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Thread: Flex Fuel - can you turn it off?

  1. #1

    Flex Fuel - can you turn it off?

    I have a 2009 GMC Sierra w/ a 5.3L V8, stock as a rock. Can I disable the flex fuel? When I hit the gas suddenly it lags. My previous GMC 5.3L (99 model) had the same engine, but had much better throttle response even though it was basically the same engine. I have a feeling its the flex fuel. Any one experience this? Can I disable the flex fuel?

  2. #2
    Advanced Tuner
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    Dec 2007
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    There is a FF master disable under fuel/general. Though if you have HPT you would be better off logging it and establishing where/why the lag is occuring. Generally the FF calibration adds timing over the base timing table depending on % ethanol.

    Dont know though if the 09's have the virtual sensing or the hard fuel composition sensor. '08's were virtual sensing.

  3. #3
    Senior Tuner
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    I tend to believe it's not the flex fuel system causing this. Why don't you log Commanded AFR and see what it's showing your stoich value to be.
    Formerly known as RWTD

    Toys: '22 Tesla Model S Plaid / '20 Chevy Duramax / ?20 Sea-Doo RXT-X (2)

  4. #4
    Tuner
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    I had the same thing and it was not FlexFuel causing it. Log the different Advance and Knock pids. Look for negative timing, most likely coming from low RPM and high load in the HO Advance table and burst knock. I changed all the negatives to 0 and it is much better, yet still has no knock.
    07 Avalanche Exhaust & CAI

  5. #5
    It's not the flex fuel. I've worked on many of those and got rid of that lag. Both my Tahoe and my Silverado have the flex fuel also, no lag here. The 07' and newer body style trucks just have a terrible tune in them for performance. They drive great, just lack performance in the tune.

  6. #6
    Thanks guys. I posted this and then forgot about it.

    But I will take a look at the timing instead.

  7. #7
    Potential Tuner
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    Did you ever get it figured out? I have a 2004 gmc yukon with the flex fuel engine was wondering how to do this my millage sucks I'm using regular fuel not e85 I'm just wondering how the hell do we turn it off?

  8. #8
    Tuning Addict blindsquirrel's Avatar
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    Quote Originally Posted by bmpalevich View Post
    Did you ever get it figured out? I have a 2004 gmc yukon with the flex fuel engine was wondering how to do this my millage sucks I'm using regular fuel not e85 I'm just wondering how the hell do we turn it off?
    screenshot.12-01-2021 21.56.17.png

    Also, '04 Yukon is a Gen3 (P59 computer), this is the Gen4 section for E38s, E40s, E67s...