diagnoseing TPI & LT1 problems


Staff member
the use of a digital volt meter ,too check ALL YOUR FUSES before you start to diagnose problems, having a test fuel pressure gauge and having a shop manual handy that's for your exact corvette or muscle car is almost mandatory

and reading thru and understanding this thread helps
be sure the fuel pressure gauge reads correctly by comparing it to a second test gauge, these fuel pressure gauges are frequently defective
The secret of effective diagnosis and troubleshooting is to have a logical plan.
verify your fuel pressure stays up in the 37-40 psi range without falling much after the cars shut off for at least 5 full minutes, and it should stay at 37-42 psi if the fuel rail pressure does drop off in the first few minutes its almost always a leaky injector or bad fuel pressure regulator, and check for vacuum leaks either could cause your symptoms, remember the fuel pressure should stay fairly consistent while the cars running regardless of how fast or long the throttles quickly blipped or held wide open,
read these threads, as theres a huge amount of info in the links and sub-links

it helps if you understand that oxygen sensors do not measure your true fuel air ratio,entering the engine, but instead measure the remaining oxygen content of the burnt exhaust gases,and there are both narrow and wide band sensors
http://www.superchevy.com/how-to/additi ... ult-codes/



http://www.youtube.com/watch?v=sSuL58YE ... re=related







TPI Torque Specifications

Below are the torque specifications as designated by GM for Tuned Port Injection intakes. Please note that some specifications are in lb. ft., and some are in lb. in.

Manifold to Runner Bolts 25 lb. ft.
Runner to Manifold Bolts 25 lb. ft.
Fuel Rail Attaching Bolts 15 lb. ft.
Throttle Body Attaching Bolts 18 lb. ft.
Throttle Position Sensor 18 lb. in.
Idle Air Control Valve 13 lb. ft.
Throttle Body Coolant Plate 27 lb. in.
Throttle Body Cover Plate 30 lb. in.
Power Brake Vacuum Fitting 108 lb. in.
Fuel Line Nuts 20 lb. ft.
Fuel Tube Bracket Bolt 25 lb. ft.
Fuel Pressure Connection 115 lb. in.
Pressure Regulator Base to Rail 44 lb. in.
Pressure Regulator Bracket 44 lb. in.
Outlet Tube to R/H Rail Retainer 44 lb. in.

http://www.superchevy.com/how-to/additi ... ult-codes/

knowing whats going on and WHY can help , so read thru the links and flow charts

http://www.corvettefever.com/techarticl ... index.html

http://members.shaw.ca/corvette86/Component Location View 86.pdf






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

this cheap fuel pressure test gauge works ok, you just need to figure out how to tap into the fuel rail or line and that varies between years but certainly not a difficult issue to either remove a shrader valve and insert an adapter barb fitting or splice a TEE into a fuel line as a temp. test point, certainly any mechanic has an assortment of the common barb fittings required in a test kit



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

SOME OF THAT INFO IS MANDATORY to use of this threads info, for testing, isolating and correcting problems, start by verifying the fuels good, there's no water in the fuel, check the fuel flow and fuel pressure and verify the spark gets to the spark plugs and check timing, verify the oils clean and up to the full mark on the dipstick, and the cars gauges are working

this should help.






The secret of effective diagnosis and troubleshooting is to have a logical plan.
The secret of effective diagnosis and troubleshooting is to have a logical, well-ordered system. Following a logical step-by-step procedure will get you to the root cause of a problem quickly and efficiently. Repair manuals will have a diagnostic tree that goes in a logical progression. It will ask a question, usually yes or no, and depending on the answer will branch off in two or more directions. The last box in the tree will have the problem and how to correct it. The example shown is a simple one. A more complicated system will have branches going into specific tests and then have 2 or more branches from there depending on test results.
If youve got an early year TPI , thats designed to use a 9th cold start injector,check the cold start injector as its a potential problem source if its not working correctly.

BTW To remove the connector from the injector, push carefully in the middle of the locking clip, with your thumb, This will make the sides, and tips move OUT WARD in their connector slots allowing the tips that hold the injector into the connector to slide out of the way from the injector body, and you can then pull the sides away. WATCH the locking clip - IT can and WILL come off and get lost, if your not careful
You will need some basic test equipment to perform your diagnosis.

take notes your memory may not hold the exact numbers & results you need to refer bacK too.





LOOK THRU THIS LINK, and all the sub linked info!!!!!!


http://www.harborfreight.com/cpi/ctaf/d ... mber=94169

http://www.harborfreight.com/cpi/ctaf/D ... mber=46972

http://www.harborfreight.com/cpi/ctaf/D ... mber=93547

http://www.harborfreight.com/cpi/ctaf/D ... mber=94190

then read this
L-98 Engine Start Sequence

Posted by Grumpyvette

Info that might help - (before you ask, yeah the LT1 is very similar).

L-98 Engine Start Sequence

When you start an L-98 engine Corvette, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.

Fuel Rail Pressurization:

When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails. There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure. The reading will go to 38-40 pounds nominal once the engine is running.test by attaching a fuel pressure gauge to the fuel rail at the shrader valve, on TPI and LT1 engines its located on the pass side fuel rail.

Initial Crank Action:

If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.

Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run. (Note that you should have a black oil pressure switch/sender. It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad units that stayed in the GM parts pipeline for some time).

The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter. These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.

ECM Reaction:

If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods. (In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).

The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to start and run unless you can’t make at least 4 PSI oil pressure. This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).

The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54 volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts during start and at idle from the TPS is very important to both starting and run performance.)

Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.

Engine "Catches":
When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how much air is being pulled through to the intake manifold. The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms nominally so as to attain a proper air/fuel mixture to insure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).

The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.

If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be maladjusted. Alternatively, there may be a leak in the intake manifold or another vacuum leak may be present. Listen for hissing sounds---there should be none.

ECM Mode:

The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.

Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.

Note that prolonged idling can force the computer back into open loop mode.

Note: In 1990, the MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.

Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly. A multitude of things can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.


If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.

A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems.


Most of the C4 Corvettes used a MAF (Mass Air Flow) sensor to determine how much air is being pulled into the intake manifold. The exceptions are the 1984 Corvette that used a speed density system--a sort of predictive method of measurement---and the 1990 through 1993 C4 models which were also speed density based. In 1994, Corvette went back to the MAF based system but used the speed density approach as a back up. (1989 Bosch MAF installation shown at right).

A Mass Air Flow sensor has an extremely fine wire inside its bore. The 1985 through 1989 C4 engines used a Bosch MAF sensor that heated the wire to 100 C. The 1994 and later C4 models used a AC/Delco MAF that heated the wire to 200 C. The amount of current required to reach the temperature is measured in each case. (Note: the LT-5 engine used in the ZR-1 used a speed density system and continued to use that system in 1994 and 1995 since the engines had already been made prior to the last two years of production. The ZR-1 therefore has no MAF even after Corvette went back to the MAF based system).

Theory of Operation

As the air travels past the heated wire en route to the intake manifold, it will cool the wire and additional current is added to again heat the wire to the design temperature. Since the amount of air moving past the sensor is directly related to the amount of cooling experienced by the heated wire, a feedback condition is established whereby the exact amount of moving air is directly related to the amount of current passing through the wire and the intake air is therefore precisely measured.

Once the amount of air is known, the computer controlling the engine can add or subtract fuel as required to maintain the magic 14.7:1 air-fuel mixture resulting in the cleanest burn possible from an emissions (pollution) standpoint.

It does this by varying the "on time" of the fuel injectors. The injectors are pulsed on and off and the width of the pulse is lengthened or shortened as required. When you first start a typical engine, the pulse width is around 4 milliseconds but as soon as the engine "catches" the pulse width is shortened to about 2.2 milliseconds for idle. During operation, the measured air flow through the MAF will cause the computer to increase or decrease the pulse width as explained above.

MAF Operating Conditions

The Bosch MAF is more complex than the AC/Delco version. Both measure the air flow but the Bosch MAF has a circuit called the 'burn-off circuit' that cycles on for about 2 seconds when you shut the engine down. This circuit heats the wire to a high enough temperature to burn off any residue that may have collected on the wire during operation. If you are in a quiet area, you can hear the relays click on and then off on a 1985-1989 C4 as the burn-off cycle occurs.

There are two relays involved with the Bosch MAF: A power relay that passes current to the MAF wire during normal operation and the burn-off relay that provides the current for the cleaning cycle. Both are located on the firewall in the engine compartment, just behind the battery on the drivers side. Bad MAF power and burn-off relays can cause hard starting problems and should be changed periodically as preventative measure and any time you experience hard starting conditions.

The AC/Delco MAF has a power relay but no burn-off relay. For this reason, you should pay even closer attention to the condition of your air filter on a later model C4 than normal since a contaminated wire in a AC/Delco MAF is going to stay contaminated for the most part and cause false signals to be passed to the computer.

Also, the Bosch MAF outputs its information as a analog signal to the computer but the AC/Delco sends its signal as a digital component of varying frequency. For this reason, you cannot measure it's operation directly.

A scan tool is generally the best way to troubleshoot engine problems and with the 1994 and later Corvette, it is virtually mandatory. (An oscilloscope will also work on the AC/Delco MAF but a regular test meter will not).

MAF Problems

Faulty MAF sensors will normally light the check engine light on the drivers information center if the problem is constant and store a trouble code. If intermittent, a trouble code will still be stored as long as the battery is not disconnected.

Normally, the problem is a poor connection at the sensor and wiggling the wires, unplugging and reinserting the connector will often cure the problem.

A faulty MAF will normally cause a no start or difficult start condition and although you can eventually get the car into the "limp-home" mode in most cases, you need to attend to the problem ASAP.

This flow chart might help.


AC/Delco sensors can become intermittent or give false readings if the wires become contaminated as explained above.

The MAF is a critical part of the emission control system and as such will cause the computer to react to problems very quickly, setting trouble codes and reducing performance in ways that cannot be ignored for long.

MAF Mods

The Bosch MAF is often modified by removing the two screens that are present in the front and rear of the cylinder. Removing these screens significantly increases the air flow through them and this results in more horsepower. Removing the screens is an old trick from the Corvette Challenge days in 1988 and 1989. It does work but is illegal in many states so be advised not to do anything that will get you arrested for a pollution violation.

The AC/Delco MAF is not readily modified. It is what it is but since it is a larger diameter than the Bosch, it responds well to changing the air filter to a free flowing type such as the K&N filter.

Welcome to C4 vette codes it is very ....repeat very important that if you are not savvy of working on your Vette ...you would be better off - taking your car to a dealership for repairs on your trouble codes. However if you feel that you want to dive right in ..than you have come to the right place. First locate your car's alcl this component is located just below the instrument panel and to the left of the center console. Remove the plastic cover the first two slots to your right are the A & B slots for a drawing of the alcl module's picture is added below.

The A slot is the diagnostic slot and the B slot is the ground slot. insert the computer key into these slots (with the engine off) this is very important...now only put the ignition key to on ( not start !!!) the check engine light will display a code 12 which is one flash followed by two flashes. This code will be flashed three times ..followed by the trouble code stored in your car's computer.

What ever the code is it will be flashed three times. Have a paper and pencil ready and write down the code.

code 13 =1 flash followed by 3 flashes =>oxygen sensor
code 14 =1 flash followed by 4 flashes =>coolant sensor
code 15 =1 flash followed by 5 flashes =>coolant sensor
code 21 = 2 flashes followed by 1 flash =>throttle position sensor
code 22 = 2 flashes followed by 2 flashes=> throttle position sensor
code 23 = 2 flashes followed by 3 flashes=> manifold air temp sensor
code 24 = 2 flashes followed by 4 flashes=> vehicle speed sensor
code 25 = 2 flashes followed by 5 flashes=> manifold air temp sensor
code 32 =>egr system
code 33 =>map sensor
code 34 =>maf sensor
code 35 => idle air control
code 41 => cylinder select error
code 42 => electronic spark control
code 43 => electronic spark control
code 44 => lean exhaust
code 45 => rich exhaust
code 51 => PROM
code 52 => fuel calpak
code 53 => system over voltage
code 54 => fuel pump circuit
code 55 => ecm
code 62 => oil temp

Please remember that if you have the computer key installed in the alcl and you start the engine (you will ruin the engine's computer) only put the ignition to on (not to start).

If you should get a check engine soon display.. you can use the above procedure and codes to buy the right part or at the very least to keep from getting taken for a ride and be made to pay hight prices for some inexpensive module that you could have installed yourself.

If your engine displays a trouble code ... your engine will go into limp mode ..it will still run but very poorly. You might be able to reset the computer if it will not start (just to get home) by disconnecting both battery cables and re-installing them ...this is not recommended ..but if you are stranded it might help unitl you get your car home or to a repair shop..good luck.

1985 TO 1991:

Code #12: Normal No Codes.
Code #13: Open Oxygen Sensor Circuit.
Code #14: Coolant Sensor Circuit Low.
Code #15: Coolant Sensor Circuit High.
Code #21: Throttle Position Sensor High.
Code #22: Throttle Position Sensor Low.
Code #23: Manifold Air Temperature Circuit High.
Code #24: Vehicle Speed Sensor.
Code #25: Manifold Air Temperature Circuit Low.
Code #32: EGR System Failure.
Code #33: Mass Air Flow Sensor High.
Code #34: Mass Air Flow Sensor Low.
Code #36: Mas Air Flow Sensor Burn-Off Function Fault.
Code #41: Cylinder Select Error.
Code #42: Electronic Spark Timing.
Code #43: Electronic Spark Control.
Code #44: Lean Exhaust indication.
Code #45: Rich Exhaust Indication.
Code #46: Vehicle Anti Theft Fault.
Code #51: Faulty Mem-Cal.
Code #52: Fuel Calpak Missing.
Code #52(1990-91 Corvette Only): Engine Oil Temperature Sensor Low.
Code #53: System Over Voltage.
Code #54: Fuel Pump Circuit Low Voltage.
Code #55: Defective ECM.
Code #62: Engine Oil Temperature Sensor Circuit High.

ECM CODES 1992 TO 1993:

Code #12: Normal No Codes.
Code #13: Left Oxygen Sensor Circuit.
Code #14: Coolant Temperature Sensor Circuit High.
Code #15: Coolant Temperature Sensor Circuit Low.
Code #16: Opti-Spark Ignition Timing System.( Low Pulse)
Code #21: Throttle Position Sensor Circuit High.
Code #22: Throttle Position Sensor Circuit Low.
Code #23: Intake Air Temperature Sensor Circuit Low.
Code #24: Vehicle Speed Sensor Circuit.
Code #25: Intake Temperature Sensor Circuit High.
Code #26: Quad-Driver Module #1 Circuit.
Code #27: Quad-Driver Module #2 Circuit.
Code #28: Quad-Driver Module #3 Circuit.
Code #32: Exhaust Gas Recirclation Circuit.
Code #33: Manifold Absolute Pressure Sensor Circuit Low.
Code #34: Manifold Absolute Pressure Sensor Circuit High.
Code #36: Opti-Spark Ignition Timing System. (High Resolution Pulse.)
Code #41: Electronic Spark Timing Circuit Open.
Code #42: Electronic Spark Timing Circuit Grounded.
Code# 43: Electronic Spark Control Circuit.
Code #44: Left Oxygen Sensor Circuit Lean.
Code #45: Left Oxygen Sensor Circuit Rich.
Code #51: Mem-Cal Error.
Code #52: Engine Oil Temperature Sensor Circuit Low.
Code #53: System Voltage.
Code #55: Fuel Lean Monitor.
Code #56: Vacuum Sensor Circuit.
Code #61: Secondary Port Throttle Valve System.
Code #62: Engine Oil Temperature Sensor Circuit High.
Code #63: Right Oxygen Sensor Circuit Open.
Code #64: Right Oxygen Sensor Circuit Lean.
Code #65: Right Oxygen Sensor Circuit Rich.
Code #66: A/C Pressure Sensor Circuit Open.
Code #67: A/C Pressure Sensor Circuit. (Sensor or A/C Clutch Circuit Problem)
Code #68: A/C Relay Circuit Shorted.
Code #69: A/C Clutch Circuit.
Code #72: Gear Selector Switch Circuit.

CODES 1994 TO 1996:

DTC #11: Malfunction Indicator Lamp Circuit.
DTC #13: Bank #1 Heated Oxygen Sensor #1 Circuit.
DTC #14: Engine Coolant Temperature Sensor Circuit Voltage Low.
DTC #15: Engine Coolant Temperature Sensor Circuit Voltage High.
DTC #16: Distributor Ignition System Low Pulse.
DTC #18: Injector Circuit.
DTC #21: Throttle Position Sensor Circuit Voltage High.
DTC #22: Throttle Position Sensor Circuit Voltage Low.
DTC #23: Intake Temperature Sensor Circuit Voltage High.
DTC #24: Vehicle Speed Sensor Circuit.
DTC #25: Intake Air Temperature Sensor Circuit Voltage Low.
DTC #26: Evaporative Emission Canister Purge Solenoid Valve Circuit.
DTC #27: EGR Vacuum Control Signal Solenoid Valve Circuit.
DTC #28: Transmission Range Pressure Switch Assembly Fault.
DTC #29: Secondary Air Injection Pump Circuit.
DTC #32: Exhaust Gas Recalculation.
DTC #33: Manifold Absolute Pressure Sensor Circuit High.
DTC #34: Manifold Absolute Pressure Sensor Circuit Low.
DTC #36: Distributor Ignition System High Pulse.
DTC #37: Brake Switch Stuck On.
DTC #38: Brake Switch Stuck Off.
DTC #41: Ignition Control Circuit Open.
DTC #42: Ignition Control Circuit Shorted.
DTC #43: Knock Sensor Circuit.
DTC #44: Bank 1 LF Heated Oxygen Sensor #1 Circuit Lean.
DTC #45: Bank 1 LF Heated Oxygen Sensor #1 Circuit Rich.
DTC #47: Knock Sensor Circuit Or Module Missing.
DTC #48: Mass Air Flow Sensor Circuit.
DTC #50: System Voltage Low.
DTC #51: EEPROM Programming Error.
DTC #52: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #53: System Voltage Low.
DTC #55: Fuel Lean Monitor.
DTC #58: Transmission Fluid Temperature Sensor Circuit Low.
DTC #59: Transmission Fluid Temperature Sensor Circuit High.
DTC #62: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #63: Bank 2 RF Heated Oxygen Sensor #1 Circuit Open.
DTC #64: Bank 2 RF Heated Oxygen Sensor #1 Circuit Lean.
DTC #65: Bank 2 RF Heated Oxygen Sensor #1 Circuit Rich.
DTC #66: A/C Refrigerant Pressure Sensor Circuit Open.
DTC #67: A/C Pressure Sensor Circuit Sensor or A/C Clutch.
DTC #68: A/C Relay Circuit.
DTC #69: A /C Clutch Circuit.
DTC #70: A/C Clutch Relay Driver Circuit.
DTC #72: Vehicle Speed Sensor Loss.
DTC #73: Pressure Control Solenoid Circuit Current Error.
DTC #74: Traction Control System Circuit Low.
DTC #75: Transmission System Voltage Low.
DTC #77: Primary Cooling Fan Relay Control Circuit.
DTC #78: Secondary Cooling Fan Relay Control Circuit.
DTC #79: Transmission Fluid Overtemp.
DTC #80: Transmission Component Slipping.
DTC #81: Transmission 2-3 Shift Solenoid Circuit.
DTC #82: Transmission 1-2 Shift Solenoid Circuit.
DTC #83: Torque Converter Solenoid Voltage High.
DTC #84: 3-2 Control Solenoid Circuit.(Auto Only).
DTC #84: 2nd And 3rd Gear Blockout Relay Control Circuit.
DTC #85: Transmission TCC Stock On.
DTC #90: Transmission TCC Solenoid Circuit.
DTC #91: One To Four Upshift Lamp(Manual Only).
DTC #97: VSS Output Circuit.
DTC #98: Tachometer Output Signal Voltage Wrong.

You really can't be effective at playing mr-fix-it with out the correct tools especially on the more modern cars that are computer controlled, the days of effectively tuning by ear and vacuum gauge and engine sound went out with carbs.

You need a few basic tools, now the list will vary, but you can,t get by by guessing, you neet to know and test now that sensors and CPUs control engine function. Here's some basic tools:

Be sure to get the specific manuals your car and EFI system and ignition system,require FIRST

more info, yes there's a real need to read thru the links and sub links as theres a great deal of info, problems, tests and solutions in the text



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


(very useful info here)

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
http://www.carcraft.com/techfaq/116_070 ... index.html

http://www.ecs.fullerton.edu/~sowell/ja ... estRig.pdf

http://www.carcraft.com/projectbuild/11 ... index.html


http://chevythunder.com/fuel%20injectio ... pg%20a.htm

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

http://www.ehow.com/how_2387429_test-fu ... n=yssp_art




http://www.gmtips.com/3rd-degree/dox/ti ... t/port.htm

http://www.tpis.com/instructions/Large% ... ctions.doc


http://www.cartechbooks.com/vstore/show ... 83&CATID=3






http://www.thirdgen.org/techboard/tpi/4 ... k-tpi.html










get out your oxy-acetylene torch or a propane torch, don,t light it but turn it on so gas is flowing and place the tip near any suspected leak points, while a buddy watches the tach, anytime the gas gets sucked thru a crack the rpms will jump higher in direct relation to the amount of extra fuel the engines getting, no, oil, fluid,or cruds or stains are left on the intake and its simple too do process

btw if you have starting problems heres a simple test, a slightly worn or leaking fuel pressure regulator diaphragm or leaking injector are high on the suspect list, put a fuel pressure gauge on the fuel rail, hit the key,but don,t start the engine, it should quickly hit 40 plus psi, and hold that for at least 10 minutes after the keys removed, if it holds replace the fuel pump pressure switch and check engine grounds, and use the trouble shooting flow chart above,
if it drops off below 40 psi in under 5 minutes somethings leaking and needs, cleaning, repair or replacement

(I) test the injectors first, remove the regulator and plug the fuel rail, if the fuel pressure dropping off problem stops with the fpr removed its a defective pressure regulator if it continues its very likely a defective injector(S) if they (injector(S) minus the FPR) hold steady pressure testing the FUEL PRESSURE regulator next, is pointless ,its defective, if the fuel pressure dropping off problem starts again once its reinstalled its a defective pressure regulator

HOME DEPOT sells rubber corks, and you hold them temporarily in place for testing with a vise grip

if its defective replace it with an adjustable version






this info may help

Last edited by a moderator:
Re: diagnoseing TPI problems
the later 1992-96 corvette lt1 used a MAT SENSOR

http://www.mamotorworks.com/corvette.ht ... %20manuals

http://www.helminc.com/helm/Result.asp? ... ted_media=







http://www.factoryautomanuals.com/p-912 ... -only.aspx
" I Need some help I have a TPI system from a 88-89 5.0 Camaro installed in my chevy. In the beginning it would not run after it warmed up. Checked the fuel pressure and it was only 20 PSI. Replaced pump and that cured 85% of my problems. Now it runs great under 2800 RPM. Anything above that it backfires and bogs down. I thought it might be the vacuum advance, but the distributor does not use a vacuum advance. Anyone with an idea want might be causing my problem??"
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)



while your at harbor freight buy some O-rings,
they come in handy on hydraulics,
fuel lines,
test gauges,
and air conditioners etc.

you can get a decent random assortment, of O-rings,
for less than $60 at HF
and you may be amazed at how often its handy to have around,
if your getting some for the shop inventory
be aware the materials vary,
some are used for oil, fuel,Freon, higher temps etc,
but they are not all interchangeable






on the TPI and LT1 efi and many other EFI intakes theres a shrader valve,
that can be easily accessed to check fuel rail pressure,
dealing in facts you verify is always preferable.
leaving a small under hood pressure gauge attached is a potential fuel leak point ,
if its not very carefully installed, and you want a fuel pressure gauge that can read,
in the 15 psi-65 psi range, many fuel pressure gauges max out at 30 psi.








and rev the engine a few times and watch the fuel pressure it should in theory remain in the 38 psi-42 psi range for most TPI and TUNED PORT efi





If youve got an early year TPI , thats designed to use a 9th cold start injector,check the cold start injector as its a potential problem source if its not working correctly.The cold start injector is between the front and rear pairs of runner tubes on the driver-side.on early TPI engines if its not working or loose causing a vacuum leak starting the engines much harder, later versions did not use these





http://www.hawksthirdgenparts.com/produ ... e-Kit.html

http://www.chevyhiperformance.com/techa ... index.html
partly clogged injectors, or injectors running at over 80% duty cycle tend to reduce power and increase emissions

verify the fuel pressure with a fuel gauge connected to the shrader valve on your TPI fuel rails,


during all rpms it should remain constant between 38psi min and 44psi max,
if its not stable the fuel pump, restrictions in the lines or fuel filter or a defective fuel pressure regulator are most likely at fault








Intake Air Temp sensor. It is located on the bottom of the inlet plenum a few inches ahead of the distributor. It is like right next to the fuel pressure regulator., it can effect fuel flow rates

The secret of effective diagnosis and troubleshooting is to have a logical plan, and to follow it without skipping any steps.when ever you get into unexplained symptoms the answer remains the same....in every case,.... you drop back to the basics and do a few tests, (and until you gain experience you need the FACTORY SHOP MANUAL TO INDICATE THE TESTS, NEEDED AND THE EXPECTED RESULTS LISTED) get out a multi meter and verify every fuse is good, and you have 13 volts at the battery and youve got at least a 1/2 tank of fresh fuel, before you start chasing your tail, thinking about more complicated problems
find the cause, you isolate, each function or system or malfunction to its related components then test, each individually, use of the shop manual is almost mandatory

http://www.classictrucks.com/tech/0404c ... index.html


The Most Asked Questions Answered
By Thomas P. Iverson

There aren't too many things more misunderstood to many of us than fuel injection. Although carburetors haven't been used on new cars since the mid-'80s hot rodders haven't moved as quickly. There's no denying that a well-tuned fuel injection will perform better than a carb, but there's still a stigma left by some of the early aftermarket kits that left a whole lot to be desired. ignition spark should be bright blue and impressive, if its, weak,narrow, yellow or red theres a problem so research the cause, verify the coil and voltage
TPS (Throttle Position Sensor). .54 Volts Idle, ~ 5 Volts Full Throttle.

IAC and TPS Adjustment

One of the best ways to retrofit fuel injection is to still take advantage of the research General Motors put into it and find a used factory system. The Tuned Port Injection (TPI) units of the mid '80s to early '90s are still some of the best and easiest to do. In an effort to take some of the mystique out of upgrade to this "new" technology, we got together with the folks at Street & Performance to get some of the most common questions we get asked answered. We'll be following this article with an actual installation, hopefully, next month.

1. Why should I swap to fuel injection?
Swapping to fuel injection has many advantages. Your engine will last longer and have better low- and mid-range torque. The computer will keep the air/fuel ratio correct even when changing altitude, which not only provides better driveability, but improved fuel mileage.
2. Can I get a Tuned Port Injection unit out of salvage?
Yes, the only part to wear out is the shaft in the throttle body which can be replaced with a larger aftermarket throttle body or a new factory unit.
3. What vehicles should I look for as good donors?
The '85-92 F-bodies (Camaros, Firebirds, and Trans Ams) will be the most plentiful, but '85-91 Corvettes are also out there now at reasonable prices. The Corvettes were all 350 V-8s. The '85-86 F-bodies were 305s and the '87-92 were either 305s or 350s.
4. Will the TPI intake manifold bolt onto my earlier heads?
The '85-91 Corvettes and '85-86 F-bodies all have the early bolt pattern. The '87-92 F-bodies have the late intake bolt pattern, but can be interchanged, or the two center bolt holes can be modified to fit early heads. Street & Performance will trade or modify them for you. If you have the new Vortec of Fast Burn heads, S&P can supply intakes to work with these as well.
5. Do I need to find a complete unit or can I piece one together?
Ideally you want to find one as complete as possible, but you can piece one together. Most parts interchange, but not all. There are lots of TPI units on ebay if you're looking to keep your hands clean, or you can look in salvage yards as the TPI-equipped vehicles are old now and are hitting salvage yards regularly. The S&P team can help you, or you can send your unit to them and they will interchange the parts with their own or new GM parts. S&P also offers complete units set-up for your engine (283-406ci or beyond) and transmission (automatic or manual) in three finishes (natural, polished, and chromed).
6. How much should I pay for a unit?
If you find a used but complete unit expect to pay from $150 on the low side to a high of $500.
7. How can I tell if it's a 305 or 350 unit?
The injector numbers are the only way to tell. A GM dealer or S&P should be able to tell you which ones you have. All Corvette units were 350s. All '85-86 F-bodies were 305s, but the '87-92 F-bodies could be 305s or 350s.
8. What's the difference between the Corvette unit and F-body unit?
On all Corvette units, the fuel rails come out on the passenger side. On all F-body units, the fuel rails turn and come out on the driver side. The fuel rails can be modified for rear exits with fittings supplied by S&P.
9. Can I reuse the old injectors, or do I need to get new ones?
The '85-88 injectors can be cleaned with S&P's ultrasound system and made like new again. They have new performance screens and all other parts to service the injectors. S&P recommends replacing the '89-92 multi-tech injectors as they have had a lot of problems.
10. Is there anything I can do about the dents in my runners?
S&P can straighten them and make them like new most of the time, or they can supply new or reconditioned ones for whichever year you need. Aftermarket larger runners are also available that will supply the engine with more air and allow you to pick up some horsepower.
11. Can the fuel-injection unit be too big for my engine?
An engine is like an air compressor. The easier it breathes, the more power it will make. Porting the intake, adding a larger throttle body and runners, and higher flow injectors will help a high performance engine. But, just as a high- flowing carburetor can be too big for a mild to stock engine, so can the fuel injection. Go too big and the torque will start to drop.
12. What is a Mass Air Flow (MAF) meter?
The MAF is an air meter that is in the air induction tube leading to the throttle body that measures the amount and the temperature of the air going into the engine.
13. Can I run the old computer?
Yes, but the '85-89 computers were all mass air flow (MAF), which is expensive and is susceptible to damage by dirt or water (new GM replacement is about $300). Plus it can get in the way when trying to fabricate a custom air intake to fit a specialized installation. The '85s used the #1226870 computer only one year, and '86-89s used the #1227165 computer. Also the '85-88 used a ninth injector for cold start. The '89s used the same computer as '86-89 but fired all eight injectors for cold start like the '90-92s.
14. What is the ninth injector?
The '85-88 units have a ninth injector on the driver side of intake that fired one injector on the side of the intake for cold start only. The fuel then has to travel down each runner under the intake. The '89-up computer fires all eight injectors right on top of the valves so that you get better cold starting.
15. Can I disengage the ninth-injector cold-start function?
Yes, by using the '90-92 speed density computer, you will use all eight injectors for cold start.
16. Why should I change to speed density?
Speed density is preferable for many reasons. First, although the TPI units through the years remained about the same, the computers kept getting smarter. GM hasn't updated the '85-89 MAF computer in years. But they just updated the speed density computer in mid 2003. Speed density, uses the Mass Air Pressure (MAP) sensor because it's more dependable and only has a four-wire hookup (key, starter, ground, and fuel pump). Speed density also provides better mileage, performance, and drivability.
17. How do I get a speed density computer?
Speed density computers were used on '87-up four-cylinder and six-cylinder cars and '90-92 V-8s. Look for computers with these numbers #1227730, #16198262, #16196344, and #8889196 and weather-proof computer #1227727 and #16198260. You can also still get one through GM or some aftermarket companies.
18. Can I use a four-cylinder or six-cylinder computer?
Yes, they're all the same but will need to be upgraded with a chip designed to work with the V-8. Just be careful not to get a V-8 chip that has the Vehicle Anti-Theft (VAT) protection built into it. VAT will only let the vehicle run for two seconds then shut down the injectors--not allowing fuel to get to the engine. Most 305 chips don't have VAT, but all 350 chips do.
19. Can VAT be taken out?
Yes, if you have a V-8 speed density chip, S&P can reprogram it for your application.
20. Can I mount the computer in the engine compartment?
Only if you use the '90-91 Corvette computer (#1227727).
21. Why should I reprogram the chip and not just use it?
Because the vehicle will perform better and have better drivability because the chip will be programmed specifically for your vehicle's transmission, rearend ratio, and tires.
22. Why use a GM computer and not an aftermarket programmable one?
The chip in the GM computer can be programmed for just about everything you want to do. If you want to build a stroker 383-406, the chips can deal with it. GM spent a lot of time and money developing computers and updating them, and they know what they are doing. They use the latest technology and you can get a replacement anywhere if the need ever comes up.
23. Can I put 305 injectors on a 350?
If your engine is not over 250 horsepower, you can put an S&P adjustable fuel-pressure regulator on it and raise fuel pressure. Plus when you get your chip programmed, you can tell the computer you are running 305 injectors on a 350, which will lengthen the pulse width to let more fuel to the engine when needed. Engines with 250-300 horsepower need more fuel, so they'll need to run 21.9 lb-hr injectors.
24. What size injectors did the factory use?
The 305s came with 19.9 lb-hr injectors. The standard 350s came with 21.9 lb-hr injectors, and the LT1 350s had 24.9 lb-hr injectors. The 300-400hp engines should run 24.9 lb-hr injectors.
25. Can I use LT1 injectors in my TPI?
Yes, but be careful. S&P has had a lot of problems with LT1 injectors when they have sat for a while.
26. Is there an advantage to chroming the TPI unit?
There are no performance gains, but it depends on if you're looking for style as well as performance. The natural as-cast finish is hard to clean. The polished finish looks good when freshly polished, but you have to maintain it to keep it looking good all the time. The chrome units offer easy maintenance--just spray on some Windex and wipe it off.
27. How big of cam can I use?
You can run a high-lift cam, but it'll need a wide lobe separation and be able to provide at least 13 inch or more of vacuum.
28. Do I need to use a special fuel pump?
An in-tank fuel pump is best because the pump is kept cool by the fuel in the tank, and they run quieter. The in-tank units don't have to work as hard as the exterior pump because they just to have to push the fuel. A lot of the late-model in-tank pumps will fit into older tanks, (S&P can help you determine if yours will work) or you can check with companies like Rock Valley, Tanks, No Limit, and others who sell new tanks designed for the in-tank pumps. You can use an external fuel pump. They work okay if properly mounted--which is close to the tank away from heat.
29. How much fuel pressure do I need?
You need 38 to 48 psi for a 300- to 400-hp engine. High horsepower engines need 40 to 48 gallons-per-hour of fuel. You don't want to use a racing-type pump because they push too much fuel, and the TPI regulator cannot push it back to the tank fast enough.
30. Can I swap to fuel injection myself or do I need a professional shop?
It depends on how mechanical you are and how comfortable you are with your skills. The wiring harness only has four wires to hook up (ignition switch, fuel pump, starter, and ground). Most of the rest of the job is the same as swapping any other intake manifold. CLASSIC TRUCKS will soon be running an install story showing a 17-year-old retrofitting his pickup with a TPI. S&P has videos available to help walk you through it as well.
31. What wiring grounds do I need to make sure I have?
The battery needs to be grounded to the engine or transmission, and the engine needs to be grounded to the frame. If a ground is bad, you will have poor starting and poor drivability. You'll also need a battery with at least 850 cranking amps and a 90-amp or higher alternator.
32. Do I need to use an oxygen sensor?
Yes, the oxygen sensor lets the computer keep the fuel ratio at 14.7:1 for good fuel economy and performance. You'll need to install one in the exhaust somewhere (single wire with short headers, three-wire heated sensor with full-length headers). The oxygen sensor needs to stay at 600 degrees, and the 12-volt wire of the 3-wire heated sensor will keep it hot so the computer goes into closed loop mode quicker--which will give you better drivability.
33. What distributor can I use?
Use the '85-91 Corvette or '85-86 F-body large distributor with the coil. If you need a smaller unit, the '87 and later F-bodies used a small distributor with a remote coil. If you are using the '85-86 distributors with a roller cam, you need to change the distributor gear to the appropriate GM gear (PN 10456413) or an aftermarket one.
34. Do I need special intake gaskets?
With aluminum heads you need the gaskets for the late-'86-91 Corvette if you are going to run the water-crossovers (the intake has outlets from the back of the heads to let water out to keep from getting hot spots). Cast iron heads can use the '85-92 F-body gaskets.

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.




http://www.thirdgen.org/service-engine- ... rror-codes

http://www.iroczone.com/2009/10/1982-19 ... -firebird/







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. IF everything seems to run decent , your not throwing trouble codes and your only real complaint is its not idling smoothly, and you've set the timing, set the plug gaps at .045 and done the basic tune up route , checked for vacuum leaks,verified the ignition wire resistance etc, etc,ID adjust the TPS and IAC and idle screw and get it to run at 650 rpm, then throw in a can of both texico fuel injection cleaner and sea foam gas treatment, yes they do tend to clean up varnish and crud in the injectors, but in most cases its WATER in the fuel thats a more common problem than crud, if your changing fuel filters about every two years or more frequently


this info may help

testing the c4 corvette fuel pressure relay

it could also be the fuel pump... the fuel pump relay.. the oil pressure switch or sending unit.. the what...

the fuel pump relay on most models.. comes on when the ecm sees cranking signal from the ignition switch... some models it stays on ... others.. it drops out of the circuit...

but how does the fuel pump get power when running down the road...

the oil pressure switch or sending unit has 3 terminals... one that is hooked to the oil pressure gauge or light..

the other 2 terminals power the fuel pump when the oil pressure is up... these have gone bad before... and do the weird start and die problem....

it takes less than an hour to change the fuel pump on a C4... right through the fuel door when you take it off and the overflow boot out..
2°F (-50 to 1000°C)

OBDI code scanner adapter driver




watch video

http://www.corvettefever.com/techarticl ... index.html


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

http://members.shaw.ca/corvette86/Compo ... w%2086.pdf

http://members.shaw.ca/agent86/Fuel%20C ... ery-8A.pdf



http://www.corvettefever.com/howto/corp ... index.html
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one thing I've always found to be rather amazing is the number of guys who own corvettes who have no idea how to check fuel pressure


most guys prefer to have a tool they keep in the tool box, they can use to check fuel pressure with once they open the schrader valve on the fuel rail,

but some guys prefer to mount a small gauge on the rear of the pass side fuel rail with an adapter after removing the inside valve section from the schrader valve, which is easily done and makes checking the fuel pressure far faster and easier



oldvettefan said:
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

larger intake manifold plenum volume TENDS to allow higher rpms
throttle body efi acts like a carb, as the injectors dump fuel/air in the plenum







direct port fuel injection has a separate, individual fuel injector feeding each cylinder, the plenum essentially runs mostly dry air free of fuel


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.
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most of the muscle cars and corvettes many of us rebuild and drive are 15-60 years old now, keep in mind the standard G.M. O.E.M. wiring and connectors,
were designed to last 15 or so years under normal use,
pigtails electrical connectors are seldom designed to stand up to long term exposure to moisture,
nor are they built to have excessive extra amp capacity, the conductive surface connector connections degrade,
and must be replaced eventually, so having a multi meter and the related test leads, and a shop manual, for your year car,
is almost mandatory and a code reader at a minimum is almost mandatory

the threads have links youll need