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Thread: GM Crate LS3-480 LTFT too high

  1. 07-06-2021 #1
    SPN
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    GM Crate LS3-480 LTFT too high

    I'm an ASE cerified tech but kinda new to tuning. Have customers '55 Chevy with brand new crate LS3-480 with supplied pre-programmed GM E67 ECM (495 adv. HP, 10.7:1 CR, .219/.228x.525/.525 cam-LSA unkonwn, 6spm trans). Engine starts well idles ok cold but quickly transitions into rough idle with extremely hi LTFT. -FYI: I'm using latest update HP Tuners MPV1.- Suspected vacuum leak but none can be found after extensive spray test. Injector size/flow is correct per par numbers. Found both LS1 and LS6 style pcv systems operational but converting to either one gives same results. My LM2 wide band is currently failed but new one on the way. I understood the old histogram form LTFT/STFT error correction but am struggling with new "charts" function to create error correction functionality-even though I've read the stickys and forum posts. Please: Before you go off on making a bunch of snarky remarks, I've been building building street and racing vehicles way before many of you were born. I'm still trying to stay "relevant". PS: I can probably help you too in some areas, if you need it! I'd appreciate your help! Lastly, cthe ar came to me with L&R bank primary O2 sensors but do I have to add another port to attach my WB w/o disconnecting the BN sensor or can I use one of the existing? Don't think so due to failed primary signal to ecm. Thanks guys!
    AS Found-COT disabled.hpl070221 Edit#3.hpt
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  2. 07-06-2021 #2
    5FDP
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    Whats your fuel pressure at the rail though?
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.
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  3. 07-06-2021 #3
    SPN
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    Oh Yeah, I forgot that. It's right at 58psi.
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  4. 07-06-2021 #4
    5FDP
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    Per the scanner or manually checked?

    How complete is the exhaust too?
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.
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  5. 07-06-2021 #5
    SPN
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    Fuel PSI via my OTC direct at rail gauge reading. System has LT headers with 2 1/2" dual exhaust system with X pipe to performance chamber style oval mufflers with tail pipes to rear bumper.
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  6. 07-06-2021 #6
    5FDP
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    You wouldn't think that a GM supplied engine and computer that is shipped as a combo would need that much extra fuel added just to make it run good.

    I'd be kinda leery of just adding 5% or more to the entire MAF airflow freq curve to bring the fuel trims back in line.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.
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  7. 07-06-2021 #7
    SPN
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    Agreed! I've not encountered this before-even with multiple crate engine tunes. Some other forum posts have suggested that this 495HP version of the LS3 has some as delivered ecm issues that the other 2 crate versions (430 and 525HP) do not. Unfortunately the issues were not identified. (Go figure). I did try loading an OEM LS3 430HP MAF airflow freq curve to see if that might get me in the ball park but it was not any better. I had to walk away for the night but will probably try incremental "multiple by" increases tomorrow. I appreciate your responses. Thanks
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  8. 11-01-2021 #8
    SPN
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    Thought I would finish out this old thread in case anyone else encountered a similar i
    issue. I discovered that the installer of the crate engine had attached multiple grounds to a single ground point. Also they installed the MAF too close to the 45* inlet tube. I individually grounded the ECM, separated the remaining grounds and installed a straight CAI tube with properly placed MAF. Instantly the LTFT/STFT were reading +2-4 instead of +15. Easy to tune following that.
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