My '92 190E 2.6 sometimes has a hard time holding its idle revs on start up. It will scream at 2000-3000 revs, then drop to 200, then scream again - over and over and over again. Sometimes it will stall outright. If I catch it in mid-transition between over-revving and under-revving, sometimes I can interrupt it and return it to normal. This happens for a few days and then it will behave itself for another few days before it has another fit. Any thoughts on what’s going on and how to fix it? My mechanic has not succeeded in 3 years!
What has your mechanic tried? EHA valve? Idle contol valve (idler solenoid valve)?
Does temperature affect it? Cold start valve?