The dealer has thrown up his hands so I'm hoping someone on this board can help. My '90 300E occasionally comes down with a low idle - about 400 rpm rather than the usual 600 rpm (in gear). It is annoying because the car then shakes and the cycling of the AC will occasionally cause it to stall. This condition can suddenly arise at any time - shortly after a start up or after an hour of stop and go. Once it's in the low idle mode it stays there - no amount of driving will have an effect. However, if I shut the engine off and immediately restart it, normal idle is re-established - as if something is being reset. Replacement of all the usual tune-up items has had no effect. I even had the dealer replace the Hall sensor (behind the speedo) after reading about it on this board. Any suggestions/insight would be greatly appreciated.

Try search feature, AIR FLOW SENSOR. The later version Cis-ECU can & will go into fixed oper. mode.

I had a very similar problem resolved with my 1993 190E 2.3 recently by replacing my idle switch. At first, the problem was intermittent, and happened once in a while. After 9 months or so, it was more consistent and reproducable at the dealer. You can search for my other posts. Earnest.