Reasons An Engine Could Backfire - Subaru Service Questions in Salt Lake City, UT

What is a "backfire?" It's when a mixture of air and fuel detonates somewhere outside the engine -- such as within the intake manifold, or in the exhaust system. Only the engine's cylinders are designed to withstand the force of repeated detonations. Over time, engine backfiring can cause damage to these systems. If your car has been making the distinctive, loud bang associated with backfiring, you'll want to get it checked out right away by the pros! Head to Nate Wade Subaru to meet with the team in our service center. Read on below to learn more about what can cause a backfire.

Fuel Pump On White Background

6. Lack Of Fuel System Pressure

Here's how the combustion cycle is supposed to work in your vehicle's engine: the intake valves open to allow the air/fuel mixture to enter the cylinders. Next, the intake valves close and the spark plug detonates the air/fuel mixture, forcing the piston down. Finally, the exhaust valves open to let the exhaust gasses out of the cylinder.

If the fuel delivery system isn't sending the right amount of fuel to the engine, a backfire condition can develop. The gasoline also needs to be atomized when it is sprayed into the cylinders and mixed thoroughly with the air. If the fuel system pressure is too weak, due to a weak fuel pump or clogged fuel filter, the air/fuel mixture in the engine might be too lean. That means the air/fuel mixture won't burn up entirely by the time the exhaust valves open, leading to a backfire.

Worn Spark Plugs

5. Incorrect Ignition Timing

On most modern cars, the ignition timing is electronically controlled by the engine's computer, so there's nothing mechanical to adjust. But if the computer is malfunctioning or getting incorrect data from a bad sensor, it could fire the spark too soon, before the intake valves have closed, or too late after the exhaust valves have opened, allowing fuel to detonate in the exhaust or intake manifolds.

4. Intake / Exhaust Leaks

If you've got a leak in the exhaust system or intake manifold, such as a leaking gasket or loose connection, additional air can get into the engine, throwing off the air/fuel mixture and compression ratio. This will alter the way the fuel burns and could lead to a backfire. The intake and exhaust systems are supposed to be sealed up tight, so that the Mass Airflow Sensor and O2 sensors can detect the actual rate of air getting to the engine. The engine's computer uses that data to fine-tune fuel delivery and ignition timing to ensure the cleanest, most complete burn.

Valvetrain With Cover Removed

3. Worn Spark Plugs

If your spark plugs are old and worn, there can be additional resistance in the ignition system. This can cause a spark to detonate late or not at all, causing an incomplete burn of fuel and allowing the excess fuel to detonate in the exhaust. Other symptoms of worn spark plugs include difficulty starting the engine and unexpectedly poor fuel economy and power. If you detect these symptoms, it might be time to have the spark plugs replaced.

2. Worn Camshaft Lobes

If your engine has hundreds of thousands of miles on the clock, there can even be internal wear that causes subtle adjustments to the timing of the valves opening and closing. As the camshaft turns under the valve cover, the camshaft lobes open and close the intake and exhaust valves. If these lobes have become worn, the valves won't stay open as long, and could cause a backfire. This could require a costly valvetrain overhaul to fix.

1. Bad Valve Seals Or Springs

If it's not one of the conditions above, it could be a problem with the valves themselves. If the valve itself has become bent, the valve seals are worn and cracked or the valve springs are worn out, the valves won't be able to close entirely -- allowing air/fuel mixture out of the engine itself and into the exhaust or intake systems. Here again, repairs to bent valve seals or springs can be a costly, involved repair.

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Nate Wade Subaru

1207 South Main St
Directions Salt Lake City, UT 84111

  • Sales: (801) 658-9379
  • Service: (801) 328-1327
  • Parts: (801) 328-0061