Soot Exhaust smell only at highway speeds?
 

This one seems like a weird one to me.

W110 200d here. Around town there is zero exhaust smell in the cabin. If I drive at speeds greater than 55mph (estimate) with the windows down, I get an exhaust/soot smell in the cabin. Roll the windows up and the smell goes away.

Any ideas on this one? Seems like I'm creating some kind of negative pressure in the cabin with the windows down at high speed. Exhaust leak at the manifold or ?? I have no idea on how to diagnose this one.

Any advice is appreciated. Thanks!

Check the tail pipe where it extends beyond the rear bumper. You are probably pulling the fumes through the trunk.

I'll not point out the fact the 200D is slow it can't get away from its own smoke!:P:P I also question your use of the term "high speed". Never seen it applied to a W110 200D except when it is in "free fall".

Given the vehicle, I think his estimate varies within a +/- range, specifically "+3/-50 mph..." :D

Could also be pulling it in through the firewall/heater inlet vents if there is a manifold leak in the engine compartment or an exhaust leak near a ventilated floorboard.

Wafting in from the rear (a familiar source of odors in my cars, too) seems likely, though. Some days, windows up or down, my kid just lets us have it.

you must have blow by at highway speeds,if you have not done so adjust valves first.Then detrimine if its valve guides wore,or piston rings

Thanks all. First, for the laughs. Second, for the the advice. :D

I've been meaning to adjust the valves. The PO did say they were in need of adjustment. It's been a year now and I've yet to get to it.

The trunk theory is possible. The tailpipe looks like it's standard length based on google image searches. Mine is just a regular exhaust pipe that extends maybe an inch past the bumper. The trunk doesn't have a rubber seal, so perhaps that's an entry point.

I'll start with the valve adjustment. I've been meaning to get a feeler gauge for awhile now.