Perhaps your timing chain has jumped a tooth and is causing the rough idle. That was the problem on my 380SE. I had dealt with a rough idle for a long time, replaced injectors, fuel head, lambda tower, plugs, wires, cap, rotor, etc...and still couldn't get it right. Finally I decided to pull the valve covers and have a look inside...

Well what I found brought somewhat of a sigh of relief to me. I saw that one of my guide rails had broken at the top and that the chain had at some point jumped on the cam sprocket, but not enough to damage the engine (the sigh came here :) )

This was last year at 110,000 miles so I decided to do all upper rails, a new chain and a new tensioner (original MB ONLY!--I've seen many problems with the Febi Bilstein tensioners). Spent the day rediong everything, and at that time switched over to Mobil 1 synthetic, jumed in the car the very next day and took off for Atlanta. Ahh, it idled so smoothly and since the switch to synthetic, it returned about 25+mpg even while travelling 85-90mph will four people and a trunk full of luggage! So, since 420SELs are known for timing chain problems, I'd pull those valve covers and check everything in there. Might save you some money and headaches.

Thanks for the tip Steve. I will take it in to my mechanic as I don't have a multi meter and ask him to test the Lambda out. Any ideas about the oil in the filter housing, or is a little normal? I don't remember seeing any in the past.

Arron, what were your other symptoms when the chain skipped the tooth? Was the idle always rough? Mine is pretty much only when cold.

Thanks

Jason,

Yes, idle was rough all the time but moreso when cold. I'd just double check your timing to be positive that's not the culprit.

Hello Jason,

Nice to run into you in here. I just joined tonight. Did you try checking the motor mounts. Sometimes they are bad and you need to get under the car to check it. Also the transmission mount. If the transmission mount is bad, it can cause the tranny to wiggle and make it seem like its coming from the front. The last resort is to do a compression test. If the compression is low, you might have these problems that you are describing. I hope this helps you some, goodluck.