How do I test a mechanical fuel pump (psi test)

Discussion in 'Troubleshooting & Diagnosis' started by Riley, Mar 17, 2016.

  1. Riley

    Riley Veteran Member

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    How exactly do I test a mechanical fuel pump to find the amount of PSI pressure it is pumping? The engine is a 355 SBC. Thanks
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Our older low pressure push rod pump equipped cars do not have a schrader valve like most of the newer high pressure EFI vehicles do. About the only way to test pressure is to tee into the line after the fuel pump with a 0 - 15 psi gauge.
     
  3. Riley

    Riley Veteran Member

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    Ok perfect. I get you so far, so I can run the car with the gauge connect to the T i install? I have a vacuum gauge that has a fuel pressure side to it. But all I can picture is fuel entering the gauge somehow.
     
  4. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    I have the same gauge, never used it for fuel pressure though, should be okay.

    Is there something that makes you think fuel pressure is low, or do you just want to check?
     
  5. Riley

    Riley Veteran Member

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    The reason i wanted to check is because sometimes i'll have what i call irregular start ups. Sometimes the car starts fine cold and hot. And other times it seems like it's starved of fuel. And the engine will sometimes run really rough. So i wanted to eliminate the fuel pump as being the problem.
     
  6. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    I highly doubt the fuel pump has anything to do with your startup issue.

    It really just moves fuel to the carburetor bowl and the carb does the rest.
    If the carb bowl bleeds down (or leaks) when the car sits for a few days... It will take several revolutions of the crank to pump fuel back to the carb and fill it to the proper level before it can feed the engine.

    A borderline pump will show up under heavy throttle when it can't supply the demands of the engine and the carb runs dry.

    Do you have a choke?
    What kind of carb?
     
  7. Riley

    Riley Veteran Member

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    Thanks for the info twisted_metal, i'm running a Edelbrock 1806, 650cfm with electric choke.
     
  8. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    Electric chokes can sometimes behave a little weird.

    Especially in a 30-45 minutes cool off period before a restart.
    The engine is still plenty warm that it doesn't need a choke but the choke coil has cooled off so it resets and tends to run rich (or even flood a bit) upon startup.

    This can be avoided by running the choke power to an IGN connection in the fuse block (instead of through the oil pressure switch as the factory did). This won't affect cold start up but allows you turn the key to the run position for a minute (prior to starting) to give the choke coil time to heat up and open the choke if the engine is still warm.
     
  9. rocket dawg

    rocket dawg Veteran Member

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    FWIW, I had fuel pump issues a few years ago. After the car was shut off, it woundn't start sometimes. The fuel was being sucked out of the the carb bowl and back to the the gas tank. The reason? i had a truck fuel pump on the engine. The engine came out of a 79 chevy short box 4X4 (400 SB) and I kept the original fuel pump on the engine. That was 100% wrong. I needed a camaro fuel pump which was quite a bit larger at least on the bottom side of the pump. I was chasing around jetting issues, ( car ran a little funny, surging, hard starting, hesitating..etc.. All along it was the wrong fuel pump. I put on a new AC Delco unit, it fired right up, went back to the baseline jetting and the car has been running great for 10 plus years. I had a Summit Tee-d fuel lines with the pressure gauge. It ran between 6.5 and 7.5 lbs ---750 Holley---
     
    Last edited: Mar 18, 2016
  10. Riley

    Riley Veteran Member

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    Thanks for all the information, I will definitley check to see if my electric choke will open with me just turning the key forward, that just may help.

    So if i was going to install a new mechanical pump just to have the new part. Would this fuel pump made by edelbrock be a ok selection?
    http://www.summitracing.com/int/parts/edl-1721/overview/make/chevrolet

    Thanks.
     

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