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Thread: Help with '99 L31 ECM Tune

  1. #1
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    Help with '99 L31 ECM Tune

    Good morning,
    I'm as noob as they come, to tuning, so any help is appreciated. I rebuilt a 5.7 Vortec from a '99 Yukon and dropped it in my CJ7. The engine came from a junk yard with only 100,000 miles, but didn't come with the ECM. I ordered a cann tuned Red/Blue/White/Black ECM and it starts, but Cylinder 8 is constantly misfiring, even though i have spark, fuel, and compression. The cam, pistons, lifters, rods, and springs are OEM. I tried reading the ECM with VCM editor, but because it's a cann tune, it's locked. However, while it was running, I played around with the VCM Scanner's Special Functions and adjusted the Absolute Timing to 0.5 and the misfiring went away Initial Run 5.11.23.hpl. Since I can't edit the cann tune, I found another ECM at a junk yard, for the same engine, but it has its own issue in that anytime the RPMs reach 1500, they automatically drop down to idle, even with the gas pedal floored and I don't know why. Is there a way to order a stock build for a '99 yukon to overwrite the junk yard ECM I have? If not, can anyone recommend a fix?

  2. #2
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    You can typically reprogram the ECM with the ACDELCO SDS system, with the correct tools and the correct VIN. The recommended fix for a one time need is to order online the correctly programmed stock ECM based upon the VIN. Ebay is often recommended for this, but I've never had the need to use it.

    Not sure what in the tune would cause it to constantly misfire in one cylinder unless they are all on the ragged edge and #8 is just slightly worse condition.

  3. #3
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    99 Black Box PCMs can't be tuner locked. Does your PCM happen to have a 5th connector in the middle?

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    Why can't you grab a stock file from the repository, use the Change VIN Wizard, then write entire?

  5. #5
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    Quote Originally Posted by SiriusC1024 View Post
    Why can't you grab a stock file from the repository, use the Change VIN Wizard, then write entire?
    I’m still learning this stuff and forget this can be an option for Gen 3 stuff. I need to dig and see if some has written a detailed list of what can be fully programmed by HPT.

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    Quote Originally Posted by 2xLS1 View Post
    99 Black Box PCMs can't be tuner locked. Does your PCM happen to have a 5th connector in the middle?
    I think so. There's a round black cap towards the middle. I tried uploading a picture, but it doesn't seem to be working.

  7. #7
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    I would, but when I checked the repository, here on the forum, I didn't find anything for that vehicle, or engine. If I missed it, can you tell me what you searched to find it?

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    What transmission?

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    Chevy NV4500 4WD 5-Spd

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    There's a 1999 Suburban 4x4 file in the repository. Grab that and change the Trans Type to Manual. Disable automatic transmission DTC's. Don't forget to put in the VIN from the junkyard ECU. VIN Change Wizard in VCM Editor. I tested this, and the Wizard accepts the change from a Suburban to Yukon VIN. Write Entire not just Write Config.

    If that doesn't work no big deal. The license you paid for is to write your ECU. You can write as many tunes as you want.

    Here, I've made the changes. DTC's adjusted. Trans settings from a 2000 Silverado 2500 manual trans (NV4500). All you should have to do is confirm gear ratio, change the VIN, then write it.
    Attached Files Attached Files
    Last edited by SiriusC1024; 05-21-2023 at 01:12 PM.

  11. #11
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    You have to license repository files. Also can't write 'your' licensed file to multiple PCMs. Otherwise HPT would be a great tool for churning out cloned, tuned PCMs to sell on Ebay.

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    Wow. I see where I went wrong. I didn't think to check other vehicle models when I searched the repository. I only searched for Yukons. I appreciate the help and I'll let you know how it goes.

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    Quote Originally Posted by blindsquirrel View Post
    You have to license repository files. Also can't write 'your' licensed file to multiple PCMs. Otherwise HPT would be a great tool for churning out cloned, tuned PCMs to sell on Ebay.
    What do you mean? I'm pretty sure that I've written files and changed VIN's multiple times on a project last year. I say pretty sure because I was sure until you mentioned something, and you usually know what you're talking about.

    The license is for the ECU. He can write any tune to that ECU as long as he's paid for the credits. I wasn't saying write to multiple ECU's.

  14. #14
    Tuning Addict blindsquirrel's Avatar
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    You can't write a file you don't have a license for, not even to a ECM/PCM that you've previously licensed. The license is for the file and ECM/PCM serial# combination. Swap out either one and it breaks the license.

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    Ok then open the junkyard file. Run a compare to the file I posted. Change everything that's highlighted green. Check everywhere, not just Engine and Trans.

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    Quote Originally Posted by blindsquirrel View Post
    You can't write a file you don't have a license for, not even to a ECM/PCM that you've previously licensed. The license is for the file and ECM/PCM serial# combination. Swap out either one and it breaks the license.
    I know I should probably know what you're talking about, but I don't completely. I've used credits once, to adjust and write a tune to my brother's jeep and it worked. I also spent credits on the ECM on my daily driver and I'm able to tune/write to it. If I need to buy more credits, that's fine, but is there something else I need to be aware of, or do before writing this tune?

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    Quote Originally Posted by SiriusC1024 View Post
    Ok then open the junkyard file. Run a compare to the file I posted. Change everything that's highlighted green. Check everywhere, not just Engine and Trans.
    Thanks Cirius. I actually know how to do that so depending on what I hear back from blindsquirrel, I might take your route.

  18. #18
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    So I decided to just buy a couple credits and licenced the file. I understand what you were talking about now blindsquirrel. I installed the junkyard ECM and wrote the file to it. I fired it up and realized I forgot to adjust the Base RPM so it was sitting at around 600. It still showed the misfires so I waited until the fuel system loop closed, and incrementally adjusted the timing-absolute, but this time I didn't see the misfire stop. It lessened, but didn't stop Yukon ECM 1GKEK13R3XJ774476 First Run.hpl. I'm still noticing the Advance timing is still in the double digits, but there isn't any Retard, so I'm going to do some research to see if that's normal for that engine, or if I need to make some adjustments based on the rest of the numbers. Thanks for the help and if there's any more experience you all can send my way, it's always appreciated.

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    Looks like it's calling for a lot of fuel. Has anything been done to the engine or is it stock? Read the numbers on the physical injectors and write them here. Need to verify injector data.

  20. #20
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    I'll fire it up this afternoon. I'm assuming there are channels for reading the individual injectors?