What is the Appropriate Range for Short-Term Fuel Trim?
2 Answers
Fuel trim is a numerical parameter with a range of -23% to 16%. Long-term fuel trim is derived from short-term fuel trim values and represents long-term fuel supply correction. A value of 0% indicates that no compensation is needed to maintain the PCM-commanded air-fuel ratio. If the value is significantly below 0% (a negative value), it indicates that the system is running rich, and fuel supply should be reduced (by decreasing injector pulse width). Below is an introduction to fuel trim: Short-term fuel trim is a real-time correction control for fuel injection based on the oxygen sensor's feedback regarding the mixture's richness or leanness in previous working cycles. If the oxygen sensor signal to the ECU indicates excessive oxygen in the exhaust (a lean mixture), the ECU multiplies the calculated fuel injection amount by a positive short-term fuel trim coefficient to increase fuel injection. Conversely, if the signal indicates a rich mixture, it multiplies by a negative short-term fuel trim coefficient to reduce fuel injection until the oxygen sensor signal reverses, at which point the sign of the short-term fuel trim coefficient also reverses. Through continuous short-term fuel trim adjustments, the mixture concentration is maintained as close as possible to the theoretical air-fuel ratio.
I remember driving for decades and often using that little OBD scanner to monitor engine data. Short-term fuel trim is best kept within the range of -10% to +10%, with values close to 0% being ideal, indicating the fuel mixture is spot on. A significantly negative value means the ECU is reducing fuel, possibly due to an air leak; a high positive value means it's adding fuel, which could signal issues with sensors or fuel injectors. Once, my old car's reading shot up to -15%, and upon inspection, it turned out to be a cracked vacuum hose. After fixing it, the values stabilized around ±5%. Don't take it lightly—long-term high values can burn more fuel and damage the engine's lifespan. I recommend car enthusiasts get an inexpensive scanner to check the data regularly and also look at long-term fuel trim trends. This habit can prevent many issues and save on repair costs.
