How Long FT Works

Long FT is a data parameter on the OBD and OBD-II data streams. It is a more permanent correction to fuel delivery because it is part of the basic injection duration calculation. Long FT changes slowly, in response to Short FT. Its normal range is ± 20%, positive values indicating rich correction and negative values indicating lean correction.

If Short FT deviates significantly from ± 10% for too long, the Long FT shifts, changing the basic injection duration. This shift in basic injection duration should bring Short FT back to the ± 10% range.

Unlike Short FT which effects injection duration calculation in closed loop only, the Long FT correction factor effects the basic injection duration calculation in open and closed loop. Because Long FT is stored in a nonvolatile RAM (NVRAM) and is not erased when the ignition is switched off, the fuel system is able to correct for variances in engine and fuel conditions even during warm-up and wide open throttle conditions.

On OBD data streams, Long FT is displayed as Target A/F. On non data stream equipped engines, Long FT is referred to as Learned Voltage Feedback (LVF) and can be accessed from the check connector VF1 terminal.

To gain a better understanding of Long and Short fuel trim, use the example given below. Referring to the graphic on the opposite page:

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