A few weeeks ago, I began to noticethat my 92 500E was idling a bit roughly, and my mpg dropped a bit. My tech suspected a throttle body, but did not have a sophisticated enough computer to verify it, but were able to read a stored code of 17. Recommended, hesitantly, taking it to the local MB (notoriously incompetent) dealer. I did, and they told me it needed new rotors and caps, which I had them replace. It ran a bit better, but still idled roughly when hot. Yesterday, all of a sudden, it began to run like it was in limp home mode, but the engine light came on. I got it to my tech's; it turned out that the dealer had not correctly routed a coil wire, and it rubbed all the insulation off against the power steering pump. Tech taped it, said to take it back to the dealer, which I did. Now the engine light was on. The dealer begrudgingly agreed to replace the wire, which they did, but they did not reset the computer, so the engine light is still on. As soon as I pulled away from the dealer, I noticed it was still idling roughly, but in a different way, where it now seems to be missing, as if it is not running on all eight cylinders. I have an appointment to take it back to the dealer to straighten it out.
I have had five different experiences with thi dealer where they screw something up. During a tune-up, they broke a resistor, during an oil change, they b angedd the Bowden cable, making it shift badly, etc.
My question, what happened, and what should they be looking for?

I believe Larry Bible has within the past month or six weeks posted information regarding 02 sensors and their failure. Is yours replaced, and how long ago? It may be going out on you.

What dealer are you going to? I need to stay away form them.

many things can cause a miss. o2 sensors do not. you really need to find someone that knows what they are doing. heck, even try a shop that is good with american cars to check the system if they have a good reputation.