carfart

(n.) an occasional smell, rancid and more putrid than normal, eminating from the exhaust of a vehicle, possibly due to the use of sulphur oxides in modern fuels.

(v.) the act of a car's exhaust giving off a smell, similar to human flatulence, namely, a strong egg-like scent.

Synonyms: vehicular fart, car flatulence, vehicular flatulence

Q: Dude, do you smell that? Was that you?
A: No, man, that's carfart.