Fuel Trims best ever explanation

Secret of Engine Problem Diagnosis- Fuel Trims Pt.1

Secret of Engine Problem Diagnosis- Fuel Trims Pt.2

Notes for myself.

Inputs:
1. Air: MAP|MAF and Temperature
2. Fuel injectors pulse width.

Engine

Ouputs:
1. O2 sensor
                 
14.7 (air) : 1 (fuel) - stoichiometry
rich at start and wide open throttle

02 sensor: 0.1-0.2V (lean) to 0.8-0.9V (rich)
not functional upto ~700 grad

STFT: immediate response
LTFT: baseline
Range -50% to +50%

Case 1. p0171 - bank1 lean
LTFT=25%, STFT=0% - vacuum leak
on idle, vac leak plays significant role. increase rpm, leak insignificant role, so LTFT should be low.
confirm by propane|carb cleaner in intake, so STFT will fall in response to rich.

Case 2. weak fuel pump, p0171 lean condition
FT can be 0% or bit above. Make load on engine as we need more fuel it will showup.
Increase RPM and see STFT, should increase too as more fuel demand and pump is weak.

Case 3. dirty MAF
Construction: heated wire to constant temperture controlled by PCM. PCM increase voltage to heat up more/less to get to set temp. When more airflow voltage vary. Also uses air temp for calculation.
LTFT around 0%.
Will behave same as in weak fuel pump. Need more inputs like fuel rail pressure or MAF readings as should be by specs.

FT +-5% regular, +-10% within spec. More than that - something wrong.

Exhaust leaks can affect O2 sensor.

Case 4. LTFT -30%. Rich condition. Maybe leaking injector.
After replacement LTFT should be updated initially with ST, later with LT.
If LTFT stays the same, problem not fixed, not injector.

Case 5. p017/4, idle, LTb1: 55%, LTb2: 2%, STb1: 55%, STb2: 0%.
Super lean at bank 1, not a fuel pump as bank 2 is ok. Vacuum hole in bank1? :-)

Also from ScannerDanner - Understanding Short Term and Long Term Fuel Trim