What is the normal range for short-term fuel trim?
2 Answers
Short-term fuel trim values typically range between -5.2% to -8.6%. How short-term fuel trim works: Short-term fuel trim performs real-time adjustments to fuel injection quantity based on oxygen sensor feedback regarding the air-fuel mixture's richness or leanness from previous working cycles. Through continuous short-term fuel trimming, the mixture concentration is maintained as close as possible to the theoretical air-fuel ratio. Factors affecting fuel trim coefficients: Factors influencing fuel trim coefficients include engine misfires, exhaust restrictions, low fuel pump pressure, or clogged injectors. These areas should be inspected first. If the combined value of long-term and short-term trim exceeds 5%, it indicates excessive deviation. Potential checks include injector clogging, spark plug erosion, fuel quality, air flow meter malfunctions, and overdue fuel filter replacement.
Last time I checked the data of my old car with the onboard computer, I felt relieved when I saw the Short Term Fuel Trim (STFT) fluctuating within ±10%. Simply put, the car's computer is fine-tuning the fuel injection—higher values mean it's adding fuel because there wasn't enough, and lower values mean it's reducing fuel because there's too much. Especially for my car with over 100,000 kilometers on it, it's normal for the values to jump more noticeably in traffic jams. As long as it doesn’t consistently exceed ±25%, there’s no need to panic. If you see it staying above 15% or below -15% for a long time, then you really need to check the oxygen sensor or vacuum lines. This parameter is healthy when it moves; if it stays completely still, that’s when big trouble arises.
