LTFT high except at idle and WOT ???

[Andrewjeffrey]

Further to my post above, Long Term Fuel Trim +STFT readings must be under 10% to be ok - the closer to zero the better. LTFT readings must not increase on acceleration. Finally, Fuel Trim readings are read first at Idle, then at 2500 RPM then at WOT before the whole picture is obtained.

 

[19963.4lsr5]

From previous experience and from Youtube, I have learnt that this problem is due to the IAC Valve being defective. It is NOT normal for the fuel trim readings to increase on acceleration.

At some point between 3/4 and WOT the engine goes open loop. Hence no fuel trim change. Below that and in closed loop a rising LTFT above what is normal range (which means you need to watch it a lot) to me can indicate a restricted or reduced fuel supply causing the fuel trim to scue the fuel mapping richer to compensate and make the O2 sensor happy.


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[Andrewjeffrey]

At some point between 3/4 and WOT the engine goes open loop. Hence no fuel trim change. Below that and in closed loop a rising LTFT above what is normal range (which means you need to watch it a lot) to me can indicate a restricted or reduced fuel supply causing the fuel trim to scue the fuel mapping richer to compensate and make the O2 sensor happy.


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To some extent I agree with you. Fuel restrictions can cause that. But if your vehicle is running fine (engine not sputtering or there is a loss of power) - no driveability issues, then it is more than likely that you have a problem with your IAC valve. In my case, I didn't know this until the IAC eventually died and started to blow my EFI fuse. Then, after changing my IAC valve, voila! The accelerating fuel trims on pedal depression stopped.

 

[19963.4lsr5]

To some extent I agree with you. Fuel restrictions can cause that. But if your vehicle is running fine (engine not sputtering or there is a loss of power) - no driveability issues, then it is more than likely that you have a problem with your IAC valve. In my case, I didn't know this until the IAC eventually died and started to blow my EFI fuse. Then, after changing my IAC valve, voila! The accelerating fuel trims on pedal depression stopped.

The idle air control valve moves such a small amount of air compared to how much the engine consumes on a whole. How can it so drastically change fuel trims? Plus it be idling way high......


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