Heavy exaust smell
 

My 1990 500Sl , 107k, has a very heavy exaust smell. It runs good, good mpg & starts everytime. The idle is rough when cold. I do notice when I clean it the rear bumper in the exaust area has a carbon like residue on the cloth.

What should I look at?

I believe that this car has KE-Jet, similar to the 560. So here are a couple of general suggestions.

1. There is an air temperature sensor in the air cleaner, and a coolant temp sensor at the front of the intake manifold. Find out what their resistance readings are supposed to be, and check them with a meter. I am not sure about the air temp one, but the coolant sensor reading is what the brain uses to determine how long to fire the cold start valve, and how much to enrich the mixture during warm-up.

2. Check or replace the oxygen sensor(s?). If worn-out ay be causing a false low reading, causing the brain to over-enrich.

3. Be sure spark plugs and other ignition stuff are in good shape.

4. Check carefully for vacuum leaks.