TrimCase
Reference

Fuel trim fault patterns: the fingerprint of every fault

A fingerprint is the short list of cues that separates one fault from every other. The same list grades the drills, so what is written here is exactly what you are marked against. A test proves that at the clean tier every fingerprint matches its own fault and no other on 500 seeded cases.

Unmetered air

Air or vapour that reaches the cylinders without passing the MAF: a vacuum leak, or a purge valve stuck open.

Vacuum leakTrims positive at idle, near zero at cruise. MAF short of the estimate at idle, fine at WOT.EVAP purge valve stuck openTrims negative at idle, near zero at cruise. MAF short of the estimate at idle, fine at WOT.

Air metering

The MAF reading low or high, and an exhaust restriction that caps how much air the engine can breathe.

MAF under reportingTrims grow more positive as airflow rises. MAF short of the estimate, and more so as flow rises.MAF over reportingTrims negative by about the same amount in every cell. MAF above the estimate everywhere.Restricted exhaustTrims close to zero in every closed loop cell. MAF short of the estimate at WOT only.

Fuel supply

Fuel pressure too low under load, too high everywhere, and an injector that drips when it should be shut.

Restricted fuel filter or weak pumpTrims grow more positive as airflow rises. Upstream sensor reads lean at WOT. MAF agrees with the speed density estimate everywhere.Fuel pressure too high (regulator stuck closed)Trims negative by about the same amount in every cell. MAF agrees with the speed density estimate everywhere.Leaking injectorTrims negative at idle, near zero at cruise. MAF agrees with the speed density estimate everywhere.

Exhaust and sensors

Things that make the upstream sensor lie: an exhaust leak ahead of it, a biased sensor, a misfiring cylinder.

Exhaust leak upstream of the upstream sensorTrims positive at idle, near zero at cruise. MAF agrees with the speed density estimate everywhere.Upstream O2 or A/F sensor biased leanTrims positive by about the same amount in every cell. Downstream sensor sits rich in closed loop.Single cylinder misfireMisfire counts on one cylinder. Downstream sensor sits rich in closed loop.

Coolant temperature

A coolant sensor stuck at a cold reading, and a thermostat stuck open that never lets the engine warm fully.

ECT sensor stuck coldECT frozen at one low value. Trims negative by about the same amount in every cell.Thermostat stuck openECT below regulating temperature, and falls at cruise.

The control

No fault (healthy control)The healthy control.