SLK32 Catalyst Defect
Symptoms: Noise from exhaust at 1900-2100 rpm when cold
Apparently this is a common problem. Here is my description:
As soon as I picked up my SLK 32 I noticed a rattle when the car hit 2100 rpm. This would occur only under load and only when the car was cold, warm but NOT when hot. I took it to the dealer and two 'a' techs took the car for a drive. They heard the sound but could not find the source. As the car gets hot the noise would disappear so they could no longer hear it.
They invited me under the car when it was on the lift to get my point of view (they know me).
I inspected the heat shields and the exhaust. Looking at the rear cats, I noticed that they were actually touching each other. I mentioned this and they did not seem to beconcerned with it.
They suggested that I leave the car overnight so they could drive it when it was cold. I said I'd make an appointment.
The noise has been driving me crazy so I figured I'd give it a try myself. I put the cold car in my lift and moved the heat shields a bit to see if I could accomplish anything.
I took it for a drive, no cure, just as noisy.
Back up on the lift. I inspected the cats in the rear. I tapped the whole exhaust with a rubber mallet. When I tapped the right rear cat I heard a noise. I wedged a screw driver in between the cats and took the car for a drive. The noise was gone!!!
Apparently the cats were so close together at the point where they are welded, that at 2100 rpm the vibration in the exhaust caused them to contact each other and make noise. As they got hot enough, they made complete contact and would no longer rattle.
I filed the welds not more than 1/2 mm to open the gap. Cured!!!
It is amazing that a 1/2 mm difference could make such a dramatic change.
I have heard of at least 5 other people with the exact same symptoms.
I used a reciprocating saw to widen the gap.
Here are some pictures:
