Exhaust smell in cabin, W126

[Joe Bauers]

My 1989 300SE has developed a smell that may be exhaust. I notice it particularly when I have been sitting at a light, then accelerate. Oddly, I do not notice it at highway speeds. I do not really think it is other cars--sometimes, I am sitting at the light alone when I accelerate and smell it.

I had the local muffler shop check out the exhaust, and all they could find was an exhaust manifold gasket leak, which they repaired. Clearly, the old gasket had been leaking--there was soot around several of the ports. There is no sign of leakage around the new gasket--no soot, and I do not hear any leaks.

Any thoughts on what might be the source of this smell, and ways though which it could enter the cabin?

Joe B.

[Joe Bauers]

One other point--I did soak the exhaust manifold nuts and studs with penetrating oil prior to the muffler shop replacing the gasket. There is no visible sign of the oil remaining--could this be a factor in my smell? And if so, how long would it take for penetrating oil to burn off?

Joe B.

[ctaylor738]

I chased this in a 420 and almost resolved it, so here are a couple of things to look at:

- the gasket that runs across the top of the cowl fits against the hood. If this is not sealing well, the blower will pull engine fumes into the cabin.

- make sure that the breather hose from the valve cover is connected to the air cleaner. Be sure that all breather hoses are sound and well-connected

- look very carefully for any valve cover leak

- see if there is any sign of a transmission seal that's leaking and getting fluid on the exhaust pipe

- same for the rear engine seal

[MikeTangas]

I know this is true for the V-8's, unknown if same for the 6's...

It is possible to install the exhaust gaskets backwards. They appear to fit either way but really only seat one way. When I had the exhaust smell in the cabin I checked and found a slight leak one the left bank. When I pulled the manifolds I found that one of the gaskets wasn't quite centered on the port. Close inspection revealed it had been put on reversed. When I install new gaskets now I always fit them to the manifold port before slipping them on the studs.

This was discussed here several years ago. Again, I don't know if it is the same for the 6's, but worth looking into.

[zcc]

Make sure that the muffler (the very end of the exhaust pipes) are not pushed to the bumper. This is a common problem in W126 if you hit something while you are reversing the car without noticing. Just ask the workshop to pull it back and hopefully you won't smell that smell again

[pwogaman]

One of the very first things I learned when working for a Mercedes dealer in 1980 was that the W126 bodies have a problem with recirculating flow through when the vents flaps are broken or defective. At certain speed the exhaust air is almost pulled back under the car and sucked up through the vents into the trunk. From the trunk the exhaust is then sucked into the cabin through the rear deck. The flap vents are under the rear bumper on the side. You can check them by pulling out the interior trunk lining. They are an easy fix and only cost about $15.