A plugged cat can cause an awful sulfur/rotten egg smell.

A common cause of a plugged cat is running rich. The cat is designed to burn unburned fuel to reduce emissions. Too much unburned fuel overheats the cat and whatever's in there melts and plugs the cat.

The cat can also be plugged by oil in the exhaust.

If your car has an oxygen sensor, check it. Do a web search to see how a healthy O2 sensor displays on a VOM.

You should be able to swap your cat for a test pipe for diagnostic purposes. I don't know if it's legal for you to do so where you are on a temporary or permanent basis.

Sixto
91 300SE
87 300SDL
81 300SD