amgmercedes

Am experiencing a drop in fuel economy, am thinking maybe the cats are blocked or need replacment. How can I tell?

E55AMG99

Mine sounded like they had rocks in them. Not very rhythmic at idle but increases with RPM. Almost sounds like a lifter or rod knock but it is not exactly in synch with RPM. Very inconsistent.

I never really checked mileage or performance but if it suffered, I didn't notice.

MeJay

I think you can smell it if your cat has gone bad. It usually smells like rotten egg.

E55AMG99

Not true. Rotten egg smell means that it is working but under some abnormal conditions. The smell is sulfur in the fuel that is being converted to hydrogen sulfide. Under very lean situations, sulfur will build up in the cats and will get burned off when the engine starts running richer.

Sometimes, different batches of fuel have higher sulfer amounts and can lead to increased "rotten egg" smell.

Greg