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Thread: Is there any such thing as spark trims in these GM ecuís?

  1. #1
    Fuel Injected! 91ss's Avatar
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    Is there any such thing as spark trims in these GM ecuís?

    This has to be a stupid question, but is there any such thing as spark trims in these GM ecuís? Much like fuel trims? It would explain a problem Iíve always had.
    Reason I ask, ever since owning this 91 caprice Ė 16136965 $62 all the way back to Ď95, it has a part throttle ping occurring just past steady state cruise. Everything I tried in the past to correct it has proven to be a short term solution that last for only one to two drive cycles. Itís as if the ecu keeps adding spark advance until the ping returns.

    Iíve tried pulling timing through the distributor, 180 thermostat, colder plugs, egr valves, latest bin, and combinations of all of these. Itís been the same with the original engine, the new replacement one, new sensors, new throttle body, etc. Iíve tried all of these and they worked, but then the ping would come right back.

    I finally got a Burn2 and TunerPro and jumped into the prom. (Been tuning mustang for a few years) As I canít yet get the ALDU working yet and just using an autoxray hand held, I took a broad stroke and pulled 6 degrees from the main spark table that covered 400 Ė 2000 RPM and bottom of the load to 50.

    It worked, but then again, on the drive home and since, it acts like I never did anything.

    So, could this ecu have logic that trims the spark advance base on the knock sensor? Donít really see anything in the definition other then attack rate and recovery.

  2. #2
    EFI GearHead ! EagleMark's Avatar
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    Sort of? It sounds like the ESC (Knock) test. Once every start up when qualifiers are met, warmed up. above 50 MPH, steady cruise the timing will advance till knock.

    Some are done in PE and add 10 degrees, if no knock it goes to 20!

    In $42 1227747 there is an ESC Done patch which removes this test. Not sure if there is one for your ECM. In 1228746 $61 if you turn off error code 43 it has been said to diable test... for yours it may take some time in hack to find and implement?

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

  3. #3
    Fuel Injected! 91ss's Avatar
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    Hmm, interesting. So the knock test can add more timing then tables and scalars indicate. Is the amount controllable? Or is it just not broken out in this definition?

    Some more clarification and questions. If it is the knock test, why does it never appear in the first drive cycle? Are the test 'results' stored in non-volatile memory afterwards? That could explain the behavior.

    Without logging, i'm flying kinda blind right now, but more detail. The just above cruise acceleration was one example. Once I achieve this ping state, it continues to happen on any light acceleration. From stand still, around town, etc. Not just highway cruise. All said previously applies. Good for 1-2 drive cycles.

    I pulled 6 degrees further into the spark table. Have it now up to 65. Morning drive was good again. Will see what happens going back.
    The main spark table is labeled RPM vs Load. But the table units show Kpa which in Ford speak is only manifold pressure, not really LOAD. Are they one and the same for GM. Some of the stuff I need clarification on. Some tables use Kpa and other speak vacuum and look like they are in inches of Hg.

    In any case, best I can tell with the hand held scanner which only read MAP voltage, the ping tends to occur around the 2.45-2.6 volt range. Is there a way to correlate that to the spark load table easily. It's about 1/2 the voltage so does that roughly equate to the 50 column in the table? If the table is not really LOAD but just MAP in Kpa, then it might be a simpler extrapolation.

    Thoughts?

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