When should you reset fuel trims?

[Widebody06]

E40 LS2-
I'm asking under what circumstances do you want to reset trims so you get an accurate result to changes? And conversely when do you want to leave them?
For example, when working on improving idle by changing VE cells, I would think you need to reset every time you make a change and to not take into account log sessions where trims werent reset.
No doubt the answer to my question involves a lot of situations so perhaps responders could tell us about the most common operation that require reset or that you dont want to reset for.

[will15]

I was thinking the same thing, if you do a log then re flash, and go again I would think reset the fuel trims so it does not take old data into account. But maybe when you re flash your learned trims are deleted?.

[MikeOD]

Any time you're tuning VE or MAF, you should force open loop and disable the open loop STFT. After that, flash the file, start the engine and reset the trims.

[will15]

Does that depend on what type of histo your using to tune, because I have used LTFT to tune my ve. My STFT are disabled in the stock tune. wouldn't you want to enable STFT for open loop?.

[Widebody06]

Im now of the opinion it only matters if you want quick answers, if your doing a couple of drive cycles, it retrims in response to the new setting and that as will15 says is good info in itself. Once I have my wideband working, that would probably be a circumstance when you'd want to remove trim influence.

[MikeOD]

Does that depend on what type of histo your using to tune, because I have used LTFT to tune my ve. My STFT are disabled in the stock tune. wouldn't you want to enable STFT for open loop?.

Your open loop STFT were disabled...but in closed loop, a LTFT can't make a change without a STFT making that change first. Tuning VE/MAF with STFT and LTFT takes a long time...it's much faster with the wideband.

Im now of the opinion it only matters if you want quick answers, if your doing a couple of drive cycles, it retrims in response to the new setting and that as will15 says is good info in itself. Once I have my wideband working, that would probably be a circumstance when you'd want to remove trim influence.

The data from the car trimming itself is "good" but you need to be careful with it. If you're going to allow it to just run with trims larger than say +/-2%, your airmass model is wrong. Closed loop will get the fuel right at light load where stoich is an acceptable AFR, but as the airmass model is incorrect, it will handle the spark advance lookup incorrectly.

[Widebody06]

MikeOD, yep I'm seeing as I get my idle air mass dialed in the advance is improving with no change in anything spark related.

[MikeOD]

I've found that I don't need to mess with the advance tables much...if at all, then tuning a car. The biggest timing changes I make are for idle/low rpm situations with bigger cams.