Under the Hood: Making sense of scan tool readings

McClatchy-Tribune Information Services

March 26, 2018

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?

Wes L.

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).

Fuel trims 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).

Other examples 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.

Given time, your scan tool will likely pay for itself many times over. Enjoy!