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Thread: Weird fuel pressure problem

  1. #1
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    Weird fuel pressure problem

    I have a 2003 truck 6.0 motor and 4L80E trans, factory computer.It has the supply and return lines on the motor. It started out as the engine running out of fuel,shifting into 2nd at WOT,it would just fall on its face and I barely made it home. Checking fuel pressure,it only had 53lb at idle but with the vacuum line off,it showed 60lb. I figured that the pump was going bad,a in tank Walbro pump,so I changed the pump to a 380 LPH Aeromotive in tank pump.After checking pressure,it still had 53lb at idle and 60lb with the vacuum off,the same pressure as the old pump. I then changed the pressure regulator and there was no change in pressure readings. I changed the new pressure regulator for another one,this one is adjustable,and still low pressure at idle. Since the new regulator is adjustable,I turned it up to the required 58lb and it runs fine now,BUT,with the vacuum line off it has 68lb. Now,off idle low speed running,it slightly bucks when cold but gets better when it warms up.I know it has too much pressure off idle causing the issue.It holds pressure when I turn it off,so I don't think there is a leak at the regulator.My question is,can something happen with the fuel rail that would cause the pressure change? I was thinking there might be a leak in the fuel rail,but when I turn it off,it holds pressure, so there can't be any leak. All of the fuel rail is factory.

  2. #2
    Could be an injector leaking into the intake runner

  3. #3
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    It idles perfect,so I don't think I have an injector leak and it holds pressure long after shut down.

  4. #4
    Senior Tuner kingtal0n's Avatar
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    Quote Originally Posted by 89 Caprice 6.0L View Post
    I have a 2003 truck 6.0 motor and 4L80E trans, factory computer.It has the supply and return lines on the motor. It started out as the engine running out of fuel,shifting into 2nd at WOT,it would just fall on its face and I barely made it home. Checking fuel pressure,it only had 53lb at idle but with the vacuum line off,it showed 60lb.
    the system set to 60psi is normal,
    vacuum brings it down 1psi for every 2" of vacuum so 60-53 = 7*2 = ~14" of vacuum at idle (typically would be 18" and 51psi)

    sounds normal to me, how do you know it was 'running out of fuel' did you see fuel pressure drop while at WOT? Because that is how you would know.
    You won't see 'running out of fuel at WOT' while staring at the fuel pressure gauge in neutral at idle.

  5. #5
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    I thought the normal pressure was 58lb at idle and 60lb with the vacuum off of the regulator at idle. That was the readings that were there from the beginning. I have been a mechanic all my life,I'm 63 now and cars have been my life,so I know it was running out of fuel. When it happened,I put the pressure gauge on it and only had 53lb and 60lb with the vacuum off. After the pump and two regulator changes it still only had 53lb at idle,I took it for a ride and it barely made it around the block. I turned the regulator up so it would read 58lb at idle and it has been running great since. This happened after 1200 mile of driving since it was built. New fuel tank and sender,AN-6 supply and return lines. The motor is stock. About a week before this happened,while driving with the scanner on, the LTF readings were going way up,15+ showing red as soon as I touched the throttle,if I remember right. The computer was trying to add fuel. To me,through a process of elimination,the only thing that it ends up is something happened to the fuel rail. I was thinking of an internal crack that is letting it bleed off pressure while running,but,it holds pressure when I turn it off,so there cant be a crack or leak. It has been running great since I turned up the pressure to where it was when I built it. The car ran 13.60 at 101 mph and it really runs a lot better now since I bumped up the the lower rpm timing.I was just wondering if anyone else had run into a problem like this,a pressure drop with no problem found.

  6. #6
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    Goes to show you don't really understand how a vacuum referenced regulator works if that's what you think is normal. No way is a 2 psi change normal between vac line connected and disconnected, that would mean you are only pulling 2 psi vacuum at idle.

  7. #7
    Senior Tuner kingtal0n's Avatar
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    fuel infectors have two sides right

    fuel flow in side "top"
    fuel flow out side "bottom"

    if the fuel flow in side is at 60psi (above atmospheric so total is 60 + 14.5psi at sea level = 74.5psi absolute)
    and the fuel flow out side is at 16" Hg (or 8psi below atmospheric)

    the total would be
    60psi + 8psi = 68psi of fuel pressure effective for the injectors

    In order to prevent the injector from flowing extra fuel when there is an intake manifold source of vacuum (running engine)
    We can connect a 'vacuum referenced regulator'
    which uses engine vacuum (same pressure drop the fuel flow out side of the injector is exposed to)
    to drop fuel pressure back to baseline in the eyes of the injectors

    ex. connect vacuum line feeding 16" Hg of vacuum to regulator
    fuel pressure drops from 60psi to (60psi - 8psi) = 52psi of total fuel pressure
    so now the injectors have:
    52psi on "top"
    -8psi on "bottom"

    The top pressure 'pushes down' with 52psi
    the bottom pressure 'pulls down' with 8psi
    together they equal 60psi,
    that is, the regulator shows 52psi but the true fuel injector pressure is 60psi because -8psi of intake manifold vacuum 'helps' the 52psi on top of the injector to create 60psi

    this works in reverse as well,
    for example: With 60psi of fuel pressure
    and running 20psi of boost pressure in the manifold
    fuel pressure reads '60psi' on the regulator gauge, yet true correct flow rate for fuel injector pressure is only 40psi
    until we connect reference for 20psi boost pressure, giving 80psi total fuel pressure and 60psi for the injector corrected.


    results:
    -Inside The engine tune file configuration, the fuel injector flow rate with the reference line connected should be the same no matter what the pressure (KPA of intake pressure)
    -If the reference line is disconnected then the fuel pressure remains locked at 60psi, which means the tune file configuration will read a different injector flow rate for each KPA cell of manifold pressure.

    suggestions:
    look inside the tune config for improperly setup fuel injector flow rates vs KPA
    If the symptom seems to have appeared from nowhere there may be other issues

  8. #8
    Hey talon you can't pull 8psi through an injector that's not pulsing. I think you missed physics in high school but thanks for the long post that led to nowhere again

  9. #9
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    When I get my pressure gauge back,son has it at work, I will look at all the readings and see what I have and go from there.

  10. #10
    Senior Tuner kingtal0n's Avatar
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    Quote Originally Posted by 85K5Jake View Post
    Hey talon you can't pull 8psi through an injector that's not pulsing. I think you missed physics in high school but thanks for the long post that led to nowhere again
    What I posted is extremely accurate and it's the reason referenced regulators exist.

    8psi absolute in the manifold is an intake vacuum of 16" , 16" of a total 14.5psi or approx 32" Hg of possible laboratory vacuum
    fuel pressure is a global variable and has nothing to do with injector pulse.

    This math is necessary for setting up injector size for referenced system, not something I invented.
    I suggest you take this opportunity to learn how it works since you obviously do not.

    since this one is edited:
    I apologize for #18. That is a automatic defense for #8 and #11, for some reason about 3 people on every forum try slander poor ol defenseless tal0n, not even a real o its a zero. i believe this is some kind of internet personality disorder where no matter what a person says, if its long enough or boring to read, they attack it. Or some shit like that (non professional setting). If there is no reckoning then it would continue forever, incomprehensibly assumed to be 'wrong cause long' and attacked must eventually end with the reverse results (other 98.89% of posts within 5% confidence interval of being 98% accurate statistically) because you can't actually believe that I might want it to happen so that I can correct those dependency behaviors? The need to attack another person for the sheer act of doing it without actually having a just cause (incomprehensible reason). What do you think my reason was #8 #11 and why did I go through with it? Because you need to see how it feels when you admit you don't know physics means nature, and that might be the cure. Its like you are telling me that you do not need to wipe your own ass. Why did you share that? Should I treat an animal like a gentleman when I am animal myself, in one of many naturally occurring jungle of interwebs...
    because I am not evil enough to say what you want to hear just to concur your favors, I am in for the defense of all our freedom of choice and would protect that while able to severe dependencies. For as long as possible (even though eventually all is gone, I can try to slow it down some) loyalty and trust are fine threads cast over large distances, they must be made of the most incredible materials not to break. uahwords finals are in couple weeks, back to work

    tldr: pointlessly provoking statements with little to no correlation, 85kjake shows up with senseless jargon to de-rail understanding of material? What troll is that I wondered, ? sorry I do not approve but will have best intentions
    Maybe you think I would have the nerve to touch a keyboard without being 98.888887% sure myself on subjects for which I pretend to just repeat because internet can last a long time,
    and all it really takes is one. once 30 years and not once
    Last edited by kingtal0n; 11-21-2018 at 12:02 AM.

  11. #11
    Quote Originally Posted by kingtal0n View Post
    What I posted is extremely accurate and it's the reason referenced regulators exist.

    8psi absolute in the manifold is an intake vacuum of 16" , 16" of a total 14.5psi or approx 32" Hg of possible laboratory vacuum
    fuel pressure is a global variable and has nothing to do with injector pulse.

    This math is necessary for setting up injector size for referenced system, not something I invented.
    I suggest you take this opportunity to learn how it works since you obviously do not.
    Physics doesn't apply to you. Right on keep entertaining us buddy.

  12. #12
    Senior Tuner cobaltssoverbooster's Avatar
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    referenced regulators exist to maintain a constant pressure delta between injector and manifold. rate of pressure increases at 1 psi fuel per 1 psi manifold or it decreases 1 psi fuel per 2" vacuum.
    when fuel pressure is held constant and the manifold is under vacuum, the injector has a larger pressure delta and can move more fuel because the lack of pressure on the discharge side isn't restricting flow by working against it. when under boost the pressure delta is lower, and the pressure in the manifold resists the fuel coming out of the injector to a degree which varies with change in manifold pressure.

    85K5Jake:
    while you would be correct in this doesnt apply to a closed injector, the fuel calibration tables are not for that; they are for modeling of active flow.

    while kingtal0n has some post you may not agree with, he worded his initial one here good enough to get the idea across.
    i by no means agree to all his posts, as some do make me wonder what he is talking about, but maybe instead of following him around and antagonizing him you can help him re-word his posts to make things more clear. Corrections he can learn from so he maybe does not end up making the same wording mistakes in the future. All im asking is to keep it productive and friendly in here, please.
    2000 Ford Mustang - Top Sportsman

  13. #13
    I'm not following him around I just want to point out his awesome high and mighty way of degrading people that ask for help. His contributions are crap, although worded very high and mighty.

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    Everything has been put on hold for a while. This wonderful weather we have been having in Pittsburgh Pa,cold temps and so much rain,is stopping me from doing anything. I also received some bad news yesterday on my lump in my left breast,yes the big C,so other things need to be done first. I thank everyone for replying to my problem. Hope to be here when everything has been sorted out. Thanks again.

  15. #15
    Senior Tuner cobaltssoverbooster's Avatar
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    Quote Originally Posted by 89 Caprice 6.0L View Post
    Everything has been put on hold for a while. This wonderful weather we have been having in Pittsburgh Pa,cold temps and so much rain,is stopping me from doing anything. I also received some bad news yesterday on my lump in my left breast,yes the big C,so other things need to be done first. I thank everyone for replying to my problem. Hope to be here when everything has been sorted out. Thanks again.
    Good luck and recover quick now so we can keep going from wherever you decide to leave off.
    2000 Ford Mustang - Top Sportsman

  16. #16
    Senior Tuner kingtal0n's Avatar
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    Quote Originally Posted by 85K5Jake View Post
    I'm not following him around I just want to point out his awesome high and mighty way of degrading people that ask for help. His contributions are crap, although worded very high and mighty.
    I re-read my post and find nothing degrading to anybody.
    The form factor of my posts is similar to how books of higher education are worded in their chapters.
    I'll read you a couple passages for example:
    Controls
    "We have shown that the design of a compensator to satisfy the given specifications (in terms of phase margin and gain margin) can be carried out in the Bode diagram in a simple and straightforward manner. It is noted that not every system can be compensated with a lead, lag, or lag-lead compensator. In some cases compensators with complex poles and zeros may be used."

    materials mech
    "The semiinverse method of Saint-Venant is comparable to the mechanics of materials method in that certain assumptions, based on an understanding of the mechanics of the problem, are introduced initially. Sufficient freedom is allowed so that the equations describing torsion boundary value problem of solids may be employed to determine the solution more completely"

    I've been reading my entire life so its rubbed off, I put "" quotes around anything I take from a book though. When you see this sort of script it is a method employed to create short-as-possible passages with maximum information and minimum math (no emotional hold ups, no pleasantries and no equations if possible). Not intended to be degrading or high and mighty, but when math is broken down into words it can become difficult to turn to English. Also Maybe it is that people not used to reading 30 pages per day as students have brains which are 'angry' at not having aptitude to decipher the seemingly endless texts that books provide, even when done second hand in a (rough/attempt) method on my part. Sorry if you think it is degrading and I'll remove anything you find offense should you truly find something that is. But I think in reality you just want to believe the information is degrading so you can feel comfortable with being angry at it and ignore it.

  17. #17
    Quote Originally Posted by kingtal0n View Post
    I re-read my post and find nothing degrading to anybody.
    The form factor of my posts is similar to how books of higher education are worded in their chapters.
    I'll read you a couple passages for example:
    Controls
    "We have shown that the design of a compensator to satisfy the given specifications (in terms of phase margin and gain margin) can be carried out in the Bode diagram in a simple and straightforward manner. It is noted that not every system can be compensated with a lead, lag, or lag-lead compensator. In some cases compensators with complex poles and zeros may be used."

    materials mech
    "The semiinverse method of Saint-Venant is comparable to the mechanics of materials method in that certain assumptions, based on an understanding of the mechanics of the problem, are introduced initially. Sufficient freedom is allowed so that the equations describing torsion boundary value problem of solids may be employed to determine the solution more completely"

    I've been reading my entire life so its rubbed off, I put "" quotes around anything I take from a book though. When you see this sort of script it is a method employed to create short-as-possible passages with maximum information and minimum math (no emotional hold ups, no pleasantries and no equations if possible). Not intended to be degrading or high and mighty, but when math is broken down into words it can become difficult to turn to English. Also Maybe it is that people not used to reading 30 pages per day as students have brains which are 'angry' at not having aptitude to decipher the seemingly endless texts that books provide, even when done second hand in a (rough/attempt) method on my part. Sorry if you think it is degrading and I'll remove anything you find offense should you truly find something that is. But I think in reality you just want to believe the information is degrading so you can feel comfortable with being angry at it and ignore it.
    This is exactly what I'm talking about, way to preach from up on your mountain. Your saying knowledge is threatening and offensive to me because I don't read 30 pages a day. I spend most of my days surround by computers and monitors running a large scale production operation. I must be a total idiot to be able to understand computer to equipment interface and automation controls. I'm just saying you word most of your posts like you're better than the guys you think you're helping. Maybe you should try not being a dick. Just saying. This is the last post from me regarding you, I'm done with your down talk and pissing match.
    Last edited by 85K5Jake; 11-18-2018 at 02:10 PM.

  18. #18
    Senior Tuner kingtal0n's Avatar
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    Quote Originally Posted by 85K5Jake View Post
    I'm just saying you word most of your posts like you're better than the guys you think you're helping.
    lol I'm legit trying to find one word or one sentence where I 'talk down' to anyone.

    The only down talk I see is the guy who said

    Quote Originally Posted by Ben C
    Goes to show you don't really understand how a vacuum referenced regulator works if that's what you think is normal. No way is a 2 psi change normal between vac line connected and disconnected, that would mean you are only pulling 2 psi vacuum at idle.
    which is why I stepped up with the explanation, I felt bad for the thread creator that he wasn't getting the help he needed and was being made fun of,
    I provided a really good, well thought out explanation because of that.

    And you call that down talk?

    Then you show up with this
    85K5Jake
    Hey talon you can't pull 8psi through an injector that's not pulsing. I think you missed physics in high school but thanks for the long post that led to nowhere again
    which honestly I thought was a troll statement. I literally thought you were just trying to confuse the thread creator with bullshit, just trying to throw him off reality by providing a nonsensical statement. I was going to ask a mod to ban you for trolling, I didn't think anybody could be that dense who posted on this site. This statement was one of the dumbest, irrelevant things I've ever read on the internet regarding fuel pressure and you should not have posted it or even thought about it. I was led to the conclusion you were either legit trolling the thread, or just plain didn't know how an engine works and somehow was mad at me for making you realize that fact.