just a thought... but i know that some cars have small tanks that filter some of the carbon and shit coming from the exhaust when being thrashed... then releases these when the engine is on idle, so as to reduce fuel consumption and to re-burn any unburnt stuff...

if your car has one, it might be full or the system not working, and its just sitting in your exhaust...

could be wrong about these... but it a thought to check out...