Long-Term Fuel Trim (LTFT)
Long-term fuel trim refers to adjustments made by the ECU based on long-term data collected over a period of time. It adjusts the fuel delivery to compensate for gradual changes in engine performance and fuel quality. The ECU monitors various sensors, including the oxygen sensor, to detect if the engine is running rich (too much fuel) or lean (too much air). Based on this data, the ECU makes gradual and permanent adjustments to the fuel injection pulse width to achieve the desired air-fuel ratio.
Short-Term Fuel Trim (STFT)
Short-term fuel trim refers to adjustments made by the ECU in real time based on immediate feedback from the oxygen sensor. It provides rapid corrections to the fuel delivery to account for short-term fluctuations in engine conditions, such as changes in temperature, humidity, or load. The ECU constantly monitors the oxygen sensor output and adjusts the fuel injector pulse width to maintain a close-to-stoichiometric air-fuel mixture.
How Long-Term and Short-Term Fuel Trims Work Together
Long-term and short-term fuel trims work in conjunction to maintain optimal engine performance and efficiency. Long-term fuel trim establishes a baseline for fuel delivery, while short-term fuel trim makes quick adjustments to ensure the engine operates at the desired air-fuel ratio under varying conditions.
The ECU continually evaluates both fuel trim values to make adjustments as needed. When the engine is consistently running rich or lean, the long-term fuel trim will be adjusted to correct for these deviations. The short-term fuel trim then fine-tunes the fuel delivery based on immediate sensor feedback.
By monitoring and adjusting fuel trims, the engine ECU can ensure that the engine operates efficiently, reduces emissions, and provides optimal performance across different operating conditions.
