How to Fuel Trim Diagnostics

To maintain the correct air/fuel mixture, the PCM adjusts Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Both of these values can be read on a scan tool that can display OBD II system data, and may be used to diagnose fuel-related problems. Normal range for both STFT and LTFT is typically plus or minus 8. If the numbers are +10 or higher for STFT and LTFT, the engine is running LEAN (not enough fuel). If the values for both STFT and LTFT are both minus 10 or more, the engine is running RICH (too much fuel). Vacuum leaks can create a lean-running engine that requires extra fuel to balance the fuel mixture. If you rev the engine to 1500 to 2000 rpm and hold it for a minute or so, and the STFT value drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. If the STFT value does not change much, the lean fuel condition is more likely a fuel delivery problem (weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum leak.