Cars backfire for several reasons, including air and fuel mixing due to leaking hoses or other mechanical problems. Faulty spark plugs or wires can cause a car to backfire. It can also occur when shifting to a lower gear in a car with high horsepower.
Backfire is a small explosion produced by a running engine outside the combustion chamber. When unburned fuel mixes with air, it can ignite when an intake valve opens. Visible flames may shoot from the car's exhaust pipe and cause a popping sound.
Backfire can result in a car losing power. The term comes from early guns, which were unreliable and sometimes fired from the back instead of from the muzzle. In common usage, the word also refers to something that causes an undesired outcome.