FP regulator you want in most cases,
and it comes in AN#8 and AN#10 sizes

its simply a strait through fuel flow with a pressure release that leads to the return line that opens if the fuel pressure exceeds a set level, thus it maintains that max pressure





remember that on any turbo or super charged engine application the fuel pressure MUST be indexed to the engine boost, to maintain the ideal fuel/air ratio, and you'll generally want most engines to run in the fairly lean 14.7:1-13.5:1 range at idle but be tuned to transition smoothly and consistently, to a fuel/air ratio closer to about 12.6:1 as the rpms AND loads on the engine are increased,


the regulator linked above can be adapted easily from carb to EFI use

http://www.amazon.com/Moeller-Separatin ... d_sbs_sg_1



yes the diagrams a bit confusing,pressure is a measurement of RESISTANCE to flow, the pump puts out , flow volume, theres no pressure until theres a resistance or restriction to flow,


potentially 6 -7 psi the true pressure is regulated to what ever the fuel pressure regulator is set too, IE, if the bye-pass spring setting is at 4.5 psi thats a high as the pressure ever gets, the fuel pressure potentially exceeding this forced the regulator to open,the spring resistance is over come and the fuel flows back too the tank, thus running out through the bye-pass, into the return line, limiting pressure.


but on some models its mounted just before the fuel log on the port labeled "CARB" and the two other ports are labeled "feed" for the pump and "RETURN", for the return line



if your, looking at the diagram,and asking what pressure the fuel rail feeding the carburetor is pressurized too its obviously controlled by the fuel pressure regulator just beyond it in the diagram, which is usually located near the carburetor and fuel rail, to maintain the indicated 5psi, beyond that is the return line feeding back to the fuel tank and that should be close to zero, the main feed line from pump to the first fuel pressure regulator which ideally is located on the inner front fender or firewall maintains the 8psi-12psi the fuel pump provides , keep in mind a fuel pressure regulator can only control the pressure between it and its pressurized feed source, by bleeding off pressure above the peak its set for, it has zero control past it, it only controls pressure between it and its pressure feed source ,in that diagram the first fuel pressure regulator is NOT mandatory in some applications, its the use of the secondary nitrous feed that makes it useful in the depicted application
http://members.shaw.ca/corvette86/FuelS ... gnosis.pdf
ok logically if your flooding fuel into the intake, its related to injectors leaking,or the fuel pressure regulator leaking or a vacuum line sucking fuel from some place like the fuel return line simply because thats how fuel can enter the plenum.
if you block the fuel return line to the tank from the fuel pressure regulator, and remove the fuel pressure regulator vacuum line temporarily, you should see the fuel pressure bump up to 40-47 psi , if not its most likely the injector(s) leaking, or fuel being sucked into a vacuum line, because your effectively eliminating a defective fuel pressure diaphragm in the regulator.
as always its a isolate and test procedure

EFI operates on a SIMILAR CONCEPT but obviously at higher pressures and generally with individual injectors spraying fuel that are usually controlled by a computer with sensor input rapidly matching fuel delivery to the engines demands

many EFI fuel pressure regulators are connected to the injector fuel rails, thus they maintain a designed fuel pressure feeding the injectors controlled by both manifold vacuum and fuel pump pressure, as plenum vacuum drops , the vacuum level above the fuel pressure regulator diaphragm has less effect on reducing fuel pressure return line psi, as it has less effect on the regulator diaphragms resistance spring, preventing the return line port opening, so fuel pressure increases slightly as demand and rpm increase.


READ THESE THREADS & linked info





http://www.circletrack.com/enginetech/c ... _pressure/




http://www.circletrack.com/enginetech/c ... _pressure/


the manual fuel pump is working in direct relation to the engines rpm levels, an electric fuel pump is working against the rapid changes in inertia as the car accelerates and brakes, if you think shifting gears at high rpms won,t alter the fuel pressure curve if you don,t have a bye-pass style fuel pressure regulator and a fuel return like in use you've never watched a fuel pressure gauge on a car without those components

ideally all lines are 3/8" minimum but 1/2" id is far better
the electric fuel pump would ideally be mounted lower than the tank and as far back as practicable INSIDE the frame rails for crash protection, with a hot lead to the electric pump that only supplies electric power ,if you have oil pressure and the ignition keys in the on position, as electric pumps PUSH fuel,far more efficiently than they PULL fuel

BTW alcohol in fuel tends to cause aluminum to oxidize over time
as you don,t want the electric pump running if the engines NOT running.
the fuel filter should be mounted for easy access,as you should change it frequently.
the regulator should be mounted as close to the carb as easy access allows, with the return line back to the tank, ideally larger than the feed line for zero resistance to fuel flow
keep in mind the by-pass style regulators work by allowing all fuel flowing above a certain threshold pressure you set the regulator too,to return to the tank, usually that's set at 4.5-6 psi for carbs[/b] [/i]

keep in mind that the pressure sensor only makes the electrical connection once theres 5 psi of oil pressure in the blocks oil passages
"Ok G.V., need your input.
I intend to wire up 1 electric pump through an oil pressure switch (GM thought of it first that's how I got the idea). If the car sits dormant for a few weeks and the fuel evaporates in the carb the engine could be hard to start. Granted, you should be able to crank 5 psi long enough to fill the carb and start the engine but this adds wear and tear to the starter. Which of the following solutions would you recommend if any?
(1) Bypass the fuel pump circuit and power it through the start circuit so the pump powers up during engine cranking. This would need a diode so the start circuit is not back fed through the fuel pump circuit after engine start up which complicates the circuit.
(2) Wire in a toggle switch that powers the pump before cranking. Once engine starts, kill the toggle and allow the oil pressure circuit take over. The switch positions would be marked START (hot)and RUN (open). In a (heaven forbid) severe collision, the toggle could be activated (unlikely but possible) and defeat the safety built in to the oil pressure power up circuit.
What do you think?"

I would NEVER use a manual toggle switch,on an electric fuel pump they have a tendency to get thrown into the wrong position at times, causing all kinds of potential problems ranging from flooded carbs,engine stalling, dead battery's, burned out pumps etc.

if you want an over ride switch for the fuel pressure you'll want a BUTTON (normally open contacts)that connects the circuit only while the spring loaded button is MANUALLY HELD in the depressed position, similar to the older cars starter circuit buttons, thus the circuit works with or without the button depressed if the engine has oil pressure,and you have the OPTION to pre-prime the carb by MANUALLY depressing and holding the over ride button, momentarily ,sending the electricity to the electric pump to pre -prime the carb. but the fuel,system still works either way

THE DEAD HEAD STYLE REGULATOR...frequently not to be trusted
works with a spring on a valve that allows the valve to open once the DIFFERENCE IN PRESSURE between the sides of the regulator valves fuel lines has changed
think of it as a door that usually has 7-10psi on the feed side and you want lets assume 5.5 psi at the carburetor feed,
youll need to understand that the dead head regulator works on the DIFFERENCE in pressure between the feed and use sides so having 10psi on the feed and 7psi on the use side is almost exactly the same as having 12psi on the feed and 9psi on the use side to that style regulator, it uses a spring and residual fuel pressure to limit flow until the difference in pressure exceeds a set value.
as the fuel pump fills the line it eventually (fractions of a second )reaches the point where theres a volume of fuel past the valve with enough pressure to allow, BOTH the SPRING in the regulators valve and the fuel pressure past the valve to close the valve.... until the fuel pressure past the regulator is reduced to the point that the SPRING and the remaining fuel pressure/volume beyond the valve can not hold the valve closed and the valve is force open and held open until, that difference in pressure is restored. now lets launch the car hard, the pump that had maintained 8-10 psi to the regulator, 5.5 psi past the valve and the spring in the regulator is now fighting the fuel in the line feeding the regulators inertia, and the sudden drop in pressure as the throttle drops full open in the carb,what the pump sees is the full 8-10 psi or MORE the regulator sees a sudden drop off to near zero and it opens wide, if the fuel pumps able too it tends to flood the fuel bowl for a second then the valve slams shut, until the pressure drops off as you hit each gear the cycle repeats, the result is a surge in pressure and a rapid drop off in volume then a rapid flood of fuel that rapidly cycles as you go down the track
if you had a accurate fuel pressure sensor at the carb you'll see a rapidly cycling pressure/flow
if some crud gets stuck in the valve it cant close and your carb FLOODS OUT, because it must fully close every few fractions of a second to work correctly

because fuel flows strait thru at all times but as soon as resistance to flow exceeds the set pressure level the adjustable spring seat is seat at, the relief valve to the return line is forced open and all excess pressure and flow exits into the return line maintaining the set pressure but preventing any further increase, but never interrupting fuel flow , one other advantage of the return style fuel pressure regulator is its generally going to provide a lower fuel pressure temp being delivered to the fuel rails, or carburetor.
your ideally routing your fuel line well away from your engine and exhaust so your in theory not dealing with fuel line temps at any point much above 200F.
keep in mind that most engine compartment fuel lines are metal except for the last 12"-18" of flex tube, engine compartment temps commonly run in the 160F-210F range but if you correctly plumbed your fuel line routing and used a return style fuel pressure regulator the constantly changing flow of fresh fuel thru the fuel lines will absorb and transport a good deal of that absorbed engine heat out of the fuel lines before it reaches the carburetor or inspector fuel rails..the rubber or synthetic fuel line is a poor conductor of heat and there are insulated reflective covers available.

ENGINE HEAT AND THE RESTRICTIVE FLOW are both good reasons to avid using fuel filters like this one pictured below in the engine compartment

braided stainless covered flex line rated at 300 psi and 300f is a good idea, adding a heat reflective tube cover also helps reduce fuel temps



#3 in the diagram, its your OEM fuel pressure regulator the vacuum line changes the effective fuel flow and pressure slightly to compensate for the engine loads
manifold vacuum changes under engine load
The spring pressure is what governs your fuel pressure/flow at any condition (pressure and flow are inversely related). Engine vacuum acts on the diaphragm which compresses or relaxes the spring, thereby opening or closing the valve. Under high engine loads (low vacuum) the spring overcomes the vacuum diaphragm, opens the valve and more fuel flows. Under low engine loads (high vacuum) the vacuum overcomes the spring and the valve closes (not all the way, obviously) and fuel flow is reduced.




swapping to an adjustable versions usually good for a couple extra ft lbs of torque
2 fuel pressure diaphragm
3 fuel flow chamber and pressure route chamber
4 boost reference connection
5 pressure adjustment
8 return line valve and exit

in the diagram fuel pressure is supplied by the fuel pump and enters the port on the upper left it fills the area ABOVE the DIAPHRAGM in area 3 , as the pressure rises it eventually reaches the point where the flexible DIAPHRAGM 2 forces the valve 8 lower and away from its seat ,allowing fuel to flow thru the return line back to the tank, until the pressure above the diaphragm drops allowing the seat spring to close the valve, this effectively causes the return line to constantly get a very rapid flow of fuel as the pressure above the diaphragm causes the valve to the return line to keep bleeding off any pressure above the springs resistance, this insures the pressure stays very consistent at the carb entrance port.
the boost or plenum vacuum pressure line 4 from the plenum adds or subtracts resistance to the diaphragm movement to open the return line as boost pressure rises or vacuum increases, thus fuel pressure stays consistent with air pressure the carb sees, on some versions, but the adjustable spring pressure makes it possible to set the minimum fuel pressure thats being maintained to feed the injector fuel rail or carb feed input port, as long as the fuel pump can at lease maintain or exceed that pressure

the far superior, bye-pass style regulator functions in a totally different manor
assuming the same set-up but you replace the regulator with a bye-pass style regulator, the bye-pass regulator works by opening a valve too a much lower pressure path for the fuel to return to the tank,,the open fuel return line. anytime the pressure exceeds the 4.0-5.5 psi,you've set it to, so the fuel line to the carb can only see a max at that 4.0-5.5 psi. now the pumps sitting there potentially supplying at 8-10psi just like before, but it can never exceed 4.0-5.5 psi because the bye pass regulator bleeds of any and all excess flow volume,the pump supplies. but lets look at your launch, if the pressure drops to 6, or 7 psi nothing changes at the carb, if it increases to 10 or 12 psi, nothing changes at the carb,if it drops to to 4.0-5.5 psi that you set the regulator bye-pass circuit too or less the valve to the bye pass line will close , but the fuel route to the carb inlet remains consistently open, but that excessive drop in pressures seldom a problem, it the sudden changes in pressure and over pressures that happen when you suddenly change the fuel flow required or the (G)loads on the system that potentially screw things up,the bye-pass regulator style regulator isolates the carb and maintains the desired 4.0-5.5 psi to the carb FAR MORE CONSISTENTLY.
this style regulator design depends on a fuel return line size large enough that there will be no significant resistance to fuel flow past the regulator, in route back to the fuel tank, because any pressure in the return line tends to reduce the regulators accuracy.
now lets assume the spring gets a bit weak over time or the adjustment gets set at 4 psi in error, with the bye-pass style you'll probably never notice ,if you had a accurate fuel pressure sensor at the carb you'll see a rock steady pressure/flow, at the 4psi.
should some crud get stuck in the valve and it can,t close 100% ,NOT MUCH HAPPENS, because its normally OPEN not closed,
if you check you'll see MOST EFI systems are BYE-PASS regulated designs also due to control and reliability issues


Dyno Comparisons -- Adjustable Fuel Pressure Regulator Settings


Here, the dyno shows a 16.6 horsepower and 14.6 ft/lbs torque difference between the stock fuel pressure (46psi) and 36psi with the LG Motorsports adjustable regulator. Actually, it was still making more power at 35psi than 36psi, but until I can measure my O2 sensors, I did not want to lean it out any further. The average gain here was 13.9 horsepower and 15.4 ft/lbs torque. Check out the Dynojet Race Routine between the stock and 36psi fuel pressure.

Something new: Dynojet is currently Beta testing their new WinPEP software for the dyno. With this software, they have the ability to graph up to 12 graphs at once. Check out the Dynojet WinPEP Graph of all the power between the stock and 35psi fuel pressures.

NOTE: One thing that I have found after some time with this mod is that the stock computer is quite smart. After lowering fuel pressure to achieve a better air/fuel ratio, the computer sees this at part throttle and adds injector pulsewidth to compensate. Basically, horsepower fell back off to where I started from. While normal trains of thought tell us that since O2 values are not measured during wide open throttle, that fuel pressure should have a long lasting effect here. Actually, it does not. The computer uses the last known long term fuel trim integer at WOT, which is to add fuel since at part throttle it has adjusted for being too lean.


return style typically have 3-4 ports

but on the dead head the cycle just gets about 20% more erratic and more frequent in the cycles, further weakening the spring over time

http://static.summitracing.com/global/i ... a-4309.pdf

btw your fuel pump tends to run under less stress and run cooler with a bye-pass style regulator also

http://static.summitracing.com/global/i ... -13301.pdf

http://aeromotiveinc.com/wp-content/upl ... ter-NN.pdf

http://static.summitracing.com/global/i ... 0036-1.pdf

most people don,t understand that the return fuel line to the tank will ideally have very low restriction to flow for the fuel pressure regulator to function correctly, if theres a restriction to flow it reduces the bye pass fuel pressure regulators accuracy and consistency.
most guys use a smaller diam. return line thinking that theres less fuel flow due to the engine use of fuel, that's not generally 100% accurate because if the fuel pressure is to remain consistent at a set pressure & volume the fuel pump must produce more fuel pressure and flow volume even at peak rpms than the engine can burn and even under high (G) loads, meaning that the return line needs to be fairly large and unrestricted to reduce the resistance, to flow in the upper rpm ranges with manual pumps that work in relation to engine rpms and all the time with electric pumps



Dead heads require a bigger rated pump than return style, hp for hp. They don't call them dead heads for nothing One source recommends -6 or 3/8 lines up to 375 hp, 550 with -8. Make sure the rest of your system is up to the task.Running a return style regulator is a pain, but worth the effort. Your pump will thank you by only working hard when it needs to, your pressure is stable, fuel cooler, and you can concentrate on the next problem. John







Assume a BSFC of 0.55 and gasoline at 6.25 lbs/gallon:

hp x 0.55 = pounds of fuel burned per hour

600hp x .55=330lbs
330lb /6.25lbs per gallon=52.8 gallons an hour minimum fuel used

http://www.circletrack.com/enginetech/c ... _pressure/


a fuel filter with an easily replaceable filter element, a clear transparent drain, remotely mounted low and near the fuel tank,for easy access and maintenance helps fuel system durability

before you get crazy chasing some problem your sure is an intermittent fuel delivery issue
(1) Do a compression test!
(2) verify your ignition advance curve, and verify the ignition systems working correctly , the spark plugs are new, properly gaped and the ignition wires in excellent condition visually and with an OHMS meter.
(3) adjust your valves, correctly
(4) carefully verify theres no vacuum leaks, in lines or gaskets
(5)Check the fuel delivery system, WITH A GAUGE, while the engines under real operational inertial loads to verify you have a consistent 5-6 psi at the carburetor inlet port
(6) change out the fuel and air filters, especially if over 3-4 months old
(7) actually check your exhaust back-pressure levels
(8) verify your return style fuel pressure regulator and fuel lines function as intended
(10) actually look for and read installation instructions and rated flow and pressure limitations on fuel pumps and filters
(11) If ALL of the above are normal, only then start looking at the carburetor, and tuning issues

I think most guys would be amazed at how often, a high flow water separating fuel filter installed near the fuel tank that,s being checked and drained, with the filter medium changed regularly will cure or prevent a good many TUNING ISSUES

http://www.harborfreight.com/38-x-38-ba ... 68213.html



http://www.amazon.com/RACOR-320R-RAC-01 ... m_sbs_sg_2

http://www.amazon.com/Moeller-Separatin ... d_sbs_sg_1
Last edited by a moderator:


Staff member
thats a good question for this forum,
Ok Ive got one and have tested several of them.





http://www.stockcarracing.com/techartic ... index.html



http://www.chevyhiperformance.com/howto ... index.html







heres my take on them, short answer, they are a big help but a P.I.T.A. to set up and use if your not going to semi permenantly install them on your car.

IVE come to use reading spark plug condition,



use of a good timing light and vacuum gauge , a fuel pressure gauge, to verify your fuel pressure under load, and use of a GOOD HIGH TEMP INFARED THERMOMETER (THIS ONE)


as a very quick to use and accurate set of tools. :thumbsup:


well your main concern when tuning an engine is to keep the all the cylinders running aproximately the same ratio and at about 12.8:1 for max power up to about 14.7:1 for low emmissions and good mileage, AND WHILE A A/F GAUGE IS A GREAT ASSET, ITS EASY TO DAMAGE, AND ITS BEST INSTALLED IN YOUR CAR , NOT USED AS A SHOP TOOL.

so whats the advantage/disadvantages

a fuel air meter uses a o2 sensor, if you place it in the header collector it gives an AVERAGE of all the cylinders on that cylinder head,If theres an (X) installed close to the dual collectors reversion pulses can occasionally even give data from the other side of the engine, so in theory and in practice you can have two cylinders run lean and two rich and the AVERAGE tends to look RICH to the O2 sensor as it SEES unburnt fuel, if you place it in the individual primary header tubes you either need eight O2 sensors (VERY EXPENSIVE, and keeping the wires from burning or grounding outs a TOTAL P.I.T.A......IF you don,t succeed you destroy the O2 sensor and need to replace it.) or you need to be constantly swapping very hot and fragile O2 sensors and bung plugs constantly, but with the IR thermometer you can almost instantly see which cylinders are running hotter or cooler and adjust the jets or look for vacuum leaks, or other CAUSED for the TEMP DIFFERANCE, ETC, far faster too get all the cylinders running at approximately the same temp, indicating the same fuel air ratio, youll be amazed at how close the temp follows the fuel/air ratio, and you can confirm it with plug condition and the other test equipment. run any cylinder too lean and detonation can break rings or melt pistons, run it too rich and you can wash the oil off the cylinder walls and ruin rings/scuff pistons, you need to verify the fuel flow rates and pressure at the carb, or fuel rails the injectors are fed from,and you need to know whats going on in EACH CYLINDER not the AVERAGE of all cylinders.

SO, if your going to install a decent wide band fuel air ratio meter on your car thats fine, its going to be an asset to your tunning skills, if you install the indicator/gauge inside the car and weld in a couple extra bungs in the collectors for tunning and wide band O2 sensors which are a big help, but you will quickly find that its a P.I.T.A. to use it for tune ups on all your buddies cars with the welding collector bungs and installing plugs and O2 sensors while the IR thermometer route is fast and very simple and you can confirm with oplug reading the condition of the engine.

YEAH! theres meters that you can stick in a tail pipe, but they read THE AVERAGE, not the individual cylinders ,
think about AVERAGEs
AS my old physics proffesor once said,
IF, I pour molten lead in your front slacks pockets and pack your butt in solid with DRY ICE,.... ON AVERAGE your comfortable:D


bits of useful info on these








IVE generally found no problem with any corvette LT1 or TPI efi if it will quickly build a minimum of 38-40PSI on start up and MAINTAIN at LEAST 38-40 psi for a MINIMUM of 10 minutes after the engines turned off, 40-42 lbs is what the factory tests want and expect so if your getting 38-40psi your pressure readings indicate a normal, perhaps even better than average pressure reading, and more than expected pressure retention,
IF you suspect a TPI has a defective injector that's leaking or not flowing fuel,or partly clogged...You can also test this, to a degree, with a fuel pressure gauge
you can install a fuel pressure gauge on the fuel rail, and turn on the ignition key then watch the gauge, it should jump to 37-42 psi then remain fairly steady for at least 5 minutes before gradually loosing pressure.
If you can - and this is a pain in the butt to do, temporarily- remove the injectors. Leave them on the rail, but remove the rail. from the intake itself, This requires removing the majority of the TPI (P.I.T.A. but required).



Remove the rail, but leave it connected to the fuel lines.

Next up, put a strip of cardboard under each bank. Then, have a buddy turn the ignition key on but not try to start the engine , you should hear the fuel pump run for 3 seconds while you look to see what happens. You should get an immediate spray of fuel.(all should be about equal and no injector remains dripping fuel) now have your buddy Turn off the key, while you watch closely to see which injectors are leaking.
in many or most cases engine dieseling after the ignition is turned of is caused by a combination of a bit too rich of a fuel/air mix being drawn in and a bit too much heat in the combustion chambers this is common when a holley carbs transfer slot is badly adjusted to expose too much slot at idle or carbs with the float level set too high, or a fuel pressure regulator pressure set too high or a blocked return line on the fuel pressure regulator, or at times indicates a vacuum leak



BTW on carburetor equipped engines 5.0psi--5.5 psi at the carb inlet port is about ideal

below you'll find a good base line starting point,
and Id suggest you monitor coolant and oil temps,
try hard to keep coolant temp under 200F and oil temp under 220F
verify your fuel delivery pressure is consistent over the whole rpm range
take the time to graph out and verify your ignition advance curve!
a good IR temp gun to use on the header primary tubes helps locate fuel/air ratio inconsistency's

GENERALLY your fuel/air ratio should be in these ranges
Idle- up too about 2500 rpm try for 14.7:1-15:1 f/a ratio,
too get max mileage and prevent spark plug fouling

from about 2500 rpm- too about 4500 rpm,
try to smoothly and predictably transition the fuel/air ratio mix richer to about 13.5:1
for good power and less chance of detonation

from about 4500 rpm- too about 6500 rpm and higher
,try to smoothly and predictably transition the fuel/air ratio mix richer to about 12.5:1,
for good peak power and less chance of detonation.

this is only a well proven starting point on the tune,
but it generally gets you in the ball park ,
and tends to reduce the chances of the engine reaching detonation conditions.

the ignition advance curve needs to be checked, the chart below is a very good starting point to work from, and USE OF A RICHER FUEL/AIR MIX, WILL AT TIMES BE REQUIRED IN THE OFF IDLE TO 3500 rpm range, but the idea here is to keep the lower rpm and lower stress operations running at efficient fuel/air ratios to reduce plug fouling and improve mileage as youll generally spend 90% of the cars operational life on the street at under 4500 rpm.


set the plug gaps at about .045, make sure the valves are adjusted correctly
Last edited by a moderator:


Staff member
if your running a carb, you need to have a correctly set up fuel system, one of the more common complaints is related to fuel pressure at the carb inlet port, many fuel pumps produce 6-12 psi of fuel pressure at higher rpm levels , some even provide that at idle speeds, that much pressure tends to flood carbs and make float adjustment difficult or useless, only 7-9 psi will almost always overcome the needle & seat and constantly flood the carb, that is probably the source of your major problem, install a RETURN STYLE fuel pressure regulator, and a line back to the tank or at least back to a TEE that's located before the pump intake point
bits of reference material.
Id try to get as close to the basic listed parameters below as a start point as you can, and the slight hesitation on acceleration might require a larger accelerator pump cam curve , or a power valve that kicks in at a bit higher vacuum level, example swapping from a 7.5 to and 8.5, make only a single change at a time and keep good detailed notes on results, and don,t discard parts, label them, keep in zip lock bags with labels
yeah I might not be up on the latest electronic data loggers but I have a good idea what an engine wants as far as a fuel/air ratio and combustion conditions and ignition advance curves and can read spark plugs, and while each engines a bit different you may be amazed at what consistent condition to start from does to help isolate any tuning problems





set the float levels and verify the fuel pressure consistent at about 5 psi entering the carb inlet port.
your fuel/air ratio should be in these ranges
Idle- up too about 2500 rpm try for 14.7:1-15:1 f/a ratio
from about 2500 rpm- too about 4500 rpm try to smoothly and predictably transition the fuel/air ratio mix richer to about 13.5:1
from about 4500 rpm- too about 6500 rpm and higher ,try to smoothly and predictably transition the fuel/air ratio mix richer to about 12.5:1
this is only a starting point on the tune but it generally gets you in the ball park and tends to reduce the chances of the engine reaching detonation conditions.
the ignition advance curve needs to be checked, the chart below is a very good starting point to work from


set the plug gaps at about .045, make sure the valves are adjusted correctly

years ago I purchased two of these assortment packs when they cost about 1/2 the current price , Ive tried hard to keep 4 jets in each size in inventory as I use those, 72 jets in assorted sizes for $52 is still a good deal , and it gets better if you buy the pack or two of them, when summit posts a discount code

IF you can read spark plugs , USE A TIMING LIGHT, and use an INFRARED TEMP GUN AND VACUUM GAUGE
http://www.harborfreight.com/cpi/ctaf/d ... mber=93547

http://www.harborfreight.com/5-in-1-dig ... 98674.html




infrared thermometers are a very useful tool to track down issues with tuning, or mal functioning sensors




these cheap ones, that are dead head style seldom work well


much better



Ive never seen a dead head style fuel pressure regulator keep consistent pressure levels.
you need to think thru your fuel supply system, line size etc. while in an ideal world you run a similar size to the pressure feed line or a larger diam. line as a return line back to the tank so theres nearly zero flow resistance,you don,t need to run the return line back to the fuel tank, on a street car if your not all that familiar with setting up a fuel system with an electric fuel pump that didn,t come from the factory with a return line PROVIDED your under about 400hop and your feed line is a 3/8" line, its not as good, or effective to jury rig it but you sure won,t be the first guy to do it either, but it can be run to a TEE
in the line on the feed side of the electric pump
Last edited by a moderator:


Staff member
if your running a carb, you need to have a correctly set up fuel system, one of the more common complaints is related to fuel pressure at the carb inlet port, many fuel pumps produce 6-12 psi of fuel pressure at higher rpm levels , some even provide that at idle speeds, that much pressure tends to flood carbs and make float adjustment difficult or useless, only 7-9 psi will almost always overcome the needle & seat and constantly flood the carb, that is probably the source of your major problem, install a RETURN STYLE fuel pressure regulator, and a line back to the tank or at least back to a TEE thats located before the pump intake point



these cheap ones, that are dead head style seldom work well


much better




High-flow for your fuel system.

When using high-pressure fuel pumps, a fuel pressure regulator must be installed to prevent carburetor flooding. These Mallory fuel pressure regulators feature housings fully machined from 6061-T6 billet aluminum, and specially designed high-flow fuel passages. These features, combined with a quick-acting diaphragm assembly, make them extremely efficient. A mounting bracket is provided to allow quick and easy mounting.




Staff member
if your running a carb, you need to have a corretly set up fuel system, one of the more comon complaints is related to fuel pressure at the carb inlet port, many fuel pumps produce 6-12 psi of fuel pressure at higher rpm levels , some even provide that at idle speeds, that much pressure tends to flood carbs and make floart adjustment difficult or useless, only 7-9 psi will almost always overcome the needle & seat and constantly flood the carb, that is probably the source of your major problem, install a RETURN STYLE fuel pressure regulator, and a line back to the tank or at least back to a TEE thats located before the fuel pump intake point (NOT NEARLY IDEAL, BUT FUNCTIONAL)

http://www.centuryperformance.com/fueli ... g-140.html



these cheap fuel pressure regulators with only an in and out port , that are dead head style seldom work well, IN FACT IVE RARELY SEEN THEM WORK AT ALL!


much better






In the fuel injector sizing, always use a safety margin between 15-20%. , if the engine requires a 40 lb injector youll want a slightly higher rated injector sizem remember carbs generally operate at about 5 psi, fuel injection runs in the 37psi-80 psi range depending on the application.


Fuel Hoses & Routing
Even with proper injector and fuel pump sizing, a fuel system will not flow adequately unless the hoses that deliver the fuel to the fuel rail are of sufficient size and are routed properly. On systems that use the PNP version of the AEM EMS, there is no need to replace the fuel delivery hoses unless the engine is heavily modified.

NEVER route fuel hoses through the interior of a car. Put bluntly, this is a dangerous thing to do. Whenever possible, use a delivery tube to make the connection from the pump discharge to the filter in the front of the car. The lines should be rated to withstand at least twice the maximum pressure of the EFI system.

Using the above parameters of our sample engine with moderate boost, we expect to see pressures in the 65-70 psi range. This will require a line with at least 140-psi rating (most AN hoses exceed this by a large margin). When routing fuel lines, it is imperative that they are protected from road hazards and the exhaust system. The fuel line should NEVER be routed near battery cables. Use clamps to secure AN hose every 15 inches, or 24 inches if a rigid tube is used.

The following table will help you determine which hose size is correct for your application: These sizes are based on a nominal fuel pressure of 40 psi.

Fuel Delivery Hose Sizes
Gasoline Powered Engines
Up to 499 HP .344” hose -6AN
500 - 799 HP .437” hose -8 AN
900 – 1100 HP .562” hose -10 AN

Methanol Engines
Up to 499 HP .437” hose -8 AN
500 - 799 HP .562” hose -10 AN
900 – 1100 HP .687” hose -12 AN


Staff member
http://www.centuryperformance.com/fueli ... g-140.html

If you don,t have an adjustable fuel pressure regulator on your fuel injection or carb equiped performance, engine.....
why not stop guessing and install a fuel pressure gauge to monitor whats really going on under the hood,and install an adjustable return style fuel pressure regulator with a return line to the tank, so YOU can control EXACTLY what pressure levels the carb input port sees!
being able to control, and verify exactly the pressure and volume of fuel at the carb, or injector fuel rail eliminates most of the guess work.
as an example
JACK recently stopped by to say his reasonably new carb would not maintain the carb float settings all the time, a quick check with a fuel pressure gauge showed he had 7 psi at the carb ,feed line, and that was overloading the carbs needle seat assembly most of the time, adding a return style regulator set at 5 psi cured is intermittent miss and occasional fouled spark plugs he had been putting up with for almost a year, it never occurred to JACK that the problem started to get worse after he upgraded his fuel pump, simply because the problem was happening with the old fuel pump, just not as frequently..

YES IN MOST CASES A CARB APPLICATION CAN RUN WITH NO RETURN LINE , OR A FUEL PRESSURE REGULATOR INSTALLED, BUT YOU WON,T TEND TO GET CONSISTENT FUEL FLOW VOLUME AND PRESSURE AT THE CARB WITHOUT A RETURN LINE STYLE FUEL PRESSURE REGULATOR, and you would be amazed at the number of tuning issues that are avoided with a decent return style fuel pressure regulator installed in a fuel supply line system , like inconsistent float levels, seemingly non-working fuel metering at idle, and high rpm lean or rich conditions.The manufacturers would NEVER spend the money on fuel return lines if it was not almost mandatory to getting the car to run consistently and pass emission testing under some conditions.
its kind of like carrying a spare tire , a jack in the trunk,or wearing a bullet proof vest, 90% or more of the time you could get bye without them, but under some conditions not having them becomes a HUGE problem


Staff member
On the TPI and LT1 corvettes, camaros, etc. theres two fairly common fuel pressure regulator problems, when the fuel pressure regulator starts causing problems, the first symptom is hard starts because the fuel pressure never gets high enough to supply full fuel pressure, the second failure results from a leaking diaphragm and you get fuel leaking in the vacuum lines
that's not un-common, Ive seen several corvettes with both TPI and the LT1 engines with fuel pressure regulators that would not maintain the standard 37-42 psi but that didn,t leak, externally, the diaphragm may be good preventing leaks but the spring allows fuel to flow below the normal minimum of 37-42 psi required for easy starts and consistent tuning on the MPFI engines
obviously a fuel pressure regulator should maintain a minimum working fuel pressure and that needs to be verified with a fuel pressure gauge. adding a small fuel pressure gauge and an adjustable fuel pressure regulator to your engine makes diagnosing and tuning far easier
IF YOU DON,T CURRENTLY have an ADJUSTABLE fuel pressure regulator on your TPI you NEED TO INSTALL ONE
In some cases a defective diaphragm in a TPI fuel pressure regulator allows fuel under pressure to enter the vacuum lines and raw fuel flows into the plenum.
installing ADJUSTABLE EFI fuel pressure regulators, which are a darn good idea on the older C4 corvettes

The stock fuel pressure for 1985 to 1987 C4 engines with TPI was 36 to 39 psi with the fuel pressure regulator vacuum hose connected, and 47 to 48 psi with the vacuum hose disconnected. For the 1988 to 1996 C4 corvettes, the stock fuel pressure is 40 to 42 psi with vacuum hose connected and 47 to 48 psi with the vacuum hose disconnected.

swapping out the stock non-adjustable fuel pressure regulator requires removing the plenum but takes only about 20 minutes and its easy to do, it allows some minor tuning adjustments which tends to help power.ESPECIALLY if the stock FPR is leaking





be VERY SURE you order a adjustable fuel pressure regulator that comes with a NEW matching diaphragm as many kits assume youll re-use the stock diaphragm


if you swap to an adjustable fuel pressure regulator on your TPI fuel rails it makes sense to install a small fuel pressure gauge on the fuel rail so you can easily verify your fuel pressure, keep in mind raising the fuel pressure tends to richen the effective fuel/air ratio slightly while reducing the pressure tends to lean it out slightly, but remember the computer and the oxygen sensors try to maintain the emission friendly 14.7:1 fuel air ratio

http://www.jegs.com/i/JET-Performance/5 ... ProductId=

http://www.holley.com/data/Products/Tec ... 0036-1.pdf

http://static.summitracing.com/global/i ... a-4315.pdf

http://www.chevythunder.com/fuel%20inje ... pg%20D.htm
All you need to do is buy any 1/8" NPT Fuel Pressure Gauge rated at 0-60 psi. to mount on your fuel rail, and a 1/16 /1/8 NPT adapter

http://www.jegs.com/i/JEGS+Performance+ ... 2/10002/-1


http://www.jegs.com/i/JEGS+Performance+ ... 3/10002/-1

http://www.jegs.com/i/BBK-Performance-P ... 4/10002/-1

http://www.harborfreight.com/fuel-injec ... 92699.html

no one anywhere can accurately predict exactly what your particular engine combo will find to be the best tune-up specs but any good dyno operation or experienced shop will start with a basic known base line and tweak the combo to get the best results, you'll want a consistent 40 lbs of fuel pressure, you'll want to keep the coolant temps under 200 F
youll want the oil temps in the 200F-210F range
and youll ant the injector pulse duration to stay under 75%-80% at peak rpms , this may require larger injectors, and youll want significantly less than 3 psi of exhaust back pressure at peak rpms.
heres a basic tune


installing a fuel pressure gauge on your LT1 fuel rail will help provide a slightly higher fuel flow from existing injectors

an adjustable fuel pressure regulator allows you the option of feeding the injectors with more pressure which results in a slightly richer fuel/air ratio at any pulse duration.(at times this helps during a tune up)










post your results and more questions




http://www.chevythunder.com/fuel%20inje ... pg%20D.htm

http://members.shaw.ca/corvette86/FuelS ... gnosis.pdf











BLACKZ51 posted this info

How to Install a Fuel Pressure gauge on an L98 TPI.

-what your going to need: -Guage (most are 1/8npt)
- An adaptor that will screw onto the -4AN schrader valve on the fuel rail and convert it to 1/8Npt.
-A tool to remove the Schrader valve.

Cost for this project:
$2 for schrader valve tool.
$17.00 for Fuel Pressure Guage
$ 20.00 for fittings (elbow and 4an to 1/8 npt.)
Total Cost $39

Step 1: remove the Fuse that says FR or FP 1. this will disable your fuel pump and relieve the pressure so you dont get fuel all over the place. (thanks oldvettefan!)

Step 2: remove the cover on the fuel rail, and then with at tool remove the schrader valve. have a rag handy to catch the little fuel that spills.

Step 3:A little teflon tape doesn't hurt, I wrapped the threads then screwed on the new fitting. then did the same for the elbow and gauge.

Tighten everything and then your done! replace vaccuum lines if you needed to move them.


I did this first so I can monitor fuel pressure settings before I go ahead with installing the AFPR.
I found that I had the stock 47 psi at idle with vaccum disconnected. and 39 or so with it connected.
Last edited by a moderator:


Staff member
Last edited by a moderator:


Staff member
Instructions, testing a stock fuel pressure regulator
Things You'll Need

Fuel pressure gauge


Locate the fuel system test port on the fuel system rail. The rail resembles a tube running between the fuel pump and the throttle body (or carburetor) and has what looks like a tire air valve stem with cap. Be certain you do not mistake the AC access tube for the fuel system rail. If in doubt, trace the line in both directions to be sure it is not connected to the air conditioning compressor.

Unscrew the cap from the fuel system test port with your fingers. If the cap sticks twist it gently with the pliers.

Screw the hose fitting of the fuel pressure gauge onto the test port. Set the gauge where it will not be vulnerable to moving parts of the engine, and start the vehicle.

Read the fuel pressure gauge, and then remove the rubber vacuum line attached to the throttle body (or carburetor) assembly. If the reading on the gauge does not increase at least 5 psi after the vacuum line is removed, the fuel pressure regulator is faulty.

FAQ - Carbureted Regulators
1. I want to convert my EFI engine to a carburetor, how can I “knock the pressure down” coming out of my EFI pump? What Aeromotive regulator should I use for this?

A common misconception about fuel pumps is that they “put out” a specific pressure. It makes more sense to think of the pump as a source of flow. A bypass regulator creates pressure by restricting flow from the pump, forcing the pump to produce pressure up to the regulator’s set point. Once enough pressure is created the regulator bypass is forced open, allowing excess flow onto the return line. From here the regulator relieves just enough excess volume to maintain pressure. The Aeromotive 13301 bypass regulator can bypass enough volume to handle most medium to larger in-tank EFI pumps, if the return line itself is large enough. Note: Most stock EFI return lines are too small for a carburetor conversion, creating more backpressure than the regulator. At minimum, carb conversions with a 13301 regulator will require a –06 AN (3/8″) return for small OE pumps and an AN –08 (1/2″) for medium to larger pumps. When in doubt install the larger, freer flowing AN -08 line to ensure good results.

If the return line is too small, the question becomes, “Besides running a bigger return line, is there any other way to use the stock, in-tank EFI fuel pump to feed a carburetor?”

There is one possibility, but it means adding another regulator, using both a bypass and a static regulator together. In this case the 13301 regulator is used to first control what is called “line pressure”. This means feeding the stock supply line into the 13301 and then running the stock return line from the bypass port back to the tank. Next, the outlet line from the 13301 is fed into the 13205 static regulator before going to the carburetor. The 13301 is set for 12-14 PSI, high enough to allow use of the smaller, stock return line and then the 13205 is used to block that down to the 5-8 PSI range for the carburetor.

2. I’m using a bypass style regulator for my carbureted fuel system. I want to install a nitrous kit. How do I set up a second regulator, either static or bypass, to control nitrous fuel pressure?

To avoid engine damage, be VERY careful with this one! Adding a second bypass regulator and attempting to set it for a lower pressure than the primary bypass regulator will default the entire system to the lower pressure. Feeding a new static regulator from an existing bypass system, either before or after the bypass regulator, will not provide adequate inlet pressure to the static regulator. Understand, a dead-head regulator needs two times inlet to outlet pressure and a bypass regulator creates the same pressure at the inlet as it does the outlet. Running a static regulator for nitrous set at 5 PSI with only 7 PSI inlet pressure is a recipe for nitrous lean-out and potentially serious engine damage.

Short of installing a separate fuel system for the nitrous (highly recommended), the only reasonable option is to raise the bypass regulator pressure up to 15-25 PSI. This is enough line pressure to feed multiple static regulators, one for the carburetor and others for nitrous stages. Aeromotive now offers stackable, modular static regulators under P/N 13217that can be bolted together for easy installation and the regulator body forms its own fuel log. Using bypass regulator P/N 11217 attached to the last stackable regulator to create line pressure necessary to feed multiple carburetors and nitrous stages. This is perfect for nitrous using inline fuel pumps like the A1000, Eliminator and Pro-Series.

3. No matter where I set the adjusting stud on my static (non-bypass) regulator, I can’t seem to control the fuel pressure consistently, why?

Most static carburetor regulators, including Aeromotive’s, require some flow through the unit while pressure is being adjusted. The proper procedure is to turn the pump on, start the engine and then set pressure. Steps for installing and setting a new regulator include: 1) Turn the adjustment screw counter clockwise to the lowest possible setting. 2.) Power and run the pump, with the engine off, until the bowls are full. 3.) Turn the pump off. 4.) Start the engine. 5.) Turn the pump back on. 6.) Adjust base fuel pressure up to the desired set-point by turning the set screw clockwise and locking the jam nut. Note: If you go to up and want to come down, make a small adjustment, turning the adjusting stud counter-clockwise, and then blip the throttle a couple of times to bring the pressure down.

4. I’m using a static (non-bypass) regulator and every once in a while the pressure spikes, going past the set point and pegging the gauge, flooding the engine. What’s wrong?

This condition is called pressure creep. It’s caused by the regulator failing to seal and stop flow when the set pressure is achieved. A static (blocking) regulator must close and stop flow perfectly to control pressure. A dead-head style pump is designed to create pressure from 14-21 PSI, in the range of 2-3 times higher than the regulator set-point. Anything that prevents the regulator valve from closing and sealing will allow it to leak, causing pressure to creep up at the needle and seat, sinking the float and flooding the engine. When pressure creep is a problem it’s common to find debris has lodged in the regulator valve. The regulator may be able to be disassembled, cleaned and restored to proper operation. In the event cleaning does not resolve the problem a rebuild kit with replacement valve will normally fix it. Aeromotive offers rebuild kits with replacement valves for all Aeromotive static pressure regulators.

5. My regulator doesn’t seem to keep consistent pressure, every time I look at the gauge it reads something different, it’s driving me crazy, what is the problem?

Chances are good you have a liquid filled fuel pressure gauge, infamous for changing its reading with temperature. Because a liquid filled gauge is sealed to keep the liquid inside, the pressure inside the gauge case may not be equal to atmospheric pressure. Once sealed shut, the liquid in the case expands and contracts as gauge temperature changes, making the internal pressure in the gauge vary up and down as it heats and cools. Case pressure can change as much as 7 psi up and down with heat, which affects the gauge mechanism and changes the gauge reading by the same amount!

Note: Even though the needle is moving up and down, fuel line pressure is NOT actually changing, the pressure in the gauge case just makes it look like it is. Testing for this problem is easy, just carefully heat the gauge, using a heat gun or blow dryer, from cold to warm and then hot, while running the fuel pump. The gauge reading will drop as the oil inside warms up and the pressure in the gauge case increases. Now pop the plug in the side of the gauge case and watch the pressure drop back down.

Aeromotive now offers a brand new, state of the art, Liquid Filled Fuel Pressure Gauges for carbureted and EFI engines P/N’s 15632 and 15633 respectively. These new “equalizer” gauges feature a pressure equalizing pin-valve in the side of the case allowing the user to compensate for heat related changes in case pressure, quickly restoring gauge accuracy at any operating temperature.

For more detailed information on problems with liquid filled gauges, and the new “equalizer” gauges, see Aeromotive Tech Bulletin TB-903 at www.aeromotiveinc.comunder the Tech Help, Tech Bulletin section.

6. What is the barb fitting in the regulator cap used for? How and where should it be connected if I use it, and what should be done with it if I don’t?

All Aeromotive, carburetor bypass regulators incorporate the necessary design to allow the regulated fuel pressure to be vacuum or boost referenced, on a 1:1 ratio. For “blow through carb”, forced induction applications, where a turbo or centrifugal supercharger pressurizes the carburetor through a hat or in an enclosure, the regulator boost port should reference to positive pressure only, not vacuum. connect the port to the carburetor box or hat, not the intake manifold. For carbureted, naturally aspirated engines, and for roots supercharged engines where the blower draws through the carburetor, the vacuum/boost reference port should be left open to atmosphere, never plugged.

There is one potential use for referencing fuel pressure to vacuum on a naturally aspirated (not blow through) carbureted engine; where alcohol is the fuel of choice. In this case a line from the regulator to the base of the carburetor, beneath the throttle blade into the intake plenum, can be used to lower idle fuel pressure and allow higher fuel pressure to feed the carburetor at wide-open-throttle.

For more detailed information on carburetion and vacuum/boost referencing fuel pressure, see Aeromotive Tech Bulletin TB-201 at www.aeromotiveinc.com under the Tech Help, Tech Bulletin section.

7. I have a dead-head (blocking) style regulator and I want to run a return line. Will it work if I connect a return line to one of the spare outlet ports on the regulator and run it back to the tank?

No, this will not work. An open return line from one of the regulator outlet ports will render the regulator unable to properly control pressure, resulting in no pressure at idle, or at best low or no pressure at WOT under high engine load. Static regulators are normally open and designed to close when the set pressure is achieved, bypass regulators work on the opposite principle.

Pressure to a carburetor normally builds when the bowls are full and the needle shuts against the seat. An open line from an outlet port on a static regulator, running back to the tank, will prevent pressure from ever building at the carburetor needle and seat to begin with. It would be the same as taking a line from the regulator and putting it into a bucket; turn the pump on and you’ll have plenty of flow out of the line, but little or no fuel pressure to register on the gauge.

The only way a return line may be connected to a static regulator is if it is through a port blocked by a highly restrictive jet, normally something with a passage in the 0.015″-0.017″ range. This is done to allow a small amount of fuel to leak through the regulator valve to prevent pressure creep on a nitrous solenoid.

There are excellent reasons to install a proper bypass regulator for use with carbureted engines and Aeromotive has perfected this technology, offering a variety of world class bypass regulator options.

For more information on Static vs Dynamic Fuel Systems and the benefits of a bypass regulator, see Aeromotive Tech Bulletin TB-801 at www.aeromotiveinc.com under the Tech Help, Tech Bulletin section.

8. I have one of your Carbureted bypass regulators, advertised to adjust from 3-14 PSI. I have a blow through carb and am setting the system up for boost reference with 7 PSI at idle and 20 PSI boost. Pressure needs to hit 27 PSI with boost reference, will it work or stop at 14 PSI?

It will work perfectly, raising fuel pressure with boost on a 1:1 ratio, all the way to 27 PSI, if the pump will support it. There’s no real limit to boost reference as far as the regulator is concerned, but it’s important to understand that as fuel pressure goes higher, the flow from an electric fuel pump trends lower, and when the pump flow is no longer sufficient to support the engine’s demand for fuel at the increased pressure, fuel pressure will flatten (stop rising) and then it will roll over and start to fall if fuel demand continues to rise. This is a pump/flow problem, not a regulator issue.

The inlets in the top left and right are parallel and can be used in a couple of different ways.

a, used inline before the carby like a normal NA reg.

b, If your carby has a return line, Run the line from the pump straight into the carby and then run the return of the carby into the inlet of the reg. Then block the other inlet/outlet off, Or you can use this port for a gauge.

Both methods will hold the selected base pressure in the fuel lines and carby, Then raise it on boost. These regs can be used with both high pressure carby pumps >20psi, Or a convention EFI surge tank set up as pictured above, But in both cases a return line must be used. When it comes to running high boost pressure, the surge tank setup is the only way to go as an EFI pump can make more pressure to keep your fuel pressure buffer. As I mentioned above, if high fuel pressure (20+psi) is not needed, but if you would like to run the surge tank setup with an EFI pump, you can run the power source for the EFI pump through a ballast resistor. This will drop the voltage to 6v and halve the output of the pump if it is too high.

A rising rate regulator works by using a vacuum/boost line off the manifold. When the boost pressure comes up in the reg, It raises the fuel pressure in relation to boost. You can get them in ratios of 1:1, 1.5:1, 1.7:1, 2:1.....12:1 etc, But most carby ones are 1:1, With a base pressure of 4-5psi (which most carby will hold) and a rising rate index of 1:1 no matter how much boost you run, that 4-5psi will always be your buffer
Last edited by a moderator:


Clicker of buttons
great information, I just picked up an "Aeromotive 13301" that I plan on running on my 84 vette (cfi).

I just need to figure out the best way to route everything now... :)


The Grumpy Grease Monkey mechanical engineer.
Staff member

8 Common Fuel Pressure Regulator Failure Symptoms

If your vehicle runs rough or does not always start smoothly, the issue can be a faulty fuel pressure regulator as it might be affecting the supply of fuel to the engine's combustion chamber.

But while fuel pressure regulator issues are quite common, they are also easy to miss if you do not know what to look for. And in some instances, you can even mistake a bad fuel pressure regulator for something else.

In this piece, we discuss some of the most common symptoms of a fuel pressure regulator failure and also explain how to replace one if you have to. But first, check out our brief overview of how a fuel pressure regulator works.

Contents [show]

How a Fuel Pressure Regulator Works
A fuel pressure regulator is tasked with ensuring that the pressure on the fuel system is right for efficient combustion, which means it is neither too strong nor too weak.

While it can come in various forms depending on the car type, it will typically feature a diaphragm attached to a ball-type valve on one side and a spring on the other. The valve side is then connected to the fuel inlet and outlet while the spring side is on the engine vacuum.

The fuel pressure regulator will work by forcing the diaphragm upward or downward depending on the driving conditions. For example, when accelerating it is forced downward which then closes the return line and increases the amount of fuel that the injectors get.

Symptoms of a Fuel Pressure Regulator Failure

As your fuel pressure regulator starts going bad, there are different symptoms that your car will display depending on the model and the extent of the problem. Knowing these symptoms will allow you to make timely interventions to avoid more serious engine problems. And they include the following.

1. Check Engine Light Turning On

One of the first symptoms of a faulty fuel pressure regulator is the check engine light turning on because the vehicle's computer is designed to use this light to alert you when there is an issue on the engine.

The light turns on because of the rich mixture condition that the problematic fuel pressure regulator will cause.

But unless you combine this symptom with others below, you cannot tell outright that you have a problem with a fuel pressure regulator since the check engine light can turn on because of several other issues.

2. Engine Does Not Run Smoothly

If your engine is not getting the right dose of fuel in its combustion chamber or it is not being supplied in the right manner, it will not run smoothly.

The engine can be stalling when you start the car or might make an odd sound which should not happen when the engine is running smoothly.

There should be no hesitation when you press on the gas or even when you decelerate. If you notice any stalling, you should have the fuel pressure regulator checked.

3. Blackening of Spark Plugs

Spark plugs play a crucial role in your vehicle despite their small size, and if you think you might have an issue with your fuel pressure regulator, the spark plug is a good place to start your inspection.

Take out the spark plug and check its condition, and if you notice the end is covered with a thick layer of black soot, this is a good sign that your fuel pressure regulator is faulty.

Blackened spark plugs are a result of oil burning in the engine head, which can be caused by fuel pressure regular failure. And in such a situation, you will need to replace both the fuel pressure regular and the spark plug.

4. Engine Misfiring or Failing to Start

An engine misfire or failure to start completely can also be symptoms of a pressure regulator failure. For the misfire, a bad fuel pressure regulator causes the engine to run rough, and in many instances end up misfiring. Also, the misfire will occur because the air to fuel ratio will not be right.

Besides misfiring, the engine can also fail to start completely. Regardless of how hard or how many times you turn the key or push the ignition button, the engine might still not start.

Because one of the main functions of a fuel pressure regulator is to ensure the right pressure is maintained, a faulty one means that this will not be achieved, and hence making it impossible for the engine to start.

5. Weaker Acceleration

When you have a bad fuel pressure regulator, weaker than normal acceleration will in many instances be your main concern as it is one of the most common symptoms of the problem.

If you press down the gas pedal and notice that the speed increase is not significant enough, you need to check this regulator as this is a sign that the engine is not able to get the right fuel to air mixture.

However, it is also worth remembering that other common engine issues such as faulty or dirty fuel injectors can also cause weak acceleration.

6. Noisy Fuel Pump

Different components of a vehicle will produce different kinds of noises, but some like the fuel pump make a hardly noticeable sound. Most motorists do not even know what the fuel pump sounds like.

However, if you notice a very loud noise coming from your fuel pump, this can be a sign that you have a faulty fuel pressure regulator.

If the fuel pressure regulator does not allow for normal fuel flow, you can have issues with the fuel pump, which is often manifested as a loud whining sound when accelerating.

But be sure to check the fuel pump thoroughly as the loud noise can also be a symptom of various other problems

7. Reduction in Fuel Efficiency

Many motorists pay a lot of attention to the quantity of fuel that their vehicle gobbles up given the ever-increasing fuel prices. But with a faulty fuel pressure regulator, your vehicle's fuel efficiency can reduce significantly.

Hence, if you notice that you are using up more fuel than usual, you need to check the fuel pressure regulator.

When the fuel pressure regulator is faulty, the engine will have to work harder to run the vehicle, which means more fuel will be required.

8. Appearance of Black Smoke

There is a long list of things that can cause your vehicle to emit black smoke, but a fuel pressure regulator is one of the main ones.

If there is a leak in the fuel pressure regulator or it has internal damage, in many instances it will cause the emission of black smoke in the tailpipe as the fuel combustion will not be properly regulated.

How to Replace a Fuel Pressure Regulator

Now that you have some idea on the symptoms to watch out for to know when you have a bad fuel pressure injector, it is only natural to also want to know how to replace it.

And while you can always take your vehicle to the auto repair shop, it is still possible to do it on your own, and here we show you how.

Things You Need

  • OBD2 scanner
  • Screwdriver or wrench
  • LED flashlight for illumination
  • Safety gloves and goggles

Step 1: Switch Off Vehicle

Although some motorists do not pay a lot of attention to this step, it is still crucial and as important as the others to follow.

Hence, before you start working on your vehicle, you need to switch it off and ensure that the battery terminals are disconnected. But, even just detaching the negative terminal alone should be enough.

Step 2: Disconnect the Fuel Pressure Regulator Vacuum Line

The next step will be to disconnect the vacuum line on the fuel pressure regulator before you can remove it. And when doing this, you should also check for any signs of wear or damage on the line, and if any, you need to replace it.

Step 3: Release Fuel Pressure

Before you can remove the fuel pressure regulator, you will also need to release the pressure. This should be easy as many engines will have a pressure test port which allows you to connect a pressure gauge and use it to release pressure.

Alternatively, use a screwdriver to relief the pressure by tapping the test port, but this method requires extra keenness to prevent accidents.

Step 4: Unscrew and Remove the Old Fuel Pressure Regulator

Now you can unscrew the fuel pressure regulator and detach it from the fuel rail. But, you should also be careful when pushing it from the fuel rail to ensure you do not damage anything as there are some delicate parts here.

Step 5: Screw the New Fuel Pressure Regulator

With the old fuel pressure regulator removed, you can now screw on the new one. Make sure that you have the right replacement part by checking the part number, and also ensure you install it correctly and tighten the screws enough.

Step 6: Clear Out ECU Memory

Because a faulty fuel pressure regulator is an engine problem, it will most likely switch on the check engine light. Hence, after replacing the faulty one, you need to clear the ECU memory to get rid of the error codes.

Here you only need to attach the OBD 2 scanner, allow it to scan then vehicles computer and then reset or clear out the error code that it will display.

Step 7: Take Vehicle for Test Drive

Even if you do everything correctly, you will not be certain the problem is fixed if you do not take the vehicle for a test drive.

Drive it for at least 10 minutes at both low and high speed and check if it runs smoothly and if other symptoms of a bad fuel pressure regulator are manifested.

If the vehicle is okay, the job is done, but if not, you need to check whether you installed the new regulator well or take the vehicle to a professional for further assistance.

It should now be clear that the fuel pressure regulator plays a crucial role in your vehicle's engine, and hence you cannot overlook it as it can affect the overall performance of your vehicle significantly.

Knowing the symptoms to watch out for when you have a bad fuel pressure regulator, which includes things like blackened spark plugs, engine not running smoothly, weak acceleration and emission of black smoke is vital.

With these symptoms in mind and you know how to replace a faulty fuel pressure regulator, you will hardly ever need to worry about this problem as you can easily fix it.