4 Cause for a Car Engine Backfiring: Things All Subaru Owners Need to Know

What Causes a Car's Engine to Backfire - Service Questions at Carlsen

Thanks to modern advances in engine technology, like variable valve timings and gasoline direct injection, cars today are far less likely to backfire than they were just a few decades ago. However, you should still know what's happening when an engine starts to backfire, so you can take steps to get it fixed. A car that's backfiring isn't burning up all the gasoline in its cylinders upon combustion, meaning you're losing performance and wasting fuel! Here are four things that might cause your Subaru to backfire, plus some helpful info on how to identify a backfire and what you should do about it.

Learn why your Subaru's engine could backfire

Reason #4: Bad Ignition Timing

Let's start with what exactly a backfire is. A backfire describes any moment when combustion of air and fuel is allowed to take place outside the engine's cylinders. Usually this happens either in the exhaust (sometimes called "afterfire") or "back" into the engine's intake.

Combustion always begins with a spark from a spark plug (at least with gasoline engines). On older vehicles, the moment when each spark plug would fire was mechanically controlled by a spark distributor. Today's vehicles feature coil-on-plug systems and computer-controlled spark plug firing. That means that the computer picks the ideal moment to fire the spark for maximum efficiency and power.

On older cars, the distributor could wear out and need service as part of a tune-up. If the ignition timing was too early, the air/fuel mixture could be ignited before the intake valves were completely shut. This allows combustion to take place in the intake manifold, where it's doing you no good for power and potentially damaging the engine.

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3. Incorrect Firing Order / Crossed Spark Plug Wires

Today's coil-on-plug ignition systems make these problems mostly a thing of the past, but if you've got an older vehicle, it's possible the spark plug wires could be hooked up in the wrong order to the spark plugs -- meaning they'll fire in the wrong order. A spark has to fire only at the right time in the correct cylinder. The firing order is especially important to smooth engine operation, too. If you recently had a tune-up or replaced spark plugs or spark plug wires, and your engine has begun to backfire, you'll want to make sure all the spark plugs were installed correctly.

2. Bent Valves

The intake and exhaust valves allow the right amount of air and/or fuel into and out of the engine's cylinders. When they're closed, they form an airtight seal so that all of the air and fuel is combusted inside and none is allowed to escape until all the gas is used and all the power possible has been extracted. Then, the exhaust valves open to allow the exhaust to escape before the intake valves open to begin the process anew.

However, if the valves can't form a good seal, gasoline can leak past bent valves or bad valve seals into the exhaust system, where it can detonate from the intense heat in that system. This a classic cause of an afterfire, even on modern cars, and it means replacing valves and valve seals -- a significant engine repair.

Things you should know about a backfiring engine

1. Incorrect Air / Fuel Mixture

Modern cars make constant micro adjustments to make sure the perfect air/fuel ratio is always being burned in the engine -- but if the sensors that feed information to the computer go bad, become corroded or get covered in carbon built-up, they can send wrong information to the computer or no information at all. Without that data, the computer could add too much or too little gasoline to the engine. When that happens, excess gasoline could go unburned in the engine and backfire in the exhaust. Or, if not enough gasoline is added, the air/fuel mixture will burn more slowly, allowing unburned gasoline into the exhaust. To fix this backfire, you can have our technicians take a close look at the exhaust system, fuel injectors, engine control unit and more to locate the problem and get it fixed.

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  1. Carlsen Subaru

    480 Veterans Blvd
    Redwood City, CA 94063

    • Sales: (888) 520-5916

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Carlsen Subaru

480 Veterans Blvd
Directions Redwood City, CA 94063

  • Sales: 650-365-6390
  • Service : 650-365-6390
  • Parts: 650-365-6390
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