What Is Wrong When My Car Backfires?

by Paul Novak
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The causes of most engine backfires fall into two categories: gases explosively expelled through the intake manifold, or explosions that take place within the exhaust system. Backfires through the intake are generally related to problems with ignition timing, and backfires through the exhaust are usually related to fuel mixture problems. In most cases these problems are fixed through simple adjustments or minor repairs to the fuel or ignition system.

Incorrect Timing

In order for an engine to run properly, spark plugs must produce a spark at the right time, or the fuel and air mixture will not be ignited properly. When ignition timing falls out of sync, the spark is produced at the wrong moment, which can result in the fuel being ignited while the intake valve is still open. This causes an explosion of the fuel and air mixture within the intake manifold, forcing the ignited mixture to exit from the carburetor or fuel injection intake.

Faulty Plug Wires

If spark plug wires are crossed or not installed on the correct spark plugs, the plugs will fire at the wrong time. This is similar to an ignition timing problem and causes the fuel mixture to be ignited while the intake valve is still open, resulting in the mixture explosively exiting from the intake manifold through the carburetor or fuel injection intake.

Vacuum Hose Leak

Leaking vacuum hoses allow excess air to enter the intake manifold. This upsets the air-fuel ratio and causes too much air to be mixed with the fuel, resulting in what is known as a lean running condition. The leaner fuel and air mixture is more volatile than the proper mixture and causes premature ignition, or pre-ignition of the mixture, resulting in backfiring through the tailpipe.

Computer Faults

Proper engine operation relies on a precise amount of fuel mixed with a specific amount of air to produce a fuel-air mixture that will ignite properly in the combustion chamber. In modern vehicles, this fuel-air ratio is monitored and adjusted by onboard computers. Problems with one of the several sensors that the computer uses to monitor the engine, such as the oxygen sensor or airflow sensor, can cause the computer to misread the fuel and air mixture requirements of the engine. When this happens, the computer may not add enough fuel to the incoming air, resulting in pre-ignition and backfiring.

Weak Fuel Pressure

Weak fuel pressure caused by a failing fuel pump or clogged fuel filter results in too little fuel being added to the fuel-air mixture entering the engine. This causes a lean running condition in which the mixture has too much air in relation to fuel, which can lead to pre-ignition and backfires through the exhaust.

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