'93 300E 2.8 check engine light??
Funny, but my 300E 2.8 doesn't have a check engine light, and I don't recall seeing one on any one I've worked on. BUT..As far as your idling problem, I would first check the overvoltage protection relay which is behind the battery with a red cap on it. There's a fuse in there which can blow, leading to problems like you describe. OR...Situation #2, (not pretty) is the wiring harness. I just replaced my harness in my 2.8 a couple weeks ago. I had been having trouble with it idling rough, not wanting to start, and misfiring under acceleration. Couldn't pull any codes, so the wiring harness was the obvious culprit. Also renewed all three coils while I was in there. Runs like a champ now! The harness is about $600, and labor times may vary, depending on diagnostic time. Since I did mine myself, I got out the "good" way!