Yes those numbers are almost certainly % + corrections to the base fueling, if You create a situation where you go lean you will see a minus sign show in front of the number (at least on my OBDII scanner and ecu)
It varies a bit by car, but most ecu's (engine management units) have about +/- 25% fuel trim authority. That means that when you get to a LTFT of about 20 -25 you will likely trip a CEL as the ecu is adding 20 -25% more fuel than it normally would run to get to its target fuel air mixture.
On my car LTFT is a lot more dynamic than I expected it to be, and I learned to totally ignore the fuel trims until the engine temp was up to full coolant temp, to avoid having issues with cold start enrichment and warm up enrichment. Also ignore it during acceleration because you are seeing acceleration enrichment which is normal.
I finally got in the habit of watching the LTFT's only when I was on light throttle highway cruise, (or at fully warmed idle conditions learn what is normal for both conditions on your car). Stable highway cruise is a condition I know should stabilize out at a stochiometric fuel air mixture, and is usually maintained long enough for you to get a feel for what the average number is.
LTFT is long term only in the perspective of the ECU it actually appears to change quite rapidly as you drive around, the STFT is almost real time, ie if you were to have a misfire on one cylinder it would see the change in mixture and try to compensate for it.
If you push the mixture too far, I would expect you would see the LTFT would peg at what ever is the maximum capability of your system, likely 25% or so, and then you would see the STFT start climbing. At some point the ecu decides this is not a transient issue but a permanent change in the engines tune and needs attention and that is when it trips the CEL. On my car I found the CEL always tripped on steady state highway cruise conditions, as apparently that is when the ecu trusts the value of the fuel trims the most.
Tuesday, August 26, 2008
"Hotrod" answers my questions about long term fuel trim
Larry, i.e. "Hotrod" over at e85 forums, answers my questions about what I'm seeing on the Scangauge on Long Term Fuel Trim (LTFT):