vapor lock, fuel percolation,cause and cure

Discussion in 'Intake Systems , Fuel Systems and Related' started by grumpyvette, Jun 19, 2010.

  1. grumpyvette

    grumpyvette Administrator Staff Member

    Re: vapor lock, cause and cure

    your not the first guy trying to track down an intermittent, drive-ability issue, or problem to find fuel quality and volume effects the engine performance.
    having less than 1/4 tank of fuel, so the fuel pump mounted in the fuel tanks kept cool and not sucking air under aggressive driving, or having older fuel with moisture in the fuel can be a source of problems.
    one really handy thing you should have is a engine compartment mounted fuel gauge to show what the fuel pressure is with EITHER injection mounted on the fuel rail or with a carburetor on the carb fuel inlet line , a carburetor should have a consistent 5-6psi, with a a fuel injection system , youll need to verify the correct pressure in the factory shop manual, but on a TPI or LT1 its supposed to be in the 38-PSI -to-42- PSI range

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    http://www.summitracing.com/parts/MRG-2975/

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    http://www.summitracing.com/parts/AEI-15632/

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    http://www.summitracing.com/parts/RUS-670353/?rtype=10


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  2. fast84vette

    fast84vette Well-Known Member

    Re: vapor lock, cause and cure

    yeah i didnt think that gas from one of those cheap arab stores were really that bad. so i guess premium gas from a know gas station is the way to go with my car? is it cuz its a carb? has to do with the pump?
     
  3. grumpyvette

    grumpyvette Administrator Staff Member

    Re: vapor lock, cause and cure

    http://www.knizefamily.net/minimopar/fuelsystem.html

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    Diagnosing Your Fuel System
    Description and Diagnosis
    VERY INTERESTING ADDITION TO THE POTENTIAL FUEL SYSTEM DESIGN
    http://www.motoiq.com/MagazineArticles/ ... rvoir.aspx
    If you are having fuel pressure-related problems, use these tests to narrow down the problem. You will need a fuel pressure gauge. You can either make one, using the instructions on the Make A Fuel Pressure Gauge page, or you can buy one for between $20 and $40. They are available at most larger auto parts stores. The better ones have brass fittings, bleed valves (to relieve the pressure), and metric adapters. Here you can see a picture of mine.

    1. Check fuel pressure using a fuel pressure gauge. Connect it to the fuel rail service valve on the fuel rail or by installing a "T" fitting between the fuel supply line and the fuel rail, as described on the Make A Fuel Pressure Gauge page.
    2. Turn the ignition key to "run" (engine off). The pressure should rise to about 34 psi to 38 psi for 1984-1985 N/A engines, 13.5 psi to 15.5 psi for 1986 and up N/A engines, or 53 psi to 57 psi for turbo engines. Try cycling the key once or twice if it is low. If you still can't get the pressure up this way, don't worry about it for now. Your battery or wiring may not be strong enough to power the pump without the engine running.
    3. Watch the gauge. The pressure should hold. If the pressure drops quickly, there may be a leaking fuel injector, a bad fuel pressure regulator, or a bad check valve on the fuel pump. If the pressure is high, the fuel pressure regulator is bad or the fuel return line is somehow blocked. Don't worry about low pressure at this point.
    4. Start the engine up. The fuel pressure regulator is designed to maintain a constant pressure on the injectors, relative to the intake pressure (or vacuum). There will be a vacuum at idle and the pressure will drop. If you have a vacuum gauge, you can verify this. For each inHg of vacuum at the manifold, you should see a 0.49 psi drop in fuel pressure. So at 15 inHg of vacuum, you should see about 47.6 psi of fuel pressure.
    5. Remove the vacuum line from the regulator. The pressure should rise back to its atmospheric level (about 36psi for 1984-1985 N/A, 15psi for 1986 and up N/A or 55 psi for turbo). If the pressure is low, there may be a fuel flow problem due to a dirty fuel filter or a worn fuel pump. Be sure to connect the vacuum line back up!
    6. If the pressure measures OK, try revving the engine with the vacuum line hooked back up. The pressure should rise to match the changing manifold pressure (a vacuum gauge or boost gauge for turbos is helpful), keeping a constant pressure difference between the two.
    7. If the pressure is OK while parked, you will need to try driving around with the gauge attached. With a long enough hose, you can run the hose through the back of the hood and clip the gauge to the windshield with the windshield wiper. You will also need a vacuum/boost gauge to compare the readings.
    8. If the fuel pressure drops off at higher engine speed or while driving, there may be a fuel flow problem or a bad regulator.

    For information on upgrades, see the Upgrading The Fuel System page.

    Fuel Flow Problems

    If you are having fuel flow problems, try replacing the fuel filter. It is fairly cheap and is mounted next to the gas tank in front of the rear, right wheel well. There are also tiny screens in each of the injectors. Most injectors can be disassembled and the screens can then be cleaned. Also, look for any pinched or kinked fuel lines. If this is not the problem, then the fuel pump may be getting weak.

    Fuel Pump Failures

    Some pumps will only fail after warming up, so they appear to work fine after they have "rested" for a while. This is the sign of a dying fuel pump motor. Some pumps have a defect where a hose clamp is missing on the short fuel hose between the pump and pump bracket, inside the tank. The hose will pop off and will suddenly produce very low pressures. Other pumps will still run strong, but the gear pump itself is worn and cannot produce enough flow at high pressure (usually accompanied by a loud whining sound from the tank). You can verify a weak pump (at idle) by pinching the fuel return line on the regulator. If the pressure does not rise, then the pump is the likely culprit.

    Fuel Regulator Failures

    Fuel regulators don't often fail, but if they do, it may result in low pressure. You can verify this by pinching off the fuel return line to see if the pressure rises. More often the diaphragm will develop a leak and while the fuel pressure is not effected, fuel finds its way into the vacuum lines causing any number of problems (usually MAP sensor related).

    Fuel Injector Failures

    To test for leaking fuel injectors, remove the fuel rail and reconnect all the fuel lines. Repressurize the fuel system by cycling the ignition key to "run" once or twice. No fuel should flow or drip from the injectors. If the injectors are leaking from the tips, you can try to disassemble and clean them. If the are leaking from the connection to the rail, remove the injector and inspect the o-rings. Replace any bad o-rings. Injectors sometimes crack along the back or sides and must be replaced. Always replace the fuel injector in sets to maintain equal fuel flow (in a pinch you can try replacing it with a used injector).
     
  4. grumpyvette

    grumpyvette Administrator Staff Member

    Re: vapor lock, cause and cure

    HEY GRUMPYVETTE?
    My friend has a 55 Chevy, almost stock 350 crate engine. The problem he's having is the engine runs fine for about 30-40 minutes. Then the Fuel in the Glass fuel filter seems to just go empty, causing the engine to quit or stumble. Ive Installed a new manual fuel Pump but problem still persisted. what could be causing this problem? the FUEL Tank was taken down and cleaned. Fuel line is 3/8" from tank to the Fuel Pump were blown out with high pressure air, and are clear and the car just started doing this lately.


    ID bet your not running a return style fuel pressure regulator fuel system?
    sounds a bit like classic vapor lock, its JULY as I write this and temps tend to be higher, Id try cooling the fuel line,and manual fuel pump as far back as you can spray it with water in the engine compartment and cool the intake , upper engine area and carb base with running hose (water) and see it the fuel returns, if you keep the heat in the fuel lines and intake reduced, if it temporarily cures the problem youll need to read the links provided and make the modifications that prevent that problem
    manual pumps and steel engine compartment fuel lines exposed to engine heat will occasionally have that issue


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    exposed steel fuel lines like in this picture are frequently a contributing factor especially if routed right next to the engine or headers

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    a return style fuel supply and a 1" thick phenolic carburetor spacer that matches your cars carburetor to insulate the carburetor from engine heat will usually help a great deal
    Id also point out many plastic carb spacers used alone are next to worthless at isolating the carb base from manifold heat.
    a combo of the aluminum shield layered gasket over the manifold with the phonelic spacer on top under the carb usually works
    http://www.summitracing.com/parts/mrg-3710/overview/
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    http://www.summitracing.com/parts/aaf-a ... /overview/
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    RELATED INFO
    viewtopic.php?f=55&t=211
     
  5. grumpyvette

    grumpyvette Administrator Staff Member

    Re: vapor lock, cause and cure

    "hey GRUMPYVETTE, ?
    Ive got a electric fuel pump mounted in my cars trunk ,Ive found that on some days it takes a damn long time after I try to start the car before I hear the electric pump change tone indicating its pumping fuel, obviously it lost its prime if it sits to long like over night but it doesn,t always happen, got any ideas??"



    Electric fuel pumps SHOULD be mounted, inside the cars frame rails for protection in a crash, and ideally lower than the fuel tank if its possible, because electric fuel pumps PUSH fuel far more effectively than they PULL fuel from the tank.
    even a minor restriction or the need to pull fuel from the tank to prime the pump frequently results in in far lower fuel pump life because a dry pump, gets hot and wears faster as fuel flow generally cools the pump motor.if your pump can,t get cooled it tends to wear out much faster, thats also why IN TANK FUEL PUMPS last much longer if you never let the tank get below 1/4 full. as sloshing fuel cools an intake fuel pump along with fuel flow.
    in an ideal set-up that includes a sump and/or fuel cell.
    mounting a fuel pump in the trunk invites fuel leaks and fires that could easily transfer rapidly into the drivers compartment
    return style fuel systems tend to have fewer problems and more consistent fuel pressure for similar reasons


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    READ THESE
    viewtopic.php?f=55&t=211

    viewtopic.php?f=55&t=733&p=1030&hilit=fuel+cell#p1030

    viewtopic.php?f=55&t=635

    viewtopic.php?f=55&t=1939

    viewtopic.php?f=55&t=4381
     
  6. DorianL

    DorianL solid fixture here in the forum Staff Member

    Would tin cut out of a Coke can work?
     
  7. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    It will flop around like a piece of paper.
    Needs to be 1/8" inch thick.
     
    DorianL likes this.
  8. DorianL

    DorianL solid fixture here in the forum Staff Member

    Thanks !
     
  9. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    They are pretty expensive the Aluminum Carburetor Heat shields.
    I see them on ebay for $125-$150.
    The Yenko Camaro, Yenko Corvette, Yenko Chevelle & Nova all had that Holley Aluminum heat shield stock.

    You can maybe make your own with a Hole saw.
    I never have tried.
     

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