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Thread: Bone Stock L96 E78 idle high spark and surging idle after ECM replacement

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    Bone Stock L96 E78 idle high spark and surging idle after ECM replacement

    Hi, I am trying to better understand and resolve a surging idle that showed up after replacing the ECM on my 2015 Sierra 2500HD 6.0L

    ECM replacement - My dumb mistake... I lost communication to the factory E78 ECM because I started a write with out the key fully on, The ECM was still powered up just long enough for the erase function on the editor and then never would talk again. I replaced the ECM with an E78 from a 2015 Buick Encore and did write entire and VIN change.

    Surging Idle - I noticed immediately the surging which was not just at idle from the start, it was happening while doing the CASE learn. As time went on the ECM corrected out most of the surge, but I would like to resolve the actual issue. comparing old logs and new then I notice MAF numbers are a bit low and idle spark is about maxed out. LTFT around idle is ~10% richer than before and spark is ~35deg when I think it should be ~20

    Tests and Checks - I first thought vacuum leak or introduction of air not caught by MAF, then spark maxed makes me wonder of some other cause. I have sprayed for vacuum leaks with not change, I even unhooked EVAP solenoid and plugged to see if it was causing it. I have hydroboost, so not other vacuum lines besides PCV valve which is not leaking, but I had not plugged it.

    testing idle surge - Scanner commanded RPM.hpl
    Short drive log before ECM replacement.hpl
    2015 sierra 6.0 stock mine.hpt
    2015 Buick Encore 1.4L.hpt

  2. #2
    Tuning Addict blindsquirrel's Avatar
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    Gen4 ECMs have the TAC inside the ECM. HPT cannot write 2 of the 8 segments in a GM ECM calibration. So, what you've done is create a 6.0 Sierra file with 1.4 Encore throttle controls left behind, unchanged.

    You can ONLY do a OS/platform change like this with GM dealer-type tools and SPS.

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    Thanks for the insight blindsquirrel. I have a VCX Nano and just paid the $40 for the 2 year SPS subscription on my VIN. Maybe if I reinstall the OS and all with SPS then it should fix it up? I also did a CASE learn using the SPS software a bit ago and the spark is down in the 20s now and it has more idle control. Still does not look 100% back to normal to me yet though, so maybe SPS programming will get me there.

    Something else I could never get the VATS to relink even with SPS. It gave me a FE03 Security Code Function Locked. Research looked like maybe GM made the E78 to where the VATS was a write once, so would have to be a new ECM to fix that?
    Last edited by Dominatorstang; 09-20-2020 at 05:24 PM.

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    Also is the E78 considered a Gen4 ECM or a Gen5? From what I understand so far, it is like Gen5 but controlling a Gen4 engine...

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    Tuning Addict blindsquirrel's Avatar
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    Get the correct calibration in it and then see what issues remain. Waste of time to even think about it until then.

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    I have now programmed it with SPS, but no changes to symptoms. VATS was still not happy with the same code too so after the SPS programming then I did a read on VCM editor and disabled VATS again then entire write using the file I just pulled out. Log attached of first start along with CASE relearn.
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    Last edited by Dominatorstang; 09-20-2020 at 03:50 PM.

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    The engine also sounds a little different under load. Used to be accelerating around town with 50% pedal or so would give what I call a throaty sound, probably like what a CAI does for most cars. I really do not hear that now. Almost makes me wonder if the timing is reading off some how.

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    Doing more research about the FE03 Immobilizer error, I found the Buick Encore is part of the GM Global A Architecture which only lets the immobilize sync happen once. So swapping one in from salvage will give this error.

    Now then, the 2500HD is not on any of the Global A lists I find. I also have a salvage yard with a E78 ECM for $25 from a 2013 2500HD, I wonder if I swap it in if the VATS relink will work?... Otherwise a replacement AC Delco from RockAuto is $140 plus $25 core. having a $25 core makes me wonder if it would be refurbished, but it is not listed as being...

    Still the salvage yard one would be almost painless to go pickup for $25, I am just not finding a verification if these 2500HD's are part of the Global A since they use the same ECM as others on the list.

  9. #9
    Tuning Addict blindsquirrel's Avatar
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    There are two different Service Numbers shown for both 2015 Sierra 2500 and 2015 Buick Encore. I am guessing from the notes that "12692865 {#12642100, 12670036}" is the one with enhanced anti-theft features, and "12692200" is the one without. Did you swap one Service Number for the other?

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    The stickers on the ECM read as follows - Sierra service#12668866 and the Buick had #12643636.

    Today I purchased the 2013 Sierra ECM and installed it. I tried to program it using SPS and change VIN code. It made it about 50% and cut out then tried and kept cutting out around 10% then displayed E4491 and E4423. I am going to research these some but I do already know that this was the exact same issue I had when I tried to program over the Buick ECM after I had loaded the tune with HP tuners. The way I got around it on the Buick ECM was to load the original Buick tune back on using the HP tuners and then the SPS loaded the Sierra program and VIN code change fine.

    At this point the software versions on the 2013 Sierra are the same as my 2015 ECM had, but the VIN code is wrong and still have immobilizer issue. Now I did the immobilizer relearn using SPS and it went to count down from 12min like it had done on the Buick, but the Buick would stop after a minute or two and give an error. This ECM counted down and was successful. Engine started fine and no more VATS light on.

    So now the VIN code is just wrong which I do not know how to change with SPS other than the ECM taking the programming or using HP tuners which needs credits to do....

    And.... I still have the surge, high idle timing, and lean at idle. Which my mind just can not wrap around how having the Buick ECM and calibration on it for a short time or somehow the ECM having major flashing done causing some defaults that bring the problem into the light. But, I absolutely had 0 surging and the idle fuel, spark, and MAF numbers are not where they have ever been.

    Positive note - I am much more comfortable with the SPS software and now know that the E78 ECM in the 2500HD is not security locked after being written once like the E78's in cars are.

  11. #11
    Tuning Addict blindsquirrel's Avatar
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    Have you done the throttle relearn? (at this point that feels kinda like asking 'do you have the battery connected?' after trying to chase down a no-crank issue).

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    Good question. I tried the cleaned throttle button on VCM scanner many times after the encore ECM and did not notice any change, or that anything really happened when I clicked it. I also had looked for a throttle relearn in the SPS configure menu, but have not yet looked on GDS2 for it. I will check out the video you linked there and dig into throttle relearn a little more. Thanks blindsquirrel.

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    Okay, I tried the learned idle airflow reset and the idle learn using GDS2 with no real change. Then I decided to physically inspect the throttle blade for something odd, which I know you would think I already should have. I remember cleaning the throttle body when I got the truck a few months ago.... Well, that was my Wifes car and the jeep that I bought in the past couple months. This throttle body was nice and filthy.

    So it appears the ECM being flashed caused the learned throttle to clear back to 0 which closed the throttle further than it had been in years which actually let it stick on the buildup. Then the motor could not precisely control idle and had to over shoot it open to get it moving again before closing down and sticking again.

    Now clean, idle spark is at 12 or so and throttle blade is at 17%. Fuel cells are also looking close to 0 again which I think the fuel compensation was from the timing compensating the stuck throttle. I guess the good old rule of trust yet verify. More verify and less trust with my memory though LOL

    Thanks again blindsquirrel for your time and pointers, take care.
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    Last edited by Dominatorstang; 09-22-2020 at 07:15 PM.