Fuel trim is a term used to describe the percentage of correction to injection duration based on oxygen feedback. There are two different fuel trim values which affect final injection duration; long fuel trim (Long FT) and short fuel trim (Short FT).
Long FT is part of the basic injection duration calculation. It is determined by how closely the fuel system achieves the design air/fuel ratio.
Long FT is a learned value which gradually changes in response to factors beyond the control of system design. For example, fuel oxygen content, engine wear, air leaks, variations in fuel pressure, and so forth.
Short FT is an addition to (or subtraction from) basic injection duration. Oxygen sensor information tells the ECM how close it comes to design air/ fuel ratio and the Short FT corrects for any deviation from this value.
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