Originally Posted by Lurkin
IMHO, I wouldn't worry about it as long as it stays consistent, but also this is where the ST trims come into play. Adding the 2 together gets you your actual fuel trims. Also check your open loop/WOT AFRs to ensure you aren't running too lean with that SC.
There is a lot more detail on how that works. This is how the LTFT and STFT work:
When the engine is running in closed loop the computer makes fast, large changes in the injector pulse width based on your short term fuel trim. This is used for changes in your driving based on instant responses such as braking, accelerating, turning on the AC, etc.
Whenever the STFT goes either positive or negative more than about 10 points for a specific amount of time, the computer then changes the LTFT number by one point in the same direction, and locks that new injector pulse width as the baseline. Then it uses STFT numbers to try to balance out the instant changes again. If the STFT again stays too far the LTFT will again increase (or decrease) to compensate.
LTFT is the baseline adapting of change to the factory injector pulse width. Things that can affect this are dirty/clogged injectors, fuel composition, airflow restriction (A dirty air filter
can and will affect your fuel trim number) etc.
STFT is the fast adaptation of change to the baseline injector pulse width. Things that affect that are: snapping open of throttle, AC compressor engaging/disengaging, alternator demand, etc.
When trying to diagnose fuel economy and other fuel related issues, including sensor codes, the primary number to look at is LTFT. If it is a high positive number (anything above 8) then you could be looking at a weak fuel pump, or clogged fuel filter or injectors. If it is a high negative number (anything above -8) then you most likely are looking at a dirty air filter, bad fuel pressure regulator, or bad EGR. LTFT Numbers in the extreme, like +25 or -25 are for gross errors: Vacuum line disconnected, injector stuck open, etc.