I own a 2006 mcs r53 that run in near perfect condition. It throws a code every now and again for the TPS sensor, but nothing major. I've replace all the hoses, and usual suspect o-rings etc. When I first got the car I sent the superchager to AutocrossX cooper and they replace and tuned it. There are no performance parts such as pulleys, however I did do the ATI crank balancer. Just a little background, the bottom of the car is bone dry, no fluids (both the coolant and power steering reservoirs are aluminum, again hoses are new).

The Problem:
The vehicle smells like rotten pickled radishes. I chased the AC condensor in case it had mold,but its not in the AC system. I've isolated the problem to the engine bay. The reason I know is that it reliably, ever since I have owned this car (4years) the moment I hit 75 mph I then get hit in the face with a constant noxious fume. I assume the pressure in the engine bay hits a certain point where it just gets in the car. Turn off the recycle on AC does nothing. Unplugging the Recycle door mechanism so the door cant physically open for the ac does nothing. I replaced the cat manifold with a known working one in case it was the cat going out. It's not, the replacement did nothing. If it was a annoying mild smell, that would be one thing, but this is eye watering/ headache inducing/dizzy when you get out of the car. It's not sulfur or burning smell. My wife even noticed it when she drove it last.

The Request:
I am open to ideas, anything I might not have though of. Something in the engine bay is generating a gas of some kind, and if I can't figure this out in the next 2 months I"m gonna trade it in for Toyota. I"m pretty savvy with a wrench (replaced the suspension, alternator, too much to list here) so fire away, as I may have already replaced parts as I tried to remedy this problem. Has anyone heard of anything like this?

 

Catalytic Converter.

 

Cat was replaced, it was my first thought too

 

Is it possible something crawled into a crevice, void or plenum somewhere and died? Like a rat or mouse? Or maybe a nest they built and later abandoned? That might explain the smell only at speed when, as you say, the under hood (bonnet) pressures may be forcing air over /around whatever it is and into the cabin.

 

hmm.. I had consider in the ac vent system... but no smell at low speed with ac. Your idea kind of makes sense... I could pull the heat shields and give the fire wall a once over. Thanks for the suggestion, any one please chime in with any other ideas while I hunt this one down.

 

Any water or moisture on the floor boards maybe from the AC drain (under the center console) or from the sun roof?

 

Did you/have you replaced the cabin filter?

The cabin air inlet is in the right front hood cowl, it goes down into the airlet inlet though the cabin filter (mostly overlooked and I have found 10 year old cars with the original filter still installed)

here is a video ..


Best of luck.

Motor On!