Does yours have a plastic pipe across the top of the air cleaner which carries exhaust? In my 300SD I smell exhaust at a stop and know that that pipe has slipped out of the rubber connector. Of course mine is a 1981 so I have no idea how MB might changed things.

On my 1982 300 SD I have the same problem but I noticed that this smelling come from this pipe that you are talking about and also from the engine air filter. As my 300 SD have a lot of blow back due the very old engine this smell is also from a blow back under the engine hood.
Anyway your reply was very good.
Thanks

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Nyck - TX - USA