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Thread: e38 bbc idle surge with ac on

  1. #1
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    e38 bbc idle surge with ac on

    Making a new thread as been trying to work out idle surge on my 97 burban. It is swapped with e38 ecu and does not currently have ac signal going through the ecu due to complications of getting ac control to go through e38. Idle is relatively decent even down to 700 rpm with ac off or in gear, but in park with ac on it will surge. Motor is 8.0l bbc with brodix race rite heads, single plane intake manifold, and cam is 233/233 @.050" .600"/.600" Valve Lift 114 LSA. 1000cc FCI injectors. There is a turbo to be installed once I get the NA tune parts sorted out. Tried to play with various changes on adjusting idle air and spark settings. Also transient mass fuel adjustments as have a very lean spot on sudden throttle change from idle. Recently tried reworking VVE and appeared to be some better but not much difference. Was trying to see if maybe there is an obvious direction that I am just missing that someone might be able to point me towards. Attaching current tune file and idle warm up log.
    2008 E38_T42_tune_45_VVE_change.hpt
    run102_tune45_idle_warmup.hpl
    Last edited by yevgenievich; 2 Weeks Ago at 09:14 PM.

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    Get your virtual torque inline will help. You're currently showing 230Nm at idle. Be good to see if you have a stock tune for that ecm to compare against even though you're engine is different. Idle spark, if you can get away with lowering it if the cam will take it, I'm no good at reading cam cards btw. There's a 10-20% difference between your MAF and VVE airflows. Good starting points for you.

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    Will work on better logs to work on the VVE and double check maf. Don't have a feel for what idle torque should be.

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    Senior Tuner Lakegoat's Avatar
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    If you are going to add a turbo, why not apply the 2 bar modification and that will give you a VE table to work with instead of VVE. That's a lot easier for me. I bet you can find that lean spot easier. If you turn off LTFT's and log STFT's only, the lean spot will show up like a sore thumb and you can add fuel to 3 or 4 cells and get rid of the lean spot.
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    Quote Originally Posted by Lakegoat View Post
    If you are going to add a turbo, why not apply the 2 bar modification and that will give you a VE table to work with instead of VVE. That's a lot easier for me. I bet you can find that lean spot easier. If you turn off LTFT's and log STFT's only, the lean spot will show up like a sore thumb and you can add fuel to 3 or 4 cells and get rid of the lean spot.
    And you also get that lovely PE based off boost which is nice.
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    ^^^ +1

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    Looking at the log again, at idle it is running speed density until 2500 rpm when maf comes in. This was done to try and fix the lean lag that is happening with quick throttle opening. But in the last log, average fuel trims are within 3 percent throughout the whole idle log. But overall thoughout the warm up cycle variance is -9 to 9 percent. For idle, then I don't really see a VVE adjustment. It is set up with a 3bar map sensor. Is it the 2bar code modifications that is being mentioned that I need to look in to?

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    Idle torque around the 30Nm mark is good for most. If you can hunt around for a stock tune for your OS you'll be able to see if it's already been messed with. Stock on your vacuum at 18 deg I'd expect to see it around 100-120Nm.

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    Appreciate all the feed back so far. Tune it self is heavily changed from stock. Scaled for injectors, updated maf and vve, different spark tables that mix big block and ls behaviors, updated coil pack tables to try and match ls1 coils used, and adaptive changes. It is mostly zl1 based 08 truck ecu with ls3 throttle body and pedal. Original ecu combo was with 6 speed, but been updated to work with t42 tcm for 4l80e. Previously I had idle torque closer to 150nm - 170nm marks but idle and off idle was worse, which is where I reworked the vve table down low. I will post prior iteration of tune and log later tonight.

    Been reading up on 2bar code modification to see if better to change to. Worried on making major changes partially due to prior changes for 4l80e application. This specific os was needed to run manual transfer case.

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    If the custom os ruins your transfer case behaviour you can always flash it back to how it is now. For all the reasons yevgenievich, Lakegoat and JayRolla pointed out 2 bar is the go.

    Getting the idle torque in range along with the maf to vve airflow mismatch will all help, or at the least you know it's right.

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    Will give the 2 bar os a try and report back. Attaching the prior version of vve for comparison.
    2008 E38_T42_tune_43.hpt
    run100_tune43_idle_warmup.hpl

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    Quote Originally Posted by yevgenievich View Post
    Looking at the log again, at idle it is running speed density until 2500 rpm when maf comes in. This was done to try and fix the lean lag that is happening with quick throttle opening. But in the last log, average fuel trims are within 3 percent throughout the whole idle log. But overall thoughout the warm up cycle variance is -9 to 9 percent. For idle, then I don't really see a VVE adjustment. It is set up with a 3bar map sensor. Is it the 2bar code modifications that is being mentioned that I need to look in to?
    That's not how that works. The 2500RPM you have set in your tune is to enable/disable dynamic airflow calculations. Both MAF and VE are active always.. The weight goes to one or the other more depending... but you are definitely not in SD up to 2500 RPM.

    Have you failed the MAF and really dialed in VVE as close as possible with a wideband? Normally fuel trims are Ok enough.. but in this situation I would use a good wideband. (AEM)


    I haven't done a GEN 4 ECM BBC but I have done a GEN 3 411. We used a edelbrock mini ram style intake on it. It had pump shot/transient issues. I think I had to go into the transient section to fix. I would save that as a last resort though.
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    Quote Originally Posted by Alvin View Post
    That's not how that works. The 2500RPM you have set in your tune is to enable/disable dynamic airflow calculations. Both MAF and VE are active always.. The weight goes to one or the other more depending... but you are definitely not in SD up to 2500 RPM.

    Have you failed the MAF and really dialed in VVE as close as possible with a wideband? Normally fuel trims are Ok enough.. but in this situation I would use a good wideband. (AEM)


    I haven't done a GEN 4 ECM BBC but I have done a GEN 3 411. We used a edelbrock mini ram style intake on it. It had pump shot/transient issues. I think I had to go into the transient section to fix. I would save that as a last resort though.
    Thank you for clarification. I have worked with wideband and vve table a while back but planning to get a newer wideband installed next week and rework with it again now with the ve table. I have a semi permanent wideband installed currently that I reference, but it has no serial output. Currently after change to 2bar os I have a very large peak right in the 80 to 100kpa and 600 to 1600rpm range. It helped with the large lean spot I had, but still have a stumble. It really feels like needing an extra pump shot of fuel with quick throttle openings. Intake is a Single plane holley intake with a 90* elbow and ls3 throttle body.

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    I obviously did things wrong in the vve table originally as a lot of changes were needed in the current iteration. Did a log and made adjustments. Had to keep the low rpm near 100kpa peak up there. Anything above 100kpa is random based on zl1 table. Will refine it and see if can get the idle to settle. Thanks for all the help.
    ve_table_30_May_24.jpg
    2008 E38_T42_2bar_os_1.hpt
    Last edited by yevgenievich; 2 Weeks Ago at 09:55 AM.

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    If you click the top column header manifold pressure.. (its underlined) you can edit this axis to suit your needs. I do this daily.

    That hump at low RPM isn't a bad thing. Lots of setups/cams will show up like that.
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    Quote Originally Posted by Alvin View Post
    If you click the top column header manifold pressure.. (its underlined) you can edit this axis to suit your needs. I do this daily.

    That hump at low RPM isn't a bad thing. Lots of setups/cams will show up like that.
    That region has a very fine separation as higher kpa on quick take off and also my current cruising rpm at 40mph with lock up on is about 1200rpm

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    Well, general response down low is better but now idle surge comes in drive and only some times in park with ac on.

  18. #18
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    Throw up a log

  19. #19
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    Reworked some more and saved a log. Back to being decent in drive, but get a surge in park or neutral.
    2008 E38_T42 8l_2bar_os_idle_retune_3.hpt
    run2_tune3_idle_surge_2baros_VE.hpl

  20. #20
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    Still high on torque.

    Small things (Ctrl + N):
    ECM 12104 Raise up to your nominal idle speed
    ECM 12087 Add 2g/s
    ECM 2198 0
    ECM 2199 0
    ECM 13540 650
    Idle spark 18
    ECM 12646 / 12648 revert to base settings (Edit -> View change / history logs -> Complete saved history logs -> Base -> Open as compare file)