Hi all, any help with this one greatly appreciated.
Symptoms:
Occasional bouts of misfiring, typically when throttle is just open at 30 to 40mph.
Engine idling normally at 600rpm, revs drop to 250 then back to 600. Repeats at random every few seconds. Sometimes engine will stop.
If this happens no faults logged at the time BUT restricted performance may appear at the next start (weird) with P0088 High Pressure Rail Too High fault, clears every time with Gap IID.
Never does this when cold, only when hot.
It may run trouble free for 2 to 3 weeks then gremlin returns.
Engine runs fine under high load so I don’t think it is a fuel blockage.
Works done:
Crankshaft position sensor changed
Turbos changed
Electrical connections in engine bay checked
High pressure rail wiring mod done
High pressure rail sensor changed
High pressure rail control valve changed
All filters including fuel filter changed twice.
I’m in Tavistock Devon this week and Oxford next week, I’d be happy for someone to take a fresh look (so far three good indies have not tracked it down due to its intermittent appearance). Home is Beds/Northants border, any recommendations for someone knowledgable with these engines?
2011 Overfinch Autobiography TDV8 4.4 with 210k. Love it and it’s the only problem it has.

Notwithstanding that the crankshaft position sensor has been changed, when the idle issue returns monitor RPM with a diagnostic tool looking for spates of zero RPM. If seen then either another dud sensor or a sensor wiring issue. Otherwise I suspect the HPFP but that's too expensive to replace on a guess.

Thank you Graeme. Do you know what it is with the HPFP that would cause the rail to go high pressure? I recall reading of someone else who was getting the same high pressure fault and changing tbe HPFP solved the problem (I can’t say I understand why, I thought a failing HPFP would cause low pressure).

I don't know the ins and out of the HPFP but the pump would have to reduce its output when the pressure gets to its high limit, but perhaps it can't because of a mechanical issue. It would be good to check the actual pressure if the operating range is known but that would require appropriate fittings and pressure gauge. However I can't imagine why excessive pressure would cause loss of idle revs.