Exhaust smell in cabin, W126
 

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.

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.

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

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.

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 :)

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.

All replies are appreciated, and I will check each suggestion out.

Not sure I understand about vent flaps in the trunk. If you could explain further, I would appreciate it.

Thanks to all.

Joe B.

Worth a thousand words! It is right behind the rear wheel. The opening also supports the rear bumper. You can check it by removing the lining inside your trunk on the sides.

Thanks pwogaman. So are these flaps supposed to move, or are they supposed to be in a certain for proper ventilation flow? What should I look for to see if they are ok?

Joe B.

They should close tight - hinging from the top. They get dried when old. I've seen them with over-spray and curl outward.

Thanks again, pwogaman. Will check them out for sure.

Meanwhile, I discovered that the muffler shop did not have the header pipe snugged tightly to the manifold after they replaced the manifold gasket. Snugged it up and, so far, do not detect any odor. But I think the flap thing is something that I ought to check on anyway--sounds like it could become a problem if it's not working properly.

Joe B.

Your picture looks like the bumper is removed. Is that necessary to replace those vents? And, are they only a dealer item? Mine on my 420SEL are wasted.

I've replaced them from the inside before, but you need to be real careful. Instead of replacing the flap with the plastic flap frame you just remove the rubber and fit in from within the trunk. Note thought that the little tabs on the rubber flap can rip easily. If you want to try doing it that way I suggest spraying some slilcone on it first. Also when considering trying it that way, remove the old flap first just to see how easily you can rip the flap tabs. If it doesn't work for you then remove the bumper to do it. In any event, liberally spray silicone on the new one when you are done.