What Causes Car Backfire?

An engine backfire occurs whenever the air-fuel mixture in your car combusts somewhere outside the engine's cylinders.

We often associate the characteristic pop and bang of an engine backfiring with old, rusty trucks but any engine can develop a backfire. Even the one under the hood of your car!

However, modern engineering has developed sophisticated engines with variable valve timings and computer-controlled spark timing, which all but eliminate backfiring in today’s passenger cars. If your engine has been making loud noises that have you suspecting a backfire, check out the information below. We’ll tell you five things that most commonly could lead to a backfire.

What is Backfire?

A backfire or afterburn is combustion or an explosion produced by a running internal combustion engine that occurs in the exhaust system, rather than inside the combustion chamber. A visible flame may momentarily shoot out of the exhaust pipe. A backfire is often a sign that the engine is improperly-tuned.

An engine backfire occurs whenever the air-fuel mixture in your car combusts somewhere outside the engine’s cylinders. This can cause damage to your car’s exhaust or intake if left unchecked, and it also means that your car’s engine isn’t making as much power as it should, and is wasting lots of fuel.

It is also sometimes referred to as an after a fire, especially in cases where the word backfire is used to mean a fuel burn that occurs while an intake valve is open, causing the fire to move backward through the system and out through the intake instead of the exhaust.

When the flame moves backward it may also be called a “pop-back.” A backfire can be caused either by ignition that happens with an exhaust valve open or unburnt fuel making its way in the hot exhaust system.

An engine backfire occurs whenever the air-fuel mixture in your car combusts somewhere outside the engine's cylinders.

So, What Causes Car Backfire?

That unburnt fuel can be caused by a variety of mechanical problems, and here are some of the most common reasons for a backfire:

1. Rich Air/Fuel Mixture

For various reasons like leaking fuel injectors or a clogged engine air filter, the engine might have too much fuel added to the cylinder, or not enough air. For proper combustion, the exact right ratio of air to fuel must be maintained.

A mixture of air and fuel that’s got too much gas in it is called, “rich.” When a Rix air/fuel mixture is ignited in the cylinder, the whole mixture won’t be burned up by the time the exhaust valves open.

Then, the combustion process will flow to the exhaust where a backfire will take place. Getting a rich air/fuel mixture fixed means replacing bad fuel injectors, a bad mass airflow sensor, or even something as simple as replacing the air filter.

2. Lean Air/Fuel Mixture

Not only can a rich air/fuel ratio cause a backfire, but a mixture that doesn’t have enough gasoline can also cause a backfire, too. A “lean” mixture is one that doesn’t have enough fuel and too much air.

Such a mixture could be caused by low fuel pressure due to a failing fuel pump, a clogged fuel filter, or clogged fuel injectors. When a lean mixture combusts, it burns more slowly, meaning there will still be some air and fuel that isn’t used up when the exhaust valves open leading to a backfire.

You’ll want to have a Chevrolet technician take a close look at your vehicle’s fuel system if the air/fuel mixture in your engine is running lean and causing backfires.

3. Bent Valve or Valves

Inside each of your engine’s cylinders, you’ll find at least one intake valve and one exhaust valve. They’re designed to let air and fuel into the cylinders, then shut when combustion occurs. Then, once the air and fuel have been combusted, the exhaust valves open to let the exhaust fumes out the tailpipe.

However, if the valves become bent, they won’t form a proper seal. That’ll allow air and fuel to flow back into the intake or into the exhaust, where it will combust.

Luckily, this is a very uncommon reason for a modern car to start backfiring. Replacing bent valves and/or bad valve seals can be a costly procedure, as it means dismantling the entire engine.

4. Bad Ignition Timing

Once both sets of valves are closed, that’s when the spark will fire inside the cylinder — or at least that’s when the spark is supposed to fire. However, if the spark timing isn’t just right, the spark could fire too early before the intake valves are closed, or too late when the exhaust valves have already opened.

When this happens, the air/fuel mixture in the exhaust or the intake can combust, leading to a backfire. Since the spark timing in a modern car is computer-controlled, this is a problem that’ll need to be addressed by a trained technician.

5. Misfiring Spark Plugs

Spark plugs that fire out of turn or not at all can also cause backfiring. Shorts in wiring, incorrect wiring, or damaged distributor caps that deliver the charge to the wrong plug at the wrong time are much more common in older vehicles.

But even in newer cars and trucks, plugs can malfunction due to carbon buildup or wear out over time.

Why Do Cars Backfire When Shifting?

Not all backfires occur when you start the engine. Sometimes one can happen when you shift gears. However, that loud pop that you hear when shifting is, in fact, an after fire.

Most of these occur on manual vehicles where a clutch is used to shift gears. As you press in your clutch and go from one gear to the next, fuel can continue to enter the cylinders, especially if you switch gears at higher RPMs.

All this unburnt fuel builds up in the exhaust and ignites when you release the clutch. Though this might sound alarming when it happens, and after a fire when shifting isn’t necessarily damaging your vehicle.

Is backfire bad for a car?

Backfires and after fires are worth paying attention to since they can cause engine damage, power loss, and decreased fuel efficiency. There’s a variety of factors that can cause your car to backfire, but the most common ones are having a poor air to fuel ratio, a misfiring spark plug, or good old-fashioned bad timing.

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