Short Term Fuel Trim Bank X:
Range: -100 to +99%
Bank: 1 or 2
Fuel Trims are correction factors applied to the base fuel map. When in closed loop mode, the O2 Sensor is used to adjust the trims based on the sensor's values. A positive trim indicates the ECU is adding fuel (a lean condition), and a negative trim indicates the ECU is taking away fuel (a rich condition) from the base fuel map. Fuel trims are not used in open loop mode.
Short Term fuel trims differ from Long Term trims in that Short Term trims are not stored between starts and they change more frequently. If there is a trend in a short term trim, eventually the long term trim will adjust to move the short term trim back to zero. This allows the ECU to adjust it's fuel map to compensate for variances in sensors or dirty injectors, etc.
The Bank indicates a group of cylinders. Inline motors typically have 1 bank. V motors have two banks. Typically there is an O2 sensor for each Bank and that is the sensor the trims use to adjust themselves. Bank 1 always contains cylinder number 1.
Trim vaules are useful for part throttle tuning of S-AFC (or similar) fuel computers.
Continued from Long Term Fuel Trim...
Now view your STFT's over the next few days keeping them at +/-0% by fine tuning with the SAFC. Be sure to note any changes to the LTFT and get the value back to 0.
That's a good rough tune to get you in the right direction for more advanced tuning (which I am not good at so please don't email asking for help...you'll just be disappointed).
From: Bill S
Car: '00 Toyota MR2 SPYDER
If the sum of STFT + LTFT exceeds plus/minus 38 then a system too rich/lean DTC is stored (PO17X).