Reasons An Engine Could Backfire - Service Questions serving Sacramento, CA

April 23, 2019

If your engine backfires, chances are it will be surprising--and it can also signify deeper problems. If the engine in your Subaru has backfired, the best way to handle the problem is to bring it to an authorized Subaru service center for an inspection. There, the technicians will take a look and determine what might be the cause of the problem. Most likely, it's one of the four causes below.

A yellow check engine light illuminated on the dash.

4. Old Engine Technology

Backfiring is exceedingly rare in modern engines; however, older cars are more prone to backfiring. Carburetors are a key culprit, and they were ubiquitous until the late 1980s. A carburetor that hasn't been properly tuned could cause a backfire, as could bad or misplaced spark plug wires. These parts are absent in modern SUBARU BOXER® engines, but may be present in older cars.

Image of engine interior showing crank, pistons, valves, spark plugs, and more.

3. The Timing of the Engine Is Off

The internal combustion engine that propels your Subaru requires precision. If the timing between the engine's spark, valves, and pistons is off, even by a little bit, it could cause trouble. For instance, ir the spark fires too early, the fuel and air mixture could be ignited while the valves are still open. If it's too late, the spark won't burn the entire mixture. Either one of these can cause an engine to backfire. Imprecise timing is also less common on modern vehicles, but it is possible.

Eight fuel injectors of various designs arranged in a circle.

2. The Air-Fuel Ratio Is Too Rich

The combustion in your engine is fed by a perfect mixture of fuel and air that's injected by the fuel injectors. An air-fuel ratio that's too rich means that there's too much fuel and not enough air. In this case, there's a chance that some of the fuel may be left unburned by the spark. It could then combust in the exhaust instead of the engine, causing a backfire. If the ratio is too rich, likely culprits include a clogged engine filter, leaky fuel injectors, or a bad mass airflow sensor.

1. The Air-Fuel Ratio Is Too Lean

The opposite problem can have a similar result. If there's too much air and not enough fuel in the mixture, the air-fuel ratio is too lean. This can cause excessive vapor, which may also combust in the exhaust. If this is causing the backfiring, our technicians will keep a particular look out for clogged fuel injectors, a bad fuel pump, or a vacuum leak when they perform their inspection.

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