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Thread: Fuel Trim Basics

  1. #1
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    Fuel Trim Basics

    Just curious about fuel trims.

    If they are showing negative, what's going on and what does the engine need more or less of?

    Same question for positive high numbers.

    Where would these changes be made?

    Of course, I'm talking here that it's been verified no exhaust or intake leaks.

    I was running my 5.3 with what I discovered to be a PCM from a 4.8 liter. I changed all settings to 5.3 stock and it's running worse. A bit of stalling and negative SFT's.

    Sorry, at work now, don't have a log file.
    Just curious about basics and where to begin to correct/tune.

  2. #2
    negative fuel trims = pulling fuel
    positive fuel trims = adding fuel

    tune the MAF to get them close to 0

  3. #3
    Tuning Addict 5FDP's Avatar
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    Both the MAF curve and VE need to be re-calibrated if the trims are either way positive or way negative.

    Of course things like checking fuel pressure for 58psi and doing the intake vacuum leaks and exhaust leak check like you did should be done first.

    Using a wideband o2 sensor is the fastest way to go about this. With that sensor, an error graph/chart can be made to log the error between the commanded AFR and the actual AFR. Either the MAF or the primary VE can be done first and both would be done in open loop with no fuel trims and closed loop disabled. The error that shows up in the graph/chart can be copied, then using the special paste by half be applied to the MAF or VE.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

  4. #4
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    Thanks guys.
    Looking at all these numbers and stuff is dizzying.....wow.
    Thank gawd, I can always put it back afterwards. I saved the last tune before changing it.

    NO WB. I called HP Tuners before ordering and was told I only need it for WOT tuning. Apparently not, correct?

    One more thing, I get what appears to be a missed/quick 3-4 shift under moderate acceleration.
    Tune and logs attached.
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  5. #5
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    Mods to the 5.3 are bored .020" over, flat top 4.8 pistons, a 6.0 liter cam and 243 heads.

  6. #6
    Tuning Addict 5FDP's Avatar
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    If the engine is modded, fuel trim tuning is more work and takes longer. You can do it but the wideband is faster and more accurate. Then you'd need it for wide open throttle anyway.

    Even though .020 over isn't that big, still calculate the cylinder volume and change that to match in the calibration.

    Disable COT, that can swing fueling if it activates on you are random.

    Typically engines that have increased flow will be lean all over. The logs show it's lean under some throttle but rich off throttle. Is fuel pressure getting higher than ~58psi at idle or coasting?

    All in all it won't take forever to dial it all in, the setup is nothing crazy. If all the other things are working good and nothing is making you run in circle, getting the fueling under control shouldn't take long.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

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    Thanks...a lot to swallow. Don't even know what COT is, let alone how to disable it. I keep googlin everything posted here.
    So, it was stalling after modding the tune to 5.3 specs. I put it back to the 4.8 specs which were running pretty good, never stalling. Well, it was stalling after putting it back.. WT?

    Then, I get to the cruise and somewhere abouts my canister, is leaking fuel. No DTC's. Tank had pressure after turning off even though the vent solenoid at canister opens on key off.


    I will change the cylinder volume, set everything back to 5.3 settings and find out if the purge solenoid/vac canister has been causing vac leaks/fuel delivery to intake, causing some of my stalling issues, etc.

  8. #8
    Tuning Addict 5FDP's Avatar
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    Cat over temp.

    If you removed the cats and rear o2 sensor, you want that disabled.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

  9. #9
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    Quote Originally Posted by 5FDP View Post
    Cat over temp.

    If you removed the cats and rear o2 sensor, you want that disabled.
    Thanks. Cats in place, rear O2's not.

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    So, I had fuel in the canister, so intake issues. Got that cleaned out, went back to the 5.3 tune and logged this.

    The trims go to red for brief times. I'm assuming this is nothing to worry about as long as they are OK during cruising.
    Correct?
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  11. #11
    Tuning Addict 5FDP's Avatar
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    You still have to go through the airflow model and re-do it all. I tried to explain that in one of my previous posts.

    The work still has to be done to correct some of your issues.

    If the engine is running good and not stalling or doing anything weird, start adjusting the MAF and VE tables to bring the fueling back in line. The cruising trims are still either adding in +20% or removing a decent amount of fuel at times. When you get further into the throttle your long term trims max out at +25%, showing that it's still needing a lot of work to fix the lean condition.

    Should probably add your throttle position channel and injector information so you can watch that too.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

  12. #12
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    Thanks.

    Huge learning curve to go with this stuff.
    Appreciate it.

  13. #13
    I find it important to note you should be disabling the mass air flow sensor when tuning the VE table because the dynamic air calculations bounce between VE and MAF depending on operating conditions.

    I would suggest starting with the VE table and doing MAF last but thats just me.

    Start by disabling the MAF sensor under engine diag -> airflow -> mass air high fail freq = 0.
    This will disable the mass airflow sensor and all dynamic air calculations and run as purely speed density based off the VE table.

    You then log LTFT in the scanner histogram with a layout that matches your VE table so you can copy you LTFT errors and paste into your VE table by right click -> paste special -> multiply by percent half.

    You also want to disable deceleration fuel cut off (DFCO) and anything else that can affect the commanded airfuel ratio in closed loop (as thats what you'll be stuck to tuning in with narrow band O2 sensors. I.e. no WOT)

  14. #14
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    Quote Originally Posted by 3strokeEngine View Post
    I find it important to note you should be disabling the mass air flow sensor when tuning the VE table because the dynamic air calculations bounce between VE and MAF depending on operating conditions.

    I would suggest starting with the VE table and doing MAF last but thats just me.

    Start by disabling the MAF sensor under engine diag -> airflow -> mass air high fail freq = 0.
    This will disable the mass airflow sensor and all dynamic air calculations and run as purely speed density based off the VE table.

    You then log LTFT in the scanner histogram with a layout that matches your VE table so you can copy you LTFT errors and paste into your VE table by right click -> paste special -> multiply by percent half.

    You also want to disable deceleration fuel cut off (DFCO) and anything else that can affect the commanded airfuel ratio in closed loop (as thats what you'll be stuck to tuning in with narrow band O2 sensors. I.e. no WOT)
    Thanks for this.
    I need to research about pretty much everything mentioned here including histograms and errors, pasting....total newb.
    But, I will start this morning.

  15. #15
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    I found the MAF and DFCO.
    For DFCO, there are several settings.
    Change them all to 0?

    Then, flash like this to PCM to do tuning?
    Attached Images Attached Images

  16. #16
    Advanced Tuner Lakegoat's Avatar
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    If you want to turn off DFCO, make the Enable temperature about 350 degrees and that will keep it from cutting fuel.
    Yes, and then write calibration.
    2005 GTO, TrickFlow 225, Magnuson 2300, 4L80, 4 link 8.8
    2013 Silverado 5.3, 6L80

  17. #17
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    Quote Originally Posted by Lakegoat View Post
    If you want to turn off DFCO, make the Enable temperature about 350 degrees and that will keep it from cutting fuel.
    Yes, and then write calibration.
    Thanks.

  18. #18
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    Quote Originally Posted by 123pugsy View Post
    I found the MAF and DFCO.
    For DFCO, there are several settings.
    Change them all to 0?

    Then, flash like this to PCM to do tuning?
    Perhaps I should have said flash the PCM like this so I could then do the logging?
    Then, use that info to do the tuning?

  19. #19
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    OK, I did that and got a log. No idea if I did it 100% correctly.

    Edit, added before log. Big difference.

    What to do with the info?
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    Last edited by 123pugsy; 06-23-2019 at 12:12 PM.

  20. #20
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    OK, I made some adjustments to the VE table twice.
    On the left of the pics is just disabling everything for the logging, the middle is the first adjustment and the right is the second adjustment.

    How close do the numbers need to be?
    Do I use the average as my guideline? Or the plus or the minus?

    Thanks.
    Attached Images Attached Images