back fire

. 1. A fire started ahead of a forest or prairie fire to burn only against the wind, so that when the two fires meet both must go out for lack of fuel. [ Webster 1913 Suppl. ]

2. (a) A premature explosion in the cylinder of a gas or oil engine during the exhaust or the compression stroke, tending to drive the piston in a direction reverse to that in which it should travel; also called a knock or ping. (b) an explosion in the exhaust passages of an internal combustion engine. [ Webster 1913 Suppl. +PJC ]