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Thread: Fuel trims

  1. #1

    Default Fuel trims

    What would cause STFT on bank one to be much higher (positive 20 to 30%) than on bank two?
    Bank two will be negatives about 10% but go away under throttle.
    Bank one will continuously show positive fuel (lean) on bank 1 and doesn't get much better just kinda hangs there.

    Activated the LTFT I'm going to log tomorrow.

    Things I've changed.
    02 sensor bank 1
    Compression good all around
    Injectors tested good.

    I do have low vacuum, I'm unsure what the vac for a cam with 110 lsa should be 234/244

    Lt1 car with 24x ...

    I'm gonna change the oxygen sensor extension also. Sometimes my trim on bank one tends to be slow and it likes to hang low in the milivolts . Then start acting "normal."

    But what would cause side specific issues.

  2. #2
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    Default

    Quote Originally Posted by anniversaryss View Post
    What would cause STFT on bank one to be much higher (positive 20 to 30%) than on bank two?
    Bank two will be negatives about 10% but go away under throttle.
    Bank one will continuously show positive fuel (lean) on bank 1 and doesn't get much better just kinda hangs there.

    Activated the LTFT I'm going to log tomorrow.

    Things I've changed.
    02 sensor bank 1
    Compression good all around
    Injectors tested good.

    I do have low vacuum, I'm unsure what the vac for a cam with 110 lsa should be 234/244

    Lt1 car with 24x ...

    I'm gonna change the oxygen sensor extension also. Sometimes my trim on bank one tends to be slow and it likes to hang low in the milivolts . Then start acting "normal."

    But what would cause side specific issues.
    o2 sensor, vacuum leak, or tune itself

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  3. #3
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    Vacuum leak I'd start there.If you have changed the o2

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    1995 Trans Am - CAI - TB coolant bypass - Performance Muffler - Poly mounts - stock LT1/A4 - 13.3 1/4 - 1.9 60 foot , driven daily. .

  4. #4

    Default

    Ok so here's a small update. Learned my tps voltsge was a little high. It was at .67 like a stock lt1.
    But I guess the ls1 pcm likes .55 ish
    I adjusted it and for the few minutes I had to let the car idle the stft were pretty good but the long term trims on the driver side were steady 20% addition pass was 0 to -3
    Noticed that when I would blip the throttle the LTFT on driver side would go to 0 then when idle settles it will come back to 20 adding.

    Any incite?

    On top of that I have a CPS code but I think it's bc of the idle misfire bc the driver side is adding fuel. But it goes away under throttle

    I have a log if anyone wants to look

  5. #5
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    LTFT's and STFT's change when you move the throttle, because there is a grid of "cells" based on RPM and MAP (as explained in my response on CZ28.com) with stored LTFT's. Move away from idle, and the numbers change. How much have you driven the car since the last PCM reset? Step on the throttle at idle, RPM increases, MAP show "no load" so you are in a cell you would very seldom actually drive in. LTFT could be near 0 at higher RPM, then as you let off the throttle, when RPM returns to the idle value, you go back to the idle cell with the very high LTFT. If the PCM can't add enough fuel using the LTFT, it starts using the STFT to eliminate the perceived lean condition. That's what appears to be happening on the driver side only. Again, based on the very limited data you have posted, which really makes it difficult to pin anything down, other than there is a problem.

    You indicate you have a CPS code (which one?). Do you also actually have a misfire code? Because a misfire would explain the huge fuel corrections being made on the driver side. If on the driver side you have a +20% LTFT (1.2X multiplier) and a +30% STFT (1.3X multiplier) the PCM is reacting to lean O2 sensor readings by adding more than 55% extra fuel on that side (1.20 x 1.30 = 1.56) - Misfire on driver side, plugged injector on driver side, huge vacuum leak SPECIFIC to the driver side (not one that could affect both banks), faulty O2 sensor or sensor wiring.

    Apparently the passenger side has no problem, given the fact you didn't point to a large + LTFT, and the fact the STFT is near 0%.

    Vacuum will be low with that cam. Overlap goes up, vacuum at idle goes down. I would guess (assuming your location is near sea level) that you are seeing 10-12"Hg vacuum, or MAP in the range of 50-60 kPa. That can cause mixture problems at idle, with high blowthrough of A/F mixture when both valves are open, making the O2 sensor read lean with the unused air.

    If you can produce a data log in either .csv or .xls format, I can look at it. I mentioned the minimum requirement in the CZ28.com post. The more items logged, the more you can learn. What software are you using?

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