Does anyone know what this fuel system part is?
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Does anyone know what this fuel system part is?

  1. #1
    Off Road Badass Bonzen's Avatar
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    Default Does anyone know what this fuel system part is?

    I'm chasing a weird fuel pressure issue at around 3/4 throttle where my pressure goes from 30-ish psi to 20psi instantaneously causing major stumbling/flat spot and then back to 30psi once I push through to WOT. It doesn't matter if I mash the gas or ease into it slow, the same thing happens. I can hold the rpms and the throttle in the flat spot for as long as I want, it never recovers until I let off or push though to WOT. I'm watching this happen on the fuel pressure gauge in real time as the issue occurs. I am now looking at this return/bypass block thing that I have no idea what purpose it serves or what it actually does. It's plumbed to the fuel pump, and the main return line from the throttle body dumps into it. I'm wondering if it has some check valve or bypass inside thats diverting fuel from the pump via the small "USCG" hose which is causing my major drop in fuel pressure in that certain RPM range.

    Any ideas?

    The plumbing goes like this: Feed line - Line out from tank(new lines) into water fuel separator(brand new) into fuel pump(old but still seems to work and obviously still works at WOT) into fuel filter(brand new) into the throttle body.

    Return line - out of the throttle body into the "block thingy" and then straight to the tank.

    Small crossover USCG tube from the "in" end of the fuel pump into the top of the "block thingy"

    Also it has been verified that the fuel pump is not experiencing any significant voltage drop during the entire RPM range. The fuel pump and fuel system setup/plumbing is identical to my previous engine which was a standard run of the mill 454 330 hp. The new motor is .030over, bigger cam, 10:1 compression. The new motor runs GREAT except in that one rpm area where the flat spot occurs.

    How could a fuel system run great, then stumble and drop pressure significantly, and then pick back up and run great again??? It is running fairly rich (another issue to get tuned) and then goes pretty lean on the wide band gauge and then back to fairly rich.

    HELP!
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    Last edited by Bonzen; 05-18-2016 at 11:03 PM.
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  3. #2
    Senior Member Hass828's Avatar
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    If the engine misses for any reason (like a real fat spot) the 02 will report lean.
    "if we keep doing it the same way we always do..we will always get the same results"
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  4. #3
    Off Road Badass Bonzen's Avatar
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    The O2 is reading lean because the fuel pressure is dropping instantly in that one spot. Why I don't know... and then it shoots back up to 30-ish at WOT. I watched it happen on the gauge while it was being driven under load at speed. Doesn't make any sense!

    What would cause fuel pressure to drop in the middle of the power band but then shoot back up towards and at the end of the power band???
    "I'd love to help you out wags, but there is larger issue concerning breast cup sizes on another thread that we have to work thru first. This subject suddenly became a non issue." - GN7
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    Quote Originally Posted by Hass828 View Post
    If the engine misses for any reason (like a real fat spot) the 02 will report lean.
    I respectfully disagree. If the engine misfires, (does not burn the fuel, ignition problem) the O2 will read rich, which will lean the mixture in a "closed loop" system.
    Last edited by Maxey; 05-19-2016 at 05:05 PM.
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    Senior Member californiadreamin's Avatar
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    Quote Originally Posted by Maxey View Post
    Quote Originally Posted by Hass828 View Post
    If the engine misses for any reason (like a real fat spot) the 02 will report lean.
    I respectfully disagree. If the engine misfires, (does not burn the fuel, ignition problem) the O2 will read rich, which will lean the mixture in a "closed loop" system.
    The o2 is looking for oxygen, a misfire causes no o2 and no fuel to burn, thus a High o2 concentration. I think hass has it right. This is a throttle body injection I assume?
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  8. #6
    Off Road Badass Bonzen's Avatar
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    Yes Throttle body injection.

    No misfire codes are being recorded, and it doesn't sound like a misfire. it sounds like the timing is abruptly retarding itself when the fuel pressure drops.
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    Senior Member californiadreamin's Avatar
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    How about some info on this fuel system? Does it run at a constant pressure or is it varied based on engine load?
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  10. #8
    Off Road Badass Bonzen's Avatar
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    Quote Originally Posted by californiadreamin View Post
    How about some info on this fuel system? Does it run at a constant pressure or is it varied based on engine load?
    Runs constant pressure at 30-ish psi until the "flat spot" at 3/4 throttle and then it drops instantly to 18-20. Then immediately back up to 30 when nearing and at WOT. I have physically watched the gauge from idle to full throttle WOT top speed.

    AFR gauge reads 14.2-14.8 during the flat spot and then high 11's immediately after. Corresponds perfectly with the timing of he fuel pressure. Something is choking off my fuel flow in that narrow rpm range...
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  11. #9
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    You need to look at your t.p.s. You can check it with a multi meter if you don't have a way to pull data
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  12. #10
    Off Road Badass Bonzen's Avatar
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    Quote Originally Posted by tbscustoms View Post
    You need to look at your t.p.s. You can check it with a multi meter if you don't have a way to pull data
    TPS has been checked and is dead on what the spec says it should be volt-wise. Trust me I was really hoping that was the initial issue as that would be an easy fix.
    "I'd love to help you out wags, but there is larger issue concerning breast cup sizes on another thread that we have to work thru first. This subject suddenly became a non issue." - GN7
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    does it use a map sensor?
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  14. #12
    Off Road Badass Bonzen's Avatar
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    Quote Originally Posted by limited View Post
    does it use a map sensor?
    Yes sir...and it's a brand new GM Delco MAP. It also has been verified to be working properly by my tuner.

    Lake tested again yesterday...bypassed the whole vapor return system thing that I posted pics of and the problem still exists. Plumbed in a low pressure boost pump into the fuel feed line before the high pressure stock pump to see if force feeding the stock pump would cure the issue, but nope. Flat spot/lean condition still exists. So to me it sounds like I have a possessed fuel pump lol or something is telling it to drop pressure in only that one area of the RPM band or there is some other issue between the fuel pump and the injectors.
    "I'd love to help you out wags, but there is larger issue concerning breast cup sizes on another thread that we have to work thru first. This subject suddenly became a non issue." - GN7
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