I've just bought a 2001 Volvo S40 D with a bit of a problem. I believe that the diesel engines in the "phase 2" (2000-2004) S40 were no longer Renault-derived. If anyone else thinks differently, let me know.

Right here's the problem: It stalls.

Sitting in neutral it holds an idle speed of roughly 550rpm, a little low for my liking but it's smooth and not struggling. Then after a couple of minutes, randomly, it'll just stall with no warning.

Also sometimes when slowing down (approaching a roundabout for example), you notice a loss of power so you increase the throttle and dip the clutch. At this point the revs drop to idle, and you can feel it's about to stall for a couple of seconds - during which it completely ignores any input on the throttle pedal. Then it stalls.

It starts instantly from cold, always restarts after stalling without a hint of hesitation, and will continue to run until you slow down or stop.

The only other thing I noticed is that after you restart it, giving it some revs it will climb to around 1500rpm, then "hesitate" for half a second before continuing to rise smoothly. After that first instance it will easily rise from neutral without this hesitation.

My guesses:
Fuel Filter is blocked. I wish, would be nice and cheap! But unlikely as I don't see why it would ignore my throttle input for a couple of seconds before stalling

Fuel Pump is dying. Again, why would it ignore my throttle input, and it would probably struggle to re-start?

MAF. I did have one fail on my S60, although that only caused reduced performance and the occasional limp-home mode. No stalling.

Idle Air Control Valve. My car has covered 138k miles so it could be rather mucky. But I would have expected this to cause a lumpy idle rather than stalling with throttle applied.

Electronic Throttle Module. This is what I initially thought was most likely. Is it true that the S40/V40 was never affected by this common Volvo problem?

ECU/CEM or some other computer.

Anyone got any ideas/suggestions?