Thanks for the responses. First of all, let me apologise for having fumble fingers on the keyboard and pressing a 4 instead of a 2 - the engine is of course an M102 (2.3 litre, 8 valve), not an M104 as per my first post. I've now edited it in case this discussion is of use in the future. The car is a 1985 W124 230E and it's Australian delivered so may not have some of the US pollution control gear.

Thanks for the suggestions - I'm not sure what a WUR so I don't know whether my car has one or not. I've checked & re-checked for vacuum leaks, all to no avail so far, and I'm not sure if a leak would allow the car to run well when cold but poorly once warm.

The coolant sensor switch may be a possibility because all the insulation on the wiring is brittle and I may well have caused it to crack & create a short or poor connection when I moved it all around in taking the head off. I'll get back into the shed on Sunday to investigate this further.

Cheers