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Thread: Wandering closed loop rpm at Idle....

  1. 06-17-2019 #1
    DanSchafer93
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    Wandering closed loop rpm at Idle....

    Alright guys,

    I need some help... I have been pulling my hair out on this one for a while and I can't seem to figure it out...

    I have a 2012 L96 6.0L out of a Sierra 3500HD. Cold air intake and Long tube headers. No cats, no post cat o2's. It has a manual transmission currently, but still working on buttoning up the detail work like the VSS. New o2 sensors.

    My issue is.. During open loop after resetting the Long Term Fuel Trims, it runs just fine... A little slow response on throttle touches as expected, but besides that it will idle and run just fine. When it goes to closed Loop the RPM wanders up, then the spark will cut for a split second and it will drop the RPM some, but then it rises back up. Idle is never the same and never comes down to the requested RPM. There doesn't seem to be a reason or rhyme to it... I have checked for obvious vacuum leaks and I blocked off the two ports I know of on the stock manifold. I am open to checking specific areas again if someone has a specific spot to check, but I feel like it shouldn't be vacuum since open loop is just fine.

    Here is my current tune and a recording of all idle with new o2 sensors.
    2012 Silverado 2500 new 02 sensors 2.hpl
    K2500_L96_Current_Tune.hpt

    Any help would be appreciated... Thanks guys!
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  2. 06-17-2019 #2
    ZincGT
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    fix your fuel trims and it will get better, also your VSS says you are going 158mph so it probably wont let you go into true idle. you are most likely in a rolling idle table.
    Last edited by ZincGT; 06-17-2019 at 07:08 PM.
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  3. 06-17-2019 #3
    5FDP
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    With the LTFT pegging out at +25-29, having the issue you have sound like it's par for the course honestly.

    If you have no vacuum leaks or exhaust leaks you have a serious fueling issue on your hands. Either low fuel pressure or the airflow model is so out of whack the computer is doing everything it can to bring the fueling can in check.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.
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  4. 06-17-2019 #4
    DanSchafer93
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    I agree with you 5FDP, I put a manual gauge on my fuel rail and its solid at 58 PSI and I have checked my tune against other "stock" tunes in the repository for the same engine and they are identical for the most part... I don't think its the tune... But nothing looks really out of whack in the Log file until the rpm goes crazy.

    ZincGT, with my fuel trims being stock I can't imagine they are that far out... also, on the Log file I have the requested RPM at the bottom of the list on the left and it is requesting 650 the entire time. In my Rolling Idle speed table, neutral is set to 0 rpm across the board as well.
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  5. 06-17-2019 #5
    5FDP
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    Does your truck use this style MAF sensor?




    If it does, how big of tube did you install it in?

    If the tube is larger than the factory MAF housing on the airbox, that will totally throw the fueling out of whack.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.
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  6. 06-17-2019 #6
    DanSchafer93
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    This is the one I have...

    Its in a 4 inch tube which I believe is the same as stock....

    I also made sure its more than 10 inches away from the throttle plate because that is what GM stated in their standalone write up.

    Attachment 90777
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  7. 06-17-2019 #7
    5FDP
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    The airbox intake size isn't 4 inches that's for sure.

    You are probably half an inch larger if I remember right.

    This isn't exact science here but just try a global fuel changes to the MAF curve as a test. Add 5% to the whole MAF curve. See if the fuel trims come way down from +25 or more.

    If the trims come down, then you know what you have to do. You'll be spending timing re-doing the MAF curve and VVE to help get fueling back in line.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.
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  8. 06-17-2019 #8
    DanSchafer93
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    I will try that and see where it takes me... I could see the MAF curve helping but I haven't spent a lot of time with the VVE stuff yet so that will be new for me. I may have some questions come up with that.... Thanks 5FDP
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