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Thread: 3800sc leaning out in WOT. Fuel pump?

  1. #1
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    3800sc leaning out in WOT. Fuel pump?

    I could use some help from this community on a tuning issue. I'll put some specifics and a log file lower down. In short, my AFR gets close to target (11.6) under WOT, but then leans out (13.5). This causes expected Knock Retard. I put a video camera on my fuel pressure gauge and then transcribed the date (blue line below). The graph shows a WOT run, 2nd and 3rd gear. The fuel pressure regulator is boost referenced so fuel pressure jumps up with the MAP. But, the fuel pressure falls off too early. Do I just need a better fuel pump? Are there other things I should look at first?
    image002.png
    I haven't messed with the fuel adder tables. Since my commanded AFR values look correct, I assume these adder tables aren't the culprit. My injector flow rate table is essentially flat, which some research shows is appropriate for a boost-referenced FPR. I haven't changed anything major on this tune, aside from MAF adjustments to get the idle and cruising LTFTs close to zero. I suspect this leaning AFR issue has been happening for a long time. I only recently got HP Tuners.

    Specifics: Pontiac Fiero with an engine from a Grand Prix GTP. 1998 3800 SC (L67) with a Gen 5 blower, Northstar throttle body, and headers. Intense Stage 1X Blower camshaft with 105# valve springs. Light DIY port on the heads. 42 lb Lucas injectors. 3.4 pulley and attempting to tune for 91 octane. The fuel pump is stock for the GTP. Here?s the log file.

    Thank you!

  2. #2
    Tuning Addict blindsquirrel's Avatar
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    Remove the regulator vacuum hose and do some WOT snaps while watching the FP gauge. It shouldn't really move any.

    p.s. Attach the logfile the same way you attached that screenshot. If the log is too long it's too long to show the specific issue you're trying to solve and needs to be cut down (zoom to the region that shows your problem happening, then 'Export', visible range).
    Last edited by blindsquirrel; 04-17-2023 at 02:50 PM.

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    Thanks for the tips. I've attached the same log file: apr14-23_WOT.xlsx
    I'm a bit hesitant to do a WOT run with the vacuum line detached.
    At start of log, injectors were flowing at an effective 37.46 PSI (40.8 FPR - 3.33 MAP psia). During WOT, injectors were at an effective 36.4 PSI then dropping to an effective 22.3 PSI. If I remove the vacuum connection to the FPR, WOT may start at an effective 19 PSI and then drop lower if the fuel pump can't keep up. At what point do I exceed the limits of the knock retard system and risk damaging engine internals?
    Is there a different step I can take to diagnose this problem?
    My injector pulse width peaked at 13.89. I have not yet figured out the calculations to connect IPW to the flow rate of the fuel pump. Research continues...

  4. #4
    Tuning Addict blindsquirrel's Avatar
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    Quote Originally Posted by Jay-ID View Post
    I'm a bit hesitant to do a WOT run with the vacuum line detached.
    In neutral. Vroom-vroom-vroom. That's all. Done thousands of times every day in repair shops all across the world to check if the needle on a fuel pressure gauge moves when it shouldn't.

    Disconnecting the reference hose is only to stop the manifold vacuum from moving the needle when you're trying to see if it will stay steady.

  5. #5
    Tuning Addict blindsquirrel's Avatar
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    One log on Google Drive and another in .xlsx format. Nobody's going to go to the trouble to look at those.

  6. #6
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    Could just be the fuel filter needing replaced.

    Post logs and tunes in hpl/hpt format, please.

  7. #7
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    Thank you both. Here's the HPL file: apr14-WOT1.hpl. Since I had to manually transcribe the fuel pressure from a camera, here's the log as a CSV: apr14-23_WOT.csv
    I tried disconnecting the FPR from vacuum. The idle fuel pressure was about 10 psi higher so 51 PSI. When revving in neutral, the gauge didn't move. There isn't much load on neutral so my problem could still be that this pump is only capable of flowing less than the maximum demand.
    Good point on the fuel filter! I haven't changed it in a couple years. I'll do that and then log more data.

  8. #8
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    Fuel filter has been replaced, but no change in behavior. The fuel pressure still falls off while the boost stays high. It could be that the wiring to the fuel pump isn't stout enough, a known issue in Grand Prixs. Before I go replacing wiring, are there any tune settings that could compensate? I could tweak the MAF to get richer at WOT, but I'm already starting WOT at roughly the desired AFR. The RPM vs. Time area is tempting. Suggestions and guidance much appreciated.

  9. #9
    Tuning Addict blindsquirrel's Avatar
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    Quote Originally Posted by Jay-ID View Post
    The fuel pressure still falls off while the boost stays high.
    That's based on what data?

    If there really is a supply problem, then asking the computer to hold the injectors open longer will only drop the pressure even more. This is like asking if there are tune settings you can change to compensate for a blown head gasket.

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  11. #11
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    Alright. I'll upgrade the fuel pump. That was on a future upgrade list anyway to support e85. Thanks again.