I had a similar problem with my M103 and the cause was a leaky valve cover gasket.

It was leaking down behind the motor, and by cylinder 6 and getting all over the exhaust manifold. The result was a noxious plume of fumes that would get sucked into the car every time I had the vents on.

Feel around behind your motor, there's a semi-circle cutout in the back where the valve cover meets the head. Feel underneath the semi-circle. If there is fresh oil there, replace the valve cover gasket.

Does the smell appear immediately or only after the car is warmed up? Does it disappear as you are driving and re-appear at stop lights? Are you running the vents at all?

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition