Fuel Trims — What the Computer Tells You

Fuel trims are the most powerful free diagnostic tool you have. They tell you exactly what the engine computer is doing to adjust the fuel mixture in real time. Understanding fuel trims lets you diagnose lean and rich conditions, vacuum leaks, fuel delivery problems, and sensor faults without buying any special equipment — just a scan tool you already have.

Short term fuel trim — STFT — is the real-time correction the PCM is making right now based on oxygen sensor feedback. Long term fuel trim — LTFT — is the learned correction the PCM has stored over time. Positive values mean the PCM is adding fuel — the mixture was running lean. Negative values mean the PCM is removing fuel — the mixture was running rich. Healthy fuel trims are within plus or minus 5 percent under most conditions.

High positive trims at idle that drop to normal at cruise — vacuum leak. The leak is a bigger percentage of total airflow at idle when airflow is low, and a smaller percentage at cruise when airflow is high. High positive trims across all RPM ranges — fuel delivery problem. Weak pump, clogged filter, restricted line. High negative trims — the engine is running rich. Leaking injector, fuel pressure too high, contaminated MAF reading too high. Always record fuel trims at idle AND at 2,500 RPM cruise. The difference between them tells you where in the RPM range the fault lives.