Definition: Backfire - The unwanted ignition (explosion) of fuel in the intake manifold, normally causing an objectionable popping noise, together with possible loss of power and forward motion.
The term was derived from experiences with early unreliable guns which could literally blow up in a shooter's face. From this came the use of the word "backfire" as a verb to indicate something that produces an unintended, unexpected, and undesired result.
Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Usually, backfiring occurs in carbureted engines that are running lean where the air fuel mixture has insufficient fuel. ("Running lean" is typically a sign of mal-adjusted carburetors or fuel injection where there is not enough fuel for the amount of air). Afterfire occurs in engines that have an emission system malfunction (air injection system diverter valve), exhaust leak or unburnt fuel in an exhaust system in which the catalytic converter has been removed. When a driver shifts up and lets off the accelerator, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system. The leak itself is the most dangerous aspect. Without it, the mixture would cool enough not to explode. A fuel injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective.
Common causes of backfires are:
In older engines, backfiring may be common. Backfire is rare in modern cars with fuel-injection and computer-controlled fuel mixtures.
Common causes of back fires in the intake manifold are bad spark timing, or incorrect (usually lean) fuel ratio.
When starting an engine, timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner. A common air filter will allow the gases to escape, but will block the flame front. On many small marine engines, no air filter is used, but a screen is placed over the intake of the carburetor as a flame arrestor to prevent these flames from escaping the intake, and potentially igniting fuel, or fuel vapors in the enclosed sump or bilge of the boat and causing a fire or explosion. Improperly adjusted carburetors that create a lean condition during acceleration can cause the air fuel mixture to burn so slowly, that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire.
In drag racing, backfires in the intake usually result in the complete destruction of the intake manifold, carburetors, supercharger, and sometimes engine.