With regards fuel trim, I've just read an interesting article in relation to this.
It would seem that the long term fuel trim (ltft) values adapt to what the sensors are doing, engine wear, fuel pressure changes, injector wear and so on.
If a sensor begin to operate at the outer edges of its parameters or it begins to fail, the ltft values shift to accomodate.
What they don't do is automatically return to base line values even though the errant component is replaced.
That needs a reset. Some vehicles such as BMW's need a hard reset using software, some including some Volvo's need a soft reset, ie disconnect the battery and discharge the system by say operating the brake pedal and leaving it disconnected long enough.
Upon reconnection and start up a drive out to allow the ltft values to adapt to the new hardware, ie replacement MAF, MAP sensor etc...
I know that is way out there in the scheme of things and I'm ready to be shot down in flames but it's worth a try and costs nothing.
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