Long term fuel trim

This is the control of Injection Pulse Time Open (ti) (also called Injection Pulse Width) over the entire range of engine operation. It is primarily calculated at idle or low load mid range engine operation and is averaged over time. In these idle/low load conditions the amount of fuel variation is small due to the relatively small amount of air input. The computer monitors the O2 sensor and ADDS or SUBTRACTS approximately 0.001msec to the injection pulse time (ti) in order to maintain a Lambda = 1. This amount of increase or decrease of the injection pulse width is known as the Adaptation Value. This is the value output by the ECU when reading the live data stream.

For example: An LTFT of 1 is the factory spec for a new injector's injection pulse width (time open). This corresponds to an LTFT Adaptation Value of 0.0. An LTFT Adaption Value of 0.100 would indicate a wider injection pulse width. This corresponds to a LTFT of 1.100. An LTFT Adaption Value of -0.020 would indicate a narrower injection pulse width. This corresponds to a LTFT of 0.980.

The LTFT is also influenced by the Short Term Fuel Trim (STFT).