What Should Be the Standard for Long-Term Fuel Trim?
2 Answers
Long-term fuel trim is a numerical parameter with a value range of -23% to 16%. Long-term fuel trim is derived from short-term fuel trim values and represents long-term fuel supply correction. More details are as follows: 1. If stored in non-permanent memory: Depending on the engine management system, adaptive correction values may be stored in the PCM's non-permanent memory or permanent memory. If stored in non-permanent memory, the memory is erased when the ignition is turned off, and the correction value returns to 0 upon restart. 2. If stored in permanent memory: Even when the ignition is turned off, the memory is not lost, and upon restart, the system returns to the previously memorized correction value. The memory is only erased when the battery is disconnected or the PCM fuse is removed, and it returns to 0.
Long-term fuel trim values typically fluctuate within ±10%, which represents the optimal operating range. When the engine control unit continuously adjusts fuel delivery to maintain air-fuel ratio balance, excessively high values indicate a lean mixture condition, potentially caused by air leaks or a faulty oxygen sensor. Conversely, values that are too low suggest a rich mixture, possibly due to dirty fuel injectors. When driving an older vehicle, I experienced unstable idle and skyrocketing fuel consumption - diagnostic checks revealed values exceeding 15%, prompting immediate replacement of sealing rings. Regular OBD scanner diagnostics, such as annual checks, are crucial to prevent issue escalation. Prolonged neglect may lead to excessive emissions or even engine damage.
