About month ago, my MY02 ML 320 started to vibrate a little when idling. It cruises perfectly on freeways. It vibration can only be felt from the floor and the steering wheel is rock solid at any time. There is not a single light lit, so I figured my dealer won’t be able to find out anything wrong anyway. I also thought it might just be an engine mount caused by my new commute route involves some real rough roads.

Last week, when I was trying to get into traffic early in the morning, I stepped on gas a little bit more than I like to on a cold engine, about 4K rpm, and suddenly the whole truck vibrates badly and almost lost power, and the CE light is on! I almost felt happy to see the light (weired I know). I gave it a little bit more gas to get the truck going. It smoothed a little, and I got to my dealer which is about half mile away.

The second day, my advisor called me and said misfire in cylinder 1, ignition coil and wire replaced under warranty. I test drove the truck and it seems perfect now. The work order says:
PERFORMED DAS TEST CODES P2050, P2051, P2043, P2044 IN ME, MISFIRE CYLINDER 1, ELECT TEST COIL AND WIRES, BAD, REPLACED #1 COIL AND WIRES

I’ve searched this forum and literally all over the internet and found no one else reported this problem on a W163. How come it happens? Open the hood; I sure can see a brand new ignition module, with the exactly same PN as the rest five. Why could this happen? What could be the reason? Should I worry about the rest five modules to fail?

One thing I thought might remotely explain the cause would be that I had the truck serviced for the first “B” which involves replacing air filter, and the technician might accidentally knocked on the ignition module #1 which is directly under the air filter box. Is it even possible?

One, the coils and supressors go bad all the time. It's an electrical part and they just wear out. To get into the rhyme and reasons would require more detailed electrial theory than I care to get into on a web forum. It is not an uncommon thing at all, we replace them at my shop with some frequency. The others will probably be fine. Two, the tech "bumping"the coil would not cause it to fail. It would require intentional force to damage a piece like that, and you would have seen a cracked housing had this been the case. I would not worry about it.

PHAEDRUS242

Thank you for your response. Since the truck vibrated about a month before the coil finally turned on the check engine light, I am wondering if this ignition coil incidence would cause some permanent damages to the cylinder or other engine parts. Should I be concerned with the reliability of engine because of this? I am thinking of keeping the truck as long as possible, but certainly do not want to have any trouble for the engine started at such a young mileage (28k).

I don't think it's something you need to worry too much about. These engines really are pretty stout and it would take a lot more than that to hurt it. The light would have been on if there was a real misfire happening.