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It's often suggested to drive 50, 100 or whatever miles for the PCM to learn the LTFTs after a re-tune.
Is it necessary for this to be a single session or, for instance, would (5) 20 mile drives = (1) 100 mile drive?
In terms of the histogram data collection, I assume values collected are based on the learnings by the PCM (since last reset) and not just from the current driving session?
Hopefully my questions make sense.
Thanks,
Scott
From what I understand long term fuel trims are saved short term not so much. So, the data you are seeing is from the last drive session ish. If you are seeing huge numbers in ltft something may be off. My truck is mostly stock I dont see big numbers ushally under double digits.
It varies from vehicle to vehicle. You would be better off to ask in the section for your particular vehicle.
Not sure if this will help since my car may work different, but on mine the LTFT are what they are in a few seconds. I can make drastic changes, or add E85, doesn't matter. Once the O2's kick on it'll adjust and that's the end of it.
There is no zeroing out over time or anything else. If it's off my 20 then it's off by 20 forever. Of course it varies all the time so when I say 20 I mean on avg. And that 20 avg can be shared or taken completely by ST, also randomly. It's simply too difficult to keep track of other than an average. It will never ever read 0 and stay there, ever. It hits 0 only as it passes from a pos to neg or vice versa.
STFT is, imo, just an extension of LT. The only point I see to LT is, at least in my car, is LT is saved for the next cold start. So if I'm off my 20, but LT is 5 and ST is 15, then 5 is what I get when I start because ST is not retained and it's 0 until the O2's kick on. So it runs lean, then the O2's kick in and say Oh crap, here's another 15, sorry buddy.
Another difference is my ST can only go to 25, then it'll fault out. I've had LT past 70 and it doesn't care.
Another is LT freezes at a certain load, or maybe it was RPM? But it freezes when you hit that point and from then on only ST works. Not a big deal unless I hit that 25 mark. I've come close because sometimes the LT is negative and the ST is pos to compensate, because as mentioned they're all over the place. So if ST is 15 when LT freezes, I got 10 left.