I took my 1994 S350D to a mechanic who specializes in German car repairs. The engine had a cracked head. I'll skip the details of the horror story but the mechanic blew the engine up twice (valve/piston interference) trying to get it right. Three was almost the charm. I've had the car back for about 6,000 miles and the engine seems fine in all respects except one. When idling, the engine shakes the whole car. I guess the best way to describe it is, "zug zug zug zug". You actually bounce up and down slightly in the seat while its doing this. The problem is minimal when the engine is cold and more pronounced when the engine is up to temp. Higher ambient air temperatures also seem to aggravate the condition.

I've noted two things about the car since the "repair".

When I first received the car back, the vacuum line were all screwed up (hard gear shifts, turbo not boosting correctly). I fortunately had photos of the engine taken prior to this misadventure. Using the photos
I was able to correct the vacuum line problems (I think).

The other thing I noted was, when replacing the valve cover gasket, I found the timing chain was very loose (sagging on the driver side) and the tensioner appeared to be near or at full extension. The mechanic had replaced the timing chain.

I'm probably going to take the car to the local Mercedes Dealer, but I'd like to have an idea as to what the problem could be. My local dealer is not exactly a whiz with diesels. Once after having the dealer replace some glow plugs, I noted the car had a very rough idle. I marched right back in and pointed out the problem. The service manager discussed the problem with the "technician" and told me I was probably looking at a new injection pump. Fortunately I have a father-in-law that's worked on everything from lawn mowers to locomotives. He said "they got air in the fuel lines, run it for a few days, it'll be fine". He was correct.

Any ideas out there as to what could be causing the rough idle? Is there something specific I should have the dealer check?

Thanks