Mercedes: mercedes 220 1993 high idle speed, litre petrol, independent mechanic

Question
a friend of mine has a 1993 2.o litre petrol mercedes lovely car , however when starting from cold the revs go to 1100-1200 rpm and stay there all day, he took it to local garage who replaced a vacuum valve at front of cylinder head (2 small pipes on it) cost about 30 dollars, its still the same as before, the problem started about 12 months ago when the battery got dry, replaced it and problems started - any ideas, local garage mechanic hides under the desk when car drives in to shop..

Answer
Neil,
A vacuum leak will fool the computer into thinking more fuel needs to be added as if the throttle has been opened more.  The vacuum operated valve may not have been the problem. I would suspect there is a leak some where else in the system.
Once you have had the vacuum system checked and verified to be leak free by an independent mechanic, then I would look for some connection with the AC unit. There is a circuit that increases the RPM to compensate for the load the AC compressor adds to the system.

Do the rpm decrease or increase when  you operate the AC on full blast?

I would turn off all accessories and the switch. Then remove the battery cables for 30 minutes. Clean the post and terminals and reinstall, (+) first.
Do not touch the accelerator pedal; start the engine. Without any input from the accelerator, allow it to warm up and see if the rpm drop back to about 850.
Good Luck Let us know what you fine out.
Rob