my 1985 500sel had the same smell as you are talking about. the fuel injector had a crack in it and so did a hose carrying fuel to the engine have you checked the fuel injetors and hoses.

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Dan Younes
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1985 euro 500sel 220k
1981 300sd 278k sold
1979 240d 298k sold
1983 300sd 272k sold
1989 chrysler 2.2l Turbo
Leabaron 130k
1996 C220(hopefully soon)