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Thread: Fuel pressure drop

  1. #1
    Registered Member chevynut's Avatar
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    Fuel pressure drop

    When I was working on troubleshooting and fixing my fuel line issues I noticed that when first starting my RJ502, it cranks for a while before it will start. I turn the key on, let the fuel pump shut off (about 5 seconds), and then immediately crank it. The fuel pressure goes to 55-60 PSI (don't remember exactly) with the key on, and stays there while running. However, when I shut down the engine the fuel pressure drops pretty fast. Shouldn't it hold fuel pressure when off?

    I assume this pressure drop is caused by either a leaky injector(s), a leaky fuel pressure regulator, or a bad pump check valve. All parts are new but I know that doesn't mean much . I can hear fuel returning to the tank when I shut the engine off, so I assume it's a bad check valve in the pump causing the pressure loss, however, I think a leaky regulator could cause that as well. I wonder if leaky injector(s) are causing the engine to have too much fuel in the cylinders and only cranking clears it out. I'm not sure how to isolate the pressure drop.

    The pump is an in-tank Walbro 255LPH high pressure model and the fuel rail is a GM part, a common single fuel rail for all 8 injectors that was also used on the ~2000 8.1L Vortec engines. The regulator is at the back of the fuel rail and bypasses fuel back to the tank and is a common regulator used on many GM engines.

    How big of a deal is this pressure loss? What else could cause the excessive cranking?
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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    56 Chevy 210 4-door sedan
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    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  2. #2
    Registered Member 55 Rescue Dog's Avatar
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    I know on my LS that the Schrader valve on the fuel rail always has pressure with the engine off, and you have to bleed it off before any maintenance. After sitting all winter for 7 months it fires right up in a couple seconds.

  3. #3
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    GM engines don't always hold fuel pressure after shutoff. I don't think a Walbro pump has a check valve. You should be good if you have proper fuel pressure prior to starting and during running. Troubleshoot for excessive cranking symptoms, you should be able to find some general guidelines on the internet. You may have to do some tuning on that now antique Commander 950 system if that's what you still have. And you may do well to convert it to Holley HP or Terminator X. You would have technical support, and it would probably make your life easier. Biggest benefit would be the "self tuning" of a modern system. Not totally self tuning but a lot less work.

  4. #4
    Registered Member chevynut's Avatar
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    My fuel pump is a genuine Walbro GSS340 and I see conflicting info on the internet about whether it should have a check valve or not. Some say it does, and others say it doesn't. I replaced the fuel hose on the pump a while ago and I wonder if I somehow lost the check valve that's supposed to be in the outlet inside the old hose when I took it off. I don't remember hearing the return fuel flow before I replaced the hose. But most guys say it's no problem as long as you have pressure when running.

    The ECU is working just fine but I haven't had a chance to do much tuning with it since I can't drive the car yet. I went through the benefits and downside of replacing it some time ago and decided it wasn't worth the effort. Any ECU just fires injectors and plugs and it's all about tuning which I believe I can do to my satisfaction. I don't need any more inputs and outputs and I have a WBO2. It probably works as good as an "antiquated" LS1 ECU and a helluva lot better than an antiquated carburetor.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  5. #5
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    To get started on troubleshooting your cranking time problem, pull the plugs. You will see if your tuneup is providing too much or too little fuel. If it's a tuning issue, they all should look similar. If it's injector problems, individual plugs will follow what the injectors are doing, as it's unlikely they all failed the same way at the same time - unless they are full of fuel line chunks.

  6. #6
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    Quote Originally Posted by chevynut View Post
    My fuel pump is a genuine Walbro GSS340 and I see conflicting info on the internet about whether it should have a check valve or not. Some say it does, and others say it doesn't. I replaced the fuel hose on the pump a while ago and I wonder if I somehow lost the check valve that's supposed to be in the outlet inside the old hose when I took it off. I don't remember hearing the return fuel flow before I replaced the hose. But most guys say it's no problem as long as you have pressure when running.

    The ECU is working just fine but I haven't had a chance to do much tuning with it since I can't drive the car yet. I went through the benefits and downside of replacing it some time ago and decided it wasn't worth the effort. Any ECU just fires injectors and plugs and it's all about tuning which I believe I can do to my satisfaction. I don't need any more inputs and outputs and I have a WBO2. It probably works as good as an "antiquated" LS1 ECU and a helluva lot better than an antiquated carburetor.
    Funny, thing those C 4 Corvettes with antiquated electronics are getting impossible to repair while the C1. C2 and C3 carbs keep on going.

  7. #7
    Registered Member chevynut's Avatar
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    Quote Originally Posted by markm View Post
    Funny, thing those C 4 Corvettes with antiquated electronics are getting impossible to repair while the C1. C2 and C3 carbs keep on going.
    You're just too stupid to understand that the only C4 Corvette parts on my car are the suspension and differential. Maybe if you learned something about electronics and modern cars you wouldn't have to use that obsolete 60's junk that you have in your junkyard.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  8. #8
    Registered Member WagonCrazy's Avatar
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    Ignore these guys Laszlo. They are wasting your valuable time and energy.
    Back to your question about fuel pressure.

    Random thoughts here
    Maybe there isn't a check valve in the system, and its designed that way to allow pressure to bleed off when the pump is not energized (ie. when you turn off the ignition key.) Maybe thats a safety thing, so that the fuel system is not constantly pressurized when the vehicle is off? Maybe that increases the life of your injector heads/seals , since they are only pressurized when the key is on? ie. not 24/7
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  9. #9
    Registered Member chevynut's Avatar
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    Quote Originally Posted by WagonCrazy View Post
    Ignore these guys Laszlo. They are wasting your valuable time and energy.
    I ignored him as long as I could. I'm done with his bullshit. Read his posts on my other threads.

    Maybe there isn't a check valve in the system, and its designed that way to allow pressure to bleed off when the pump is not energized (ie. when you turn off the ignition key.) Maybe thats a safety thing, so that the fuel system is not constantly pressurized when the vehicle is off? Maybe that increases the life of your injector heads/seals , since they are only pressurized when the key is on? ie. not 24/7
    I did a lot of research on this and MOST cars maintain fuel pressure. Then I found out that SOME Walbro pumps have a check valve and some don't. I don't think it matters as long as you allow the system to pressurize before cranking, which I usually do. I'm not going to do anything at this point and the long cranking (it's not that long) may be due to something else, like timing.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  10. #10
    Registered Member 55 Rescue Dog's Avatar
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    You could always add a momentary pushbutton to prime it before cranking and use it to test the pump too. My engine primes automatically for about 3 seconds after turning key to the on position. When filling my whole empty fuel system, I just turned it on for a bit and back off several times and had to once after running out gas from a bad level sender. I had to empty the tank to fix that, and just clipped my remote starter button on the terminal strip for the pump to quicky drain the tank dry. I would think you could add a check valve to the fuel line if that's the issue. My 18-year-old truck engine starts in less than 2 seconds after sitting all winter.
    Last edited by 55 Rescue Dog; 01-27-2023 at 05:59 PM.

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