190E Slow idle when cold

[13022]

My elderly 190E is only idling at about 800rpm from cold. THis means that it nearly cuts out whenever you take your foot off the throttle. I suspect that the problem is it doesn't know its cold, i.e. a sensor somewhere has failed or is disconnected. There are no other adverse effects and when warm the car behaves perfectly.

I can't find anything in my Haynes, has anyone come across this before? (All routine servicing is in place and up to date)