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Thread: Major issue with neg STFT at idle, help please

  1. #1
    Tuner in Training 08VRSS's Avatar
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    Major issue with neg STFT at idle, help please

    Hi guys, I've been having issues lately with my 08 cobalt ss. I've been noticing the STFT at idle will at times want to peg at -30% and stay there for no reason. MAF was dialed in beginning of the year, nothing has changed. I re dialed it in again, and everything is good except for this weird idle rich trim.

    Engine mods:

    EFR6758
    K&N short ram
    ZZP IC
    ZZP UCP
    Catless DP
    AEM alky injection
    One step colder plugs

    What I've done so far:

    Boost leak test
    Vac leak test
    Cleaned MAF
    Checked plugs
    Retuned MAF curve and base multiplier table
    Disabled the EVAP system isolating that from the engine
    Had a clogged alky injector, cleaned that out, didn't even know it was clogged


    Today with the attached log, i was going to do another MAF base multiplier table edit, after the second small pull, you can see the AFR drop like a brick all the sudden. Then at the end i pull into work and just sit at idle, again drops like a brick and stays there at -30% STFT. I can't find anything in the tune, but could use another set of experienced eyes to look it over incase i miss anything.

    I'm at a loss here to what to try next to fix this issue. I'm going to try disconnecting the alky injector today, maybe that check valve is sticking open, or maybe its an actual fuel injector sticking. I need ideas please. Thanks everyone.
    Attached Files Attached Files
    Last edited by 08VRSS; 11-06-2015 at 11:43 AM.
    The Daily: 08 SS/TC, fully built 2.0L LDK, Wiseco pistons, K1 rods, ARP head studs, upgraded main bolts, S257sx-e, Opels, ZZP stage 2 cams, BRC 5th injector, E47, AEM E85 fuel pump, custom 3.5" intake and PCV system, custom catless downpipe, 4.05 LSD, custom KY frankenclutch, ZZP full 3" catback with res and muffler, lowered on CM springs, lots of other ZZP parts, built and tuned by BRC Tuning.

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  2. #2
    Advanced Tuner
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    Did you find anything wrong with methanol system? I have seen injectors leak by. But I can not say that is what your issue is.

  3. #3
    Tuner in Training 08VRSS's Avatar
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    Besides a clogged injector i didn't know about, no. I even disconnected and plugged the hole to completely eliminate that variable. No change. However.....

    I'm convinced its an injector sticking. At idle it'll either be perfect one minute, +20% the next and -15% the next, then back to perfect. It varies each day too. What I'm doing now is running an entire bottle of lucas injector cleaner through the system to see if it clears up. Its starting to make a difference after one tank so far. Trims have definitely improved and become more stable at idle. We'll see after the next few tanks how it goes. If no improvement, I'm pulling the injectors and having a look at them.

    Cruising and under load, its spot on with trims.
    Last edited by 08VRSS; 11-10-2015 at 09:24 AM.
    The Daily: 08 SS/TC, fully built 2.0L LDK, Wiseco pistons, K1 rods, ARP head studs, upgraded main bolts, S257sx-e, Opels, ZZP stage 2 cams, BRC 5th injector, E47, AEM E85 fuel pump, custom 3.5" intake and PCV system, custom catless downpipe, 4.05 LSD, custom KY frankenclutch, ZZP full 3" catback with res and muffler, lowered on CM springs, lots of other ZZP parts, built and tuned by BRC Tuning.

    PICK TWO: CHEAP, FAST, RELIABLE

  4. #4
    Advanced Tuner
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    I would go ahead and just pull injectors and find a local injector service guy that can test direct injection injectors. And have them do a leak test on them. That way you know for 100% sure.

  5. #5
    Advanced Tuner
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    MAF in the stock location or relocated?

  6. #6
    Tuner in Training 08VRSS's Avatar
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    Yeah thats what i plan on doing in the near future. I'm going to put the car on my lift this weekend and double check all exhaust connections too. MAF is in stock location in the K&N short ram.

    Its gotten better, but even hot starts its sometimes pegged rich or lean depending on its mood lol, until i start driving then its fine again.

    Probably a good idea to walnut blast too when i pull the intake, 135,000 miles and never been done.
    Last edited by 08VRSS; 11-13-2015 at 08:49 AM.
    The Daily: 08 SS/TC, fully built 2.0L LDK, Wiseco pistons, K1 rods, ARP head studs, upgraded main bolts, S257sx-e, Opels, ZZP stage 2 cams, BRC 5th injector, E47, AEM E85 fuel pump, custom 3.5" intake and PCV system, custom catless downpipe, 4.05 LSD, custom KY frankenclutch, ZZP full 3" catback with res and muffler, lowered on CM springs, lots of other ZZP parts, built and tuned by BRC Tuning.

    PICK TWO: CHEAP, FAST, RELIABLE