Rough idling

[JW01]

I have a 1990 MkIII 2.0CDi.
Recently it has started to exhibit rough idling when I switch on which usually goes away when it's on the road. It's been seen by three garages and they can find no fault (it had a new air idler control valve fitted in December when it was seviced).
They have checked for air leaks (none), removed and blocked off pipes, checked around the air intake system, removed the spark plugs, checked compressions, carried out a smoke test and checked MAF sensor and coolant sensor readings without finding any faults.
I am concerned because it could develop into something more serious.
Does anybody have any ideas?
Many thanks.

[Robsey]

2.0CDi...so a 2 litre 8 valve unit.
On the whole good reliable units.

Problems with a rough idle.
If it is a 20NE, it could be something as daft as the wrong octane coding plug.
A 20NE should have the yellow plug, as this raises the idling rpm by 100 rpm.
Normal idling rpm is 800 +/- 80 rpm.
Many NE units were fitted with black coding plugs, causing a lumpy idle at just over 600 rpm.

If the idling smooths out when the car warms up, it sounds almost like a mis-fire caused by damp. I presume the garages checked for a clean and dry distributor cap and rotor arm. And what about corrosion on the ends of the HT leads.?

Another potential item not yet mentioned is the engine ECU in the driver's kick panel.
Or more likely, the connector to it.
Again thinking of moisture or corrosion on the connector pins.

[JW01]

Thanks for your suggestions. I'll follow them up.

[James McGrath]

Just to add to the ECU bit that Rob mentioned.
Have a look at the fuel pump relay while you are there, it's right next to the ECU.
I had some rough idling on my Diplomat which seems to have been cured by changing the Fuel pump relay.