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Thread: MAF causing lean misfire?

  1. #1
    Tuner in Training
    Join Date
    Feb 2021
    Location
    Parker County, TX
    Posts
    44

    MAF causing lean misfire?

    Guys I?ve been chasing my tail forever on this truck. I thought we had a good tune going. Tuned VE then MAF. Now I will say this. A few times during MAF tuning, sometimes it would run lean and sometimes rich. Felt like it was flopping back and forth. I figured it was just a tuning issue. When I felt they were both decent under 5% error, I turned everything back on normal.
    Then we started having a random misfire issue on #3. My son swapped the coil pack with a spare and it drove great. Once.
    After that it was misfiring on #3 and I noticed the AFR went lean when it was misfiring. I tried swapping the injector and it didn?t change. A brand new coil and plug didn?t change anything. Then #1 showed to be misfiring at idle. When I revved it up #1 stopped misfiring and #3 started misfiring. Other cylinders showed to be missing a little here and there.
    I had the thought maybe it?s a bad MAF. I failed it and went SD again and the misfire PIDs either aren?t reading anything or it?s absolutely not misfiring anymore.
    It doesn?t feel smooth but there is not pop or sputter anymore so I?m thinking it?s not misfiring and the MAF is bad.
    I will run a log on the way to work today and see what all it?s doing. We did have a little vacuum leak at the regulator but it is fixed and didn?t seem to change anything one way or the other.

  2. #2
    Tuner
    Join Date
    Mar 2014
    Location
    Australia
    Posts
    98
    The VE table has more cells than the MAF table, so it's more tolerant of bad injector data. It is possible that you have a bad MAF sensor, but it's more likely that you have bad injector data. Sometimes injectors come with bad data... in some situations even GM's stock injector data is crap. If you don't believe me, then compare the stock tunes in a 2001 Camaro and a 2001 Corvette... same engine, same injector, but the GM factory tunes have different injector data.

  3. #3
    Senior Tuner
    Join Date
    Mar 2011
    Location
    Ripley, MS
    Posts
    1,865
    Is it a P01 PCM? If so, it won't detect misfires in SD mode.