I have been getting the engine warning lights with a P0171 fault code (lean fuel) on an ongoing basis.

The fault only seems to occur at relatively high speed and revs (last fault at 50mph and 1955 rpm). After a check and given when the fault is registered I have ruled out a vacuum or exhaust leak. There is no link to when petrol is put in (before anyone says its the filler cap) and the engine does not run erratically at idle. I have already replaced the Mass Airflow Sensor so that is not the problem. The absolute manifold pressure at the time of the last fault was 19.8 inHg, which to my reading is what the manual says it should be.

I am thinking its a fuel supply issue, either filter blocked or pump not working properly. The filter, which sits on the pump, is a 60 000 mile replacement item according to the workshop manual but the local agents tell me its not on their service schedule at all. Lifetime filter was the term used, the last one ordered was nearly a year ago.

Any advice as to what could be the fault and if I am on the right track.

Thanks in advance

If fuelling I would expect more symptoms at wider throttle range and even higher rpm when fuelling really being pushed.

I would be looking at oxygen sensor on that bank, ideally want graph the sensors and monitor the fuel trims and see how the oxygen sensor behaves

I would probably plum in a fuel pressure gauge just to see pressure in spec and regular while monitoring sensor data .

Not unusual see oxygen sensor issue or wiring issue, lots of other things can be so don't guess and throw parts at it, take time and test .

I forgot to mention in my first post that you can use the car almost endlessly in town driving without the fault coming up. Even at faster speeds, it takes about an hour of highway driving before the fault registers. Why would an O2 sensor only play up under those circumstances? My experience with them (which is limited) was more low rev problems than high (bad idle etc). I will monitor the fuel trims under live load and see if that shows anything.

Anyone else have suggestions?