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Thread: '95 LT1 tip-in Stumble in SD mode?

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  1. #1
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    '95 LT1 tip-in Stumble in SD mode?

    Hi guys, I put my '95 LT1 Caprice into speed density mode awhile back to fine-tune the VE tables and ever since I did that, I've had a nasty tip-in stumble from idle. I also disabled PE and DFCO at the same time. I'm attaching a BIN and a Datamaster file.

    I datalogged today and captured the stumble in samples 5350-5550 (toward the end of the run). What I see:
    1. O2 sensors go lean but after RPMs start to climb. Is this due to (normal) delay in the O2 sensor readings?
    2. Spark timing goes to zero degrees (400-600rpm is where the engine is at while stumbling).
    3. EGR appears to be turning on during this time!

    My questions:
    1. Should I add fuel first to the tip-in region of the VE tables and see if that cures it? This is my hunch.
    2. Zero degrees seems low but I suppose at that rpm (400-600) and MAP (96kPa) that's expected?
    3. Should EGR really be coming on this low?

    BTW, my left side O2 sensor occasionally looks lazy off load but then under load it perfectly mirrors the right side. You can see it earlier in the collection. Okay, or need to be replaced?
    Attached Files Attached Files

  2. #2
    RIP EagleMark's Avatar
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    What are cam specs @.050 ?

    Need WAY more timing and higher idle. 96 MAP is nuts.

    Do need to disable EGR...

    HTH!

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  3. #3
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    From LS1 tech's website: stock B-car cam: 191/196 0.418/0.430 lift 111 LSA. (don't know for sure but assume durations are at 0.050" measurements).

    This car has a 2005-vintage pcm for less tune in it. It was always punchy off idle but never had the stumble until I disabled the MAF and whatnot.

  4. #4
    RIP EagleMark's Avatar
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    MAF was adjusting fueling. SD your VE is way off.

    2. Zero degrees seems low but I suppose at that rpm (400-600) and MAP (96kPa) that's expected?
    zero degree timing and 96 MAP at idle?????? come on? Seriously?

    I haven't looked at the log, will do so in AM. Is there any DTC/Error codes?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  5. #5
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    that sounds like the "stumble" coming off of idle...

    also, the ONLY tuning you do with EGR enabled is EGR fuel compensation and EGR added spark. having it on at any other time will skew everything else.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  6. #6
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    Quote Originally Posted by EagleMark View Post
    zero degree timing and 96 MAP at idle?????? come on? Seriously?
    As Robert pointed out, this is during the tip-in event itself. Throttle opens, engine RPM goes down, then MAP goes way up. Normal idle is around 30-ish kPa.

    Quote Originally Posted by EagleMark View Post
    I haven't looked at the log, will do so in AM. Is there any DTC/Error codes?
    None of any significance. This car never locks the TCC for some reason (haven't hunted that down yet) so I wired in a manual TCC override switch that occasionally triggers a TCC Stuck On code.

  7. #7
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    Update - based on what the short term fuel trims (STFTs) were doing during the stumble, I added 13% fuel across the board to 400-1000rpm and 45-100kPa and then smoothed the 1200rpm line 45-100kPA into the 1400rpm line. I also changed the EGR min/max rpms from 600/2400 to 900/2500. The stumble has gone away although I did discover the car has a cold misfire under load that was masking the solution. Gotta go chase that now too. But.. once warmed up, the stumble appears to be gone.

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    RIP EagleMark's Avatar
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    Have to clear that MALF 85, it says the TCC is stuck on, also a bit is set that says Trans MALF has occured, never know what PCM is going to do differantly for each error/MALF code?

    Also saw a bit set for TCC temp, it came on and off a couple times, as does low oil bit when you take off. But these are nor errors, just indicators.

    If TCC is stuck on... well it will cause a stumble off idle, no slip, or partial slip.

    Your STFT and LTFT look great. O2 look fine. I can't see anything to cause the stumble, but I did see the stumble.

    1995 has a PWM TCC, so direct on switch is not a good idea...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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