What are the standards for long-term and short-term fuel trim?
2 Answers
Short-term fuel trim is a real-time correction control of the fuel injection quantity based on the oxygen sensor's feedback regarding the air-fuel mixture richness or leanness in the previous working cycles; long-term fuel trim is an adaptive learning process performed by the electronic control unit based on the engine's long-term operating conditions. 1. Short-term fuel trim: Short-term fuel trim is an immediate strategy formulated by the electronic control unit based on the oxygen sensor's timely detection of exhaust gases. This correction is temporary, and its value changes instantly with the variation in oxygen content in the exhaust gases. 2. Long-term fuel trim: The change in the long-term fuel trim coefficient is a qualitative transformation formed by the electronic control unit based on the quantitative accumulation of consistently correct feedback results from short-term fuel trim. The establishment of long-term fuel trim aims to maintain all short-term fuel trim values within specific parameter ranges. The long-term fuel trim value is stored in the computer's memory for long-term use.
I've been repairing cars for over a decade and often use diagnostic tools to check fuel trim values. Short-term fuel trim (STFT) is the computer's real-time adjustment of fuel injection while driving, like adding more fuel when you stomp on the accelerator. Long-term fuel trim (LTFT) represents the computer's learned average adjustment over time, such as adapting to different gas station fuel qualities. The normal range for both is ±10%. At idle, STFT may fluctuate up and down, but as long as LTFT stays stable around 0%, there's no issue. If LTFT exceeds ±20%, say reaching -25%, it indicates the air-fuel mixture is too rich, and you should check if the oxygen sensor is clogged with carbon deposits. If it's above +25%, there might be a vacuum leak in the intake system. Analyzing these two values together can uncover many hidden problems, helping car owners save fuel and avoid major repairs.
