What is the long-term fuel trim value?
2 Answers
Long-term fuel trim values should normally be within the range of -23% to 16%. Here are the relevant details: Long-term fuel trim and short-term trim: Actual fuel injection duration = Basic injection time × Injection correction coefficient + Invalid injection duration. Short-term fuel trim refers to the injection correction coefficient, which is influenced by various components such as coolant temperature and intake air temperature. Long-term fuel trim is the correction of the basic injection time. The engine ECU controls long-term fuel trim by learning from short-term fuel trim values. Once long-term trim changes, the engine will use this injection time for fuel delivery. The meaning of a long-term fuel trim value of 0%: Long-term 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 the fuel supply does not require any compensation to maintain the PCM command.
Long-Term Fuel Trim is essentially the ECU's covert log of fuel injection adjustments, much like how I habitually glance at the dashboard while driving. This percentage value reveals whether the engine has been burning more or less fuel recently – positive values indicate the ECU is stealthily adding extra fuel, while negative values signify fuel economy. The normal range stays within ±10%; exceeding this warrants attention. Once when my car had unstable idling, I discovered the trim value had spiked to +20% – turned out to be a vacuum leak causing inaccurate air intake. Significant deviations from standard values most noticeably cause sudden fuel consumption spikes and pungent exhaust odors, with severe cases triggering warning lights. I recommend checking this data with an OBD scanner every six months, making it as routine as monitoring tire pressure.
