D4 stuck idle, then all ok. Hot day. Your thoughts please

[Meagain]

Hi all
Was enjoying a lovely sunny day today, hottest this year ? (28) and no wind, not something usual for uk! Engines (D4-260) ran fine to a cove and have been all week having cruised SW uk coast. When time to come home, both started fine but one wouldn’t go above idle. Gears working, no alarms, dials reading normal. Restarted engine but problem continued, nothing obvious looking into engine bay. Continued on one engine 1000 revs, left faulty one idling as runs steering. Decided to leave engine bay open a bit to keep an eye on things. After about 10 mins, tried to increase revs “to see” engined rev upto 1000, then I increased both upto 1300, then a bit more both acting like there was never an issue……. Any thoughts appreciated.

 

[julians]

My immediate gut feeling is some problem with the throttle potentiometer, rather than an issue with the engine. I reckon you'd get a fault code thrown if the throttle was correctly registering a request for more revs, but the engine couldn't deliver, whereas if the throttle wasn't even registering a demand for more revs then you'd get no error.

But the fact that engine went into and out of gear kind of calls my theory into question,as the same potentiometer is used to determine the gear required.

 

[Irish Rover]

I had a similar issue with a D4 260 a good few years ago. Turned out to be a faulty oil pressure sensor but I don't remember if it was registering a fault code.

 

[Meagain]

Julian’s, a couple more guessed it was the pot, but equally as it went into gear FNR exactly as should so dismissed, another vote was boost sensor stopping fuel demand but the fact it cleared and seems ok now puts this into question. Had the VP centre looking at it today, no historical fault codes when plugged in and no new, all parameters appeared ok. Bit of a left field, but alternator belt a bit loose, squeals at first, wondered if temp voltage drop did something!! So far no repeated fault despite numerous runs, so fingers crossed for Lyme bay crossing at weekend (weather depending).
Thanks for comments