My volvo estate 2002 petrol v40 is working again !
Many thanks to Pierremcalpine and Bobthecabbage for your support (and anyone who thought about this!).
So here’s the very simple and free fix :
I turned my AC on 6 (full) and full heat to test if the engine would still surge (I expected this not to be the case, as I thought the problem was probably in the clutch or compressor as had been logically suggested). I was surprised when the engine surged and noticed that the compressor clutch was being engaged – I thought this could only occur when the compressor was needed to produce cold air !
I turned the engine off and repeated the procedure. This time the engine didn’t surge.
I then turned off the engine and on again, this time with the AC on 6 and Full cold.
There was no more engine revs surging ! I then tried to drive which had previously led to surging (as well as when idle) and there was no abnormal surging (hunting). The drive was as smooth as before… problem solved!
Cause? I don’t know, but it looks like the ECM might have been in a « stuck » state and simply turning on full heat then back to full cold (possibly in conjunction with turning the engine on and off) reset a stuck ECM. Please let me know if you have a more precise or different idea what caused this…I’d love to hear !
So before re-shimming, or removing an old AC clutch (my clutch gap is 0.63mm/ 0.025 imperial and still seems to work…), or replacing a compressor – try this simple fix first !
...me? I'm off to sit in a lovely cool car.
Mickey74