'90 420sel, 175,000 miles, car has been FANATICALLY maintained with all services and problems corrected promptly. has been in the shop for 2 months and they can't figure out what it is. i have had a mercedes tech look at it - no idea. i am about to shove it into the ohio river.....

the problem: hot day, car had been used just prior. started up the car and heard a "whump" and from that point on it idled like crap - idles almost as if a motor mount had collapsed.

what has been done so far: checked for vacuum leaks, complete tune up including rotor and dist. cap, new fuel distributor ($$$$ - at the advise of the first mercedes tech who just knew that is what the problem was), new fuel pressure regulator ($$$ - the end had blown out evidently) , checked to make sure all cylinders were firing, checked to make sure all injectors were good and not clogged, checked EHA to make sure it was functioning properly.

still idles like crap and blows black smoke. runs o.k. when at speed.

the mechanic is going to try to have one more person (mercedes tech.) look at it on monday and then take it to the dealership. i frankly don't have much faith in the dealership - that is why i didn't take it there in the first place. i am afraid they are going to start it up and say, "well, looks like you need a new crate engine....." they do not like to work on anything pre '99.