I had the same thing happen to me when I first got the bimmer. My mom is really asmatic(sp?) and it would really bother her. I took a good look under the car and found a dime size hole on the passenger side of the muffler near the top, easy to miss. I replaced this and the smell is mostly gone but I still can't put my backpack in my trunk or it'll come out smelling like exhaust. I'm almost positive my valve cover is dripping oil onto my exhaust manifold and contributing to the smell. Good luck finding your problem.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo