Back-fire

Flames bursting from the exhaust pipe of an Audi rally car

A backfire or afterburn is combustion or an explosion caused by a running internal combustion engine that happens in the exhaust system, rather than inside the combustion chamber. When the flame moves backward it may also be called a "pop-back." A visible flame may shoot out of the exhaust pipe for a short while. A backfire is often a sign that the engine is improperly-tuned.