Q: I took your
advice and bought an inexpensive scan tool. Itís been fun trying
it out on several family cars, and Iím almost eagerly awaiting a
problem so I can try to fix it! Iím curious about the "STFT"
and "LTFT" percentage numbers ó and what they represent.
Iíve also noticed one car shows a reading for "MAP" and
the other shows it as "MAF". Whatís the difference?
A: Cool stuff!
STFT and LTFT are abbreviations for short-term fuel trim and
long-term fuel trim. MAP and MAF are short for manifold absolute
pressure and mass air flow (sensors).
provide an informative view of fuel/air delivery system happiness,
reflecting the level of correction needed beyond the original fuel
delivery mapping when abnormal conditions occur, to make the exhaust
oxygen/air-fuel sensor happy once again (normal readings).
Short-term fuel trim is a direct response to oxygen/air-fuel sensor
readings and is typically expressed as either a positive (rich
correction ó adding fuel) percentage or a negative percentage
(lean correction ó subtracting fuel). The range of correction
typically tops out at perhaps 25 percent to 30 percent positive or
negative. Readings within plus or minus 5 percent of zero are
considered normal. Multi-bank engines (V-6, other) may have trims
for each bank of cylinders (STFT-1, STFT-2) as each cylinder bank
has its own oxygen/air-fuel sensor. Long-term fuel trim(s) are a
response to a continuous and presumed permanent short-term
correction, allowing short-term trim to return back to a
quick-on-its-feet neutral value. There will be a differing trim
value for every speed/load, so itís best to observe fuel trims
during a variety of driving conditions.
Letís say a
vehicle has a dirty/moderately clogged fuel filter. It may run OK at
low speed/low load as fuel flow through the injectors is minimal,
but it may lose power at high speed/load when flow is high. As the
engine struggles at high speed/load, the exhaust oxygen/air-fuel
sensor(s) will indicate a lean mixture (not enough fuel) and the PCM
(powertrain control module) will call for a short-term correction, a
positive number perhaps in double digits to fix it. If the fault has
been learned via repeated episodes and isnít severe, it might show
up mostly in the long-term trim(s). If the filter was really clogged
and the long-term trim is topped-out, short-term trim will also help
with the fault (both are positive percentages in this case).
include negative percentage trim(s) (fuel subtracted) due to a dirty
air filter during high airflow conditions, or perhaps positive
trim(s) only at idle, due to a manifold vacuum leak. On vehicles
equipped with a MAF sensor, improperly measured air, due to a
leaking hose connection or dirty sensor would result in a positive
fuel trim correction due to the skewed original fuel mapping. MAF
sensors are typically a cartridge in the large/black intake duct
between the air filter and engine throttle and measure the
mass/quantity of air entering the engine. Smaller, lower-tech
engines may use a MAP sensor instead, which measures engine vacuum,
an excellent load indicator. The PCM crunches the MAP reading, along
with other sensor values to infer intake airflow.
your scan tool will likely pay for itself many times over. Enjoy!