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Understanding long term fuel trim

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This is probably obvious to everyone except me but what is the explanation for a more or less continuous long term fuel trim of about - 7%.

I am using an OBDII scanner to extract tuning values from a 2009 SOHC NA Subaru Forester (OEM ECU) for use in a similar engine in a non-auto application (Motec M130 ECU) but am a bit perplexed about the more or less permanent long term trim value in the car.

If anyone could comment on the likely reason or point me in the direction of the resource which deals with this subject I would be grateful.

Hopefully it will not affect the lambda and timing values which I am transposing.

Thanks in advance.

Rupert Clarke

Production tolerances in the sensors and degradation.

Thanks Raymond.