850 engine failing to idle on de-clutch

[Nick850GLT]

My 850 runs sweetly, despite covering over 200,0000 miles.
Recently it has developed a tendency for the engine to 'drop out' when declutching at a road junction.
The idle is normally fine, it will sit and tick-over without a glitch, but at a junction (often, but by no means always) the revs just drop to zero and it stalls - which is quite a nuisance as power steering obviously fails too.
There are no warning lights or service indications at all - so it is not showing a fault code.

Any suggestions as to the likely cause much appreciated.

[SwedishBus]

Idle control valve sticking. This is the little cylindrical Bosch unit that sits across the throttle body, and effectively bypasses the throttle at idle. When you over-run the engine, the valve is being controlled to a very small oriface, then is sticking there. It will not happen at normal idling, as the control never goes that small.

These valves can be carefully cleaned to free then up. I'm sure that there is a thread here somewhere on that subject!

[byootox]

These rotary valves were very common on a lot of cars back in the late eighties and throughout the nineties, and as said a good spray inside them and shaking them like you were lapping valves can help.
If the fault just goes away for even a little while you know you've found your fault.
Then you can order a new valve accordingly if the fault returns.
Bear in mind sometimes these valves are too far gone to successfully clean up.

[scutyde]

Check also the thick air hose which connects the airbox with the throttle body. Mine was broken and it exibited the same symptoms.