4 Causes of Engine Backfires | Subaru Service Info in Thornton, CO

Common Reasons An Engine Could Backfire - Service Information for Thornton, CO

June 19, 2019

A backfiring engine can be startling and alarming, but that's just the beginning. If the engine in your Subaru has backfired, it could be due to serious engine issues. After all, backfires are incredibly rare in modern engines. The best way to deal with a backfiring engine is to head to an authorized Subaru service center. The technicians there will inspect your vehicle to see the cause of the problem. Most likely, it will be due to one of the four issues below.

An older vehicle. Cars before the late 80s had carburetors.

4. Older Engine Tech

We mentioned that backfiring is uncommon in modern engines. Older cars are more likely to backfire. If you're rocking a truly vintage Subaru, a backfire may be due to a carburetor. These stopped being used by the late 1980s, but were present in cars before then. If your Subaru has a carburetor that hasn't been tuned lately, that may cause the backfire. Other older engine tech, like bad spark plug wires, may also be the cause.

Several spark plugs outside of an engine.

3. Bad Engine Timing

In order for combustion to happen as it should, the timing between the engine's spark, valves, and pistons must be absolutely precise. If timing is even slightly off, it could lead to problems. A spark that fires too late may lead to the fuel-and-air mixture not being burned completely, while a spark that's too early could cause this mixture to ignite before the valves close. In either case, a backfire could then ensue. This is an issue that's not common on modern SUBARU BOXER® engines; however, unlike a bad carburetor, it is still technically possible.

A BOXER engine like the kind that powers Subaru vehicles.

2. The Air-Fuel Ratio Is Too Rich

The fuel injectors in your Subaru vehicle's BOXER® engine produce a perfect mixture of fuel and air. However, there are many things that could throw this mixture off. A clogged engine air filter, bad mass airflow sensor, or leaky fuel injectors could lead to an air-fuel ratio that's too rich. This means that there's too much fuel and not enough air, which could cause some of the fuel to remain unburned by the spark. It could then combust in the exhaust rather than the combustion chamber, causing a backfire.

1. The Air-Fuel Ratio Is Too Lean

On the other end of the spectrum, a bad fuel pump, vacuum leak, or clogged fuel injectors could cause an air-fuel ratio that's too lean; that is, it has too much air and not enough fuel. Though this is the opposite problem, it can also cause a backfire as vapor escapes into the exhaust and combusts there.