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Thread: 99 GM 4.8L basic tuning

  1. #1
    Tuner in Training
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    99 GM 4.8L basic tuning

    Hello all, I have a 99 GMC 4.8L 5M truck that I have disabled the EGR, plugged the manifold and also bumped the idle rpm up a bit, removed the low OEM speed limiter and disabled rear 02s. I am getting abysmal mpg now with the egr disabled. Is there any way someone from here can get me dialed in? I have a data log of a 6 mile driving loop, cruising, acceleration and WOT all mixed in. Any help and guidance much appreciated!

  2. #2
    Tuning Addict 5FDP's Avatar
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    Post the tune and logs.

    Just removing the EGR shouldn't tank the fuel mileage. The 2001+ trucks don't even run EGR's.
    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.

  3. #3
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    I have them here, the OG stock tune, slightly tweaked one and the datalog.
    Attached Files Attached Files
    Last edited by FieSTafury; 07-26-2021 at 07:05 PM.

  4. #4
    Tuning Addict 5FDP's Avatar
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    The EGR does add several degrees of timing while at cruising speeds, I would try to add that timing back in to see if it helps.

    You could even pull up a 2001-2002 4.8 timing table to see how it differs in timing for area's at cruising rpm/speed. Maybe even just copy that over and see if that helps your fuel mileage.
    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.

  5. #5
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    I thought about doing that, I know the newer ecu is a faster unit and didn't know if the same map was compatible or not. What area should I look/compare to?

  6. #6
    Tuning Addict 5FDP's Avatar
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    1999-2002 are the same computer. Each year just has a different operating system number with newer calibration stuff in it. 2001-2002 being the newest because of the changes made after the 2000 model year trucks.

    Look at your stock tune, there is a EGR spark adder table. That is where the EGR is active and adding timing. Then look at your stock high octane timing compared to a newer 2001-2002 timing table to see the difference. You'll probably see that they added timing in many spots at cruising rpm.
    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.