This is a result of working equipment, and is unlikely to cause damage.Ī 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. This causes an incomplete burn which causes the fumes to explode in the exhaust system along with an audible pop or bang sound. In some high-performance vehicles, when a driver shifts up and lets off the accelerator, the engine has a moment of running rich. However, for troubleshooting, engine mechanics more strictly define an ignition of fuel within the engine exhaust system as a "backfire", while a "popback" is this same process taking place in the induction system.Įxhaust system backfires occur in engines that have an emission system malfunction, like an air injection system diverter valve problem, an exhaust leak, or when the catalytic converter has been removed. In both cases (combustion occurring before and after the combustion chamber), the result is a sharp pop, which is colloquially referred to as a "backfire". In this situation it is conceivable that there is a popback occurring in the intake manifold and exhaust manifold simultaneously. The flame front can then travel up the intake and cause a popback. Alternatively, the ignition timing may be late, in which case the combustion is not completed by the time the exhaust valves open, allowing the combustion to propagate into the exhaust system.Īdditionally, 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. On many small marine engines, 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, causing a fire or explosion. If the timing is too early, the spark plug fires before the intake valves close, causing the combustion to propagate into the intake manifold, further igniting the air-fuel mixture there the resulting explosion then travels out of the carburetor and air filter. Pop-backs are usually caused by problems with timing. That is the origin of the use of "backfire" to indicate producing an unintended, unexpected, and undesired result.Ĭommon causes of backfire are running rich (too much fuel going into cylinders) or faulty ignition, possibly a fouled (dirty) spark plug, coil, or plug wire. The term derives from parallel experiences with early unreliable firearms or ammunition in which the explosive force was directed out at the breech instead of the muzzle. A backfire is often a sign that the engine is improperly tuned. A visible flame may momentarily shoot out of the exhaust pipe. A backfire can be caused either by ignition that happens with an exhaust valve open or unburnt fuel making its way in the hot exhaust system. When the flame moves backward it may also be called a "pop-back". It is also sometimes referred to as an afterfire, especially in cases where the word backfire is used to mean a fuel burn that occurs while an intake valve is open, causing the fire to move backward through the system and out through the intake instead of the exhaust. Explosion in the exhaust of an engine Flames bursting from the exhaust pipe of an Audi Sport Quattro S1 E2 Pikes PeakĪ backfire or afterburn is combustion or an explosion produced by a running internal combustion engine that occurs in the exhaust system, rather than inside the combustion chamber.
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