Why do engines on cars sometimes give off a sulfur like rotten egg smell?

Answer

Because your motor is running too rich and the unburnt fuel is being burned in the catalytic converter, it makes the sulfur smell. At least this is what I've been told.

http://www.autozone.com/servlet/UiBroker?UseCase=S001&UserAction=viewSimpleDiagInfo&Parameters=info

you didn't say what make of car u have, but a lot of Japanese cars smell like rotten egg just because of the fuel additives in the USA