[1999 4Runner, 6-cylinder 5VZ engine. Not supercharged.]
- STFT1: Short Term Fuel Trim, bank 1
- LTFT1: Long Term Fuel Trim, bank 1
While trying to diagnose a P0171, I decided to observe my fuel trims to get a feel for how they relate to what I'm doing, and how they relate to each other.
When idling (w/ AC compressor off,) my LTFT1 sits rock-steady at 0.8, and STFT1 slowly hunts around between -3 and +3.
But while driving at e.g. 40mph, if I accelerate any amount at all, LTFT1 will immediately spike up into the mid-to-high +20s. STFT1 will also increase , but not as high as LTFT1.
I had thought that long-term fuel trim was the average of a bunch of prior instantaneous (short-term) fuel trim values. But it seems now like that can't be right, otherwise it would not change faster than STFT1.
If it is not a running average, what is it?