Mercedes Forum - Mercedes Benz Enthusiast Forums - 126

Hello there - I am new!

I have a 1989 500 SE with a high idle problem. I have done a search on the site but not found what I need to know. When cold, it idles perfectly. But within about 1 - 2 minutes it creeps up to about 1200 - 1500 revs and stays there. I have replaced the Idle valve, idle ECU, Idle temp sensor, checked the throttle switch and checked for air leaks and electrical connections - none of this has made any difference! The idle valve is getting a signal from someware to operate when it shouldn't - I would have thought this would be from the throttle switch, but this is fine. Every now and then, usually on a hot day [not too many of these in northern UK] it will run at the normal idle, but it does not stay this way. The euro spec cars of this age do not have the idle oil temp sensor the US cars had. When the car is suffering from the high idle, it is a lot noisier than when the idle is normal, as if the fan is going round [which it isn't, unless it is very hot etc] - a sort of whooshing noise. The only other fault with the car is the headlight beam range adjuster does not work, and the lights used to go up and down on their own as the engine reved. I disconnected this and set the lights. Cannot think how this could be related to the idle problem but who knows - it is a vacuum system after all.
I am at a loss to progress this any more! Does any one have any other ideas as to the cause - anything! My garage cannot sort it, the UK MB owners club are not interested and various other forums have no answer. I can truly say I belong to the 126 High Idle Club!
Many thanks in anticipation:D