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

  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
    EFI GearHead ! 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!
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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
    EFI GearHead ! 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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    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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    EFI GearHead ! 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!
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  9. #9
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    I should clarify - I built the transmission in this car, and I installed a TransGo HD2 kit and physically disabled the PWM control of TCC lockup in the valvebody while I was in it. I put the switch in because at the time I was not aware of how much you coul do via programming. The TCC never automatically locked on this car ever since the day I bought it. Maybe I should start another thread on that?

    On a side note, honestly, if I had to do it all over again, I'd put the Sonnax fix in and re-enable PWM control as the part-throttle TCC apply can get pretty harsh. The other thing I found out is that the stock TCC regulator valve limits TCC internal pressure to 80psi, but when you lock it open like the common redneck fix was, you let full line pressure hit this 12" diameter converter - that's a lot of surface area for that pressure to act on and push between the transmission and the engine.

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    Sherlock9c1, looks like when you hit the throttle at record 5361 you basically stalled for fraction of a second MAP shot up and drove you onto the upper right of main timing table to very low or negative timing settings. Your VE main table looks stock at 400-599 RPM row but then goes 8-10% on the lean side (high settings) starting at 600 RPM row. EG: at 40 Kpa 600 RPM you VE cell is 70.7 efficient when stock is 62.1 so you are (70.7-62.1)/70.7= 12.1% leaner than stock in that cell. Since adding fuel helped looks like you got this fixed. As far as EGR emission test is 15 and 25 MPH on dyno. Just figure out what gear and RPM you will be in and make sure EGR is enabled at that RPM. Otherwise it may fail the NO test. I set mine around 1200 RPM enable (back in the days when required). Usually EGR disables at low vac (high MAP) your was engaged at 87.5 Kpa, that's abnormal.
    Last edited by bobdec; 01-30-2013 at 01:42 PM.
    I'm Bob 1994 Z28, A4, CAI, Cam, 1.6RR's, LT's, stall, cat-back, tuned w/TunerCat $EE and TTS Datamaster. 1995 Chevy G20 conversion van, 5.7L A4 stock no tune..

  11. #11
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    Thanks Bob - I think you're right on the momentary stalling - that's very likely, and given that the data rate is only 10Hz it's possible that it got missed.

    Interestingly, once I put more fuel in there, I could never make the MAP go above 51 kPa since rpms picked up so quickly it moved out of the tip-in region. A friend offline mentioned that I might want to cruise under light load down to 1000rpm and if so, adding fuel all the way up to 100kPa in the 800-1200rpm bands would not be beneficial. I may go back and datalog that area and pull some fuel out based on what the O2s say.

    Upstate NY does not require emissions testing (downstate and NYC area does though; are you from there?); but as I was thinking about EGR, I never cruise at 600rpm so why not have it activate at 900rpm and be fully in by 1100-1200?

  12. #12
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    Updated my profile to Atlanta, GA area. Moved here 26 years ago from NY, Westchester County. Locations surrounding Atlanta require emission testing. One other thing I forgot to mention your IAC count at idle is low for an LT1, your TPS voltage (set screw) 0.61 is good, but IAC counts 6-13 at idle will work but may cause slight stumble when hitting the throttle. The LT1 IAC valve feeds the intake air plenums, and the plenums distribute idle air evenly to all cylinders. The engine likes that much more than idle air entering the TB blades up front. Your IAC before the stumble was at 6, saying you are getting a lot of idle air from the TB blades. Cracking the screw back a bit will drop voltage and raise IAC counts. It will move the TB/IAC balance of air towards the IAC side. Voltage specs. of closed TBS is 0.45-0.75, going to 0.55 will be in spec and may help off throttle hesitation.
    Back to EGR.. it's purpose is to cool the combustion temperature (adding co2) thus reducing NO a by-product of hot combustion. The hot combustion occurs at the lower RPM load conditions (1000-2100), higher RPM starts flowing a lot or air into the cylinder cooling the combustion, so EGR cuts off at that point. And lower RPM below 1000 does not result in a hot cylinder situation. Key is to activate EGR so you don't feel it or know it's working, in a stock setup 1000 turn on and 2100 turnoff s/b fine and will keep some poisons out of the air. It has high MAP cutoff so when you hit the throttle for performance it should be out of the picture as far as performance. It's really only on during DD street cruising.
    Last edited by bobdec; 01-31-2013 at 08:22 AM.
    I'm Bob 1994 Z28, A4, CAI, Cam, 1.6RR's, LT's, stall, cat-back, tuned w/TunerCat $EE and TTS Datamaster. 1995 Chevy G20 conversion van, 5.7L A4 stock no tune..

  13. #13
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    Thanks for the advice Bob - I did notice the low IAC counts on this motor, even at low RPMs. Questions:
    1. Am I right to think that I should first verify no other vacuum leaks are present before tweaking the throttle body set screw?
    2. I have not cleaned the IAC on this throttle body in awhile. A cruddy IAC should result in higher IAC counts, not lower, correct?
    3. Is it worth trying to clean the idle air spider plenum (without removing the intake manifold, preferably, or even possible)?

  14. #14
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    1. Correct any other air entering the intake will increase idle speed and PCM will close IAC (lower counts) to compensate.
    2. The setup is a DC stepper motor and worm gear that moves a plunger valve. If valve sticks in open or allows air to bypass the valve seat then the PCM will continue to try and close the valve using -steps that will lower the count.
    3. Been a few years since I installed my cam, was pretty busy at that time I hosed water through the idle air plenums, nothing came out and did not really look into the actual plenum design. I have heard of using a pressure washer, etc. But nothing when intake is installed. Maybe something like a super long pipe cleaner. But in reality if plenums were obstructed then air would be starved and the PCM would open the IAC and counts would increase to compensate. In modded engines incorrect idle air distribution causes split BLM's at idle and low TB position conditions.

    A little more trivia PCM controls idle two ways 1-fast action milliseconds by using idle underspeed/overspeed timing tables (adds or pulls 2-3* to speed/slow idle) 2- slower action seconds, it starts incrementing/decrementing IAC stepper motor (IAC counts) and that could take seconds. When PCM sees ign-on it sets IAC according to park position table (15-125 depending on coolant temp) after startup IAC steps from park position setting to obtain desired RPM per temp and that can take seconds.
    Last edited by bobdec; 02-01-2013 at 12:10 PM.
    I'm Bob 1994 Z28, A4, CAI, Cam, 1.6RR's, LT's, stall, cat-back, tuned w/TunerCat $EE and TTS Datamaster. 1995 Chevy G20 conversion van, 5.7L A4 stock no tune..

  15. #15
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    If I can hijack my own thread...

    Cold-start tip-in stumble in MAF mode is the subject of this post.

    I'm working on an LT1/T56 setup for a friend. This is an "interim" setup so yes, the parts are a little mismatched - long story. Stock 5.7L motor with high-quality fully-characterized injectors. Supercharger on low boost with an intercooler. Headers and huge exhaust. 3.5" F-body LT1 MAF is between intercooler and throttle body downstream of blowoff valve, in a nice straight section of pipe. I've got the MAF dialed in perfectly. Car runs awesome when fully warmed up at part throttle and WOT. BUT - Before the car gets fully warmed up, there is a tip-in stumble from idle up to 2000rpm. I've added 5% more fuel to the VE tables in those areas thinking it was a transitional fueling problem. He says the cold stumble has not improved. What am I missing here? Do I need to adjust the open-loop AFR vs. coolant vs. MAP table?

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