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Thread: Need some help. Excessive negative fuel trims...

  1. #1

    Need some help. Excessive negative fuel trims...

    Try to make this short. G8 with cam, no cats, LS7 MAF. I run MAF only. The VVE is tuned, I just choose not to use it. On my way home from work today it felt like the engine was dropping out. Steady cruise at 60 mph and it would hiccup. It kept doing it so I tried to pay attention to anything since I did not have the labtop with me. Only thing I noticed is the RPM would drop, and the wideband would go FULL RICH. its only a split second when it does it. So I got home and did some logging. Drove for an hour and of course it doesn't do it but my fuel trim were WAY rich. In the cruising range they were -20's. Both banks were pretty equal. Weird thing is, IDLE fuel trims were perfect. So I failed the MAF and went to Speed Density. Again, Idle was perfect but anything in the part throttle, cruise range was -20%. Just confused at what might be going on. I have some Logs but I don't think it would help to post them as the anomaly did not present itself. Needing some suggestions on what the knock off the checklist. Spark plugs? leaking Injector? something else. Thanks all.

  2. #2
    Forgot to add. I went WOT in 2nd real quick and while in PE it was perfect @ .85 lambda. If that help or not, I don't know. Thanks

  3. #3
    Advanced Tuner ttz06vette's Avatar
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    Sep 2008
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    Think those things are a good place to start as well as EVAP.

  4. #4
    Purge valve leaking?

  5. #5
    Tested after work. Put purge valve duty cycle in logs. It’s between 80 and 100% in the areas where I am getting large negative fuel trims. So I unplugged it. Fuel trims were absolutely perfect. Can I disable purge valve?? Thanks.

  6. #6
    Advanced Tuner ttz06vette's Avatar
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    Sep 2008
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    Lot of people plug them...I replaced mine and it fixed the problem.

  7. #7
    I replaced it about 4 months ago. Trying to read up on it now. Thanks