Why Does a Car Backfire? (4 Common Causes)

Last Updated on September 22, 2022

If you’ve heard anything that sounds like a gunshot when on a walk along a busy street, and it wasn’t from a firearm, then you’ve probably heard a car backfiring.

It might just be one solitary *pop*, a series of pops followed by a crackle, or a long string of loud, rapid pops if you’re at the dragstrip looking at the tuner cars.

One amazing thing about backfires is, if it’s night time, you may even see fire shooting out of the back of your exhaust system.

Related: How to Make Your Exhaust Louder

What is Backfire?

Backfire is caused when unburnt fuel still in your exhaust system is heated to the point of ignition, causing an explosion at a point that’s not in your engine.

This explosion is what causes that popping sound when accelerating, decelerating, or when starting up your car for the first time on a cold morning.

When that burst of fuel gets too hot, it will explode just like a Molotov cocktail and sometimes even send flames shooting out the back of your car. (Please no burnouts near a field during a fire ban, kids)

Read Also: 5 Signs You Have a Hole in Your Muffler

What Does it Sound Like?

Backfires sound a little like gunshots, with maybe a touch of fireworks thrown in there for good measure. A good backfire is like fine wine… it should be loud, but not too loud where you blow out your buddy’s eardrums.

If you’re working on tuning your car for backfires, you might have a little more fuel than is required for normal combustion so you get excess thrown out the back of your exhaust system. 

You might want just a tad bit of burble as you let off the accelerator, causing the RPMs to drop, but leaving unburnt fuel flowing through the exhaust system. This will leave your heart with the amazing sensation of *burbles* that you can hear from the driver’s seat.

Still not sure what a backfire sounds like? This may help:

Reasons for Engine Backfire

#1 – Bent Valves

In each of your cylinders, you’ll usually find at least one intake and one exhaust valve. These intake valves open and close to let fuel and air in, while the exhaust valves open at the end of each combustion cycle to let out the emissions/excess gas and fuel out.

As with most moving parts in your vehicle, these valves are designed to create a good seal when they are closed to facilitate the most efficient combustion cycle possible. So, if the seal gets worn out or the valves are bent, the seal will no longer prevent fuel from entering the cylinder when you don’t want it to.

Now, if too much fuel is in the cylinder during compression, you’ll have extra unburned fuel present in your exhaust gases. This amount might be irregular if your valve is bent, unlike in a “rich” air/fuel mixture. Luckily for you, the cause of your backfire is probably not bent valves as this is a very costly fix at your local mechanic.

Also, since bent valves are usually the result of an engine timing problem, you’ll find that your timing belt or chain is broken before your valves lead to a backfire.

#2 – Bad Ignition Timing

Hopefully, your spark plugs will only fire when both of your valves are closed so you can get the most efficient burn possible.

However, if your ignition timing is off, you’ll burn your fuel when one of your valves are still open or closing, leading to a disproportionate amount of fuel expelled through your exhaust system.

If you have too much unburned fuel moving through your red-hot headers, they’ll ignite causing an explosion to pop through your exhaust system.

#3 – Rich Air/Fuel Mixture

If you have too much fuel in your air/fuel mixture, it’s called running “rich”. In order to maintain the most-efficient combustion in your engine, you need to have the perfect air to fuel ratio. If you maintain this, you will improve the longevity of your engine and it will cost you less down the road.

However, if you have a rich air/fuel ratio, then you’ll send too much fuel into the cylinder which will result in an incomplete burn. As with the above examples, too much fuel will be left unburned and sent out the exhaust valve. This excess fuel will ignite in your exhaust, causing a loud *pop* that you can hear from your driver’s seat. 

When this pop occurs, you’re creating a break in the exhaust flow which will give you a momentary lapse in acceleration, which you can definitely feel. Your car might feel sluggish, and you won’t be able to street race as well. Remember, an efficient engine is smooth, and a smooth engine is fast.

#4 – Lean Air/Fuel Mixture

If your fuel injectors are clogged, your fuel filter is dirty, or your fuel pump is failing, you will probably develop a “lean” air/fuel mixture in your combustion cycle. By “lean”, I mean there isn’t enough air to burn all of the fuel in time for proper combustion.

It’s all about maintaining the proper balance of air, spark, and fuel. If one of those elements is lacking in any way, your engine will throw a Check Engine Light, and you’ll have excess unburned fuel popping its way through your exhaust system.

Is Backfire Bad for a Car?

With any vehicle, anything outside of the normal manufacturer’s specification will usually decrease the life of your powertrain, suspension, and reliability. Specifically, a backfire will decrease your fuel efficiency, meaning you can’t get to where you want to go without spending more money at the pump.

Unfortunately, a backfire can also cause damage to your catalytic converters and will also cause you to lose power. It will damage your catalytic converter simply because the unburned fuel is now going to be burning up your “Cat” instead of burning up inside the cylinder walls, where it’s meant to burn.

With the increase in catalytic converter theft on the rise, prices of new catalytic converters have been skyrocketing across the US. Because of that, you’ll want to take the best care of this component as possible.

If you have excessive backfiring on acceleration, your engine will think that there is something wrong (which there is) and start to dial back your engine’s power. This is a failsafe mechanism that your car’s ECU will enact to prevent any further damage that might occur with a backfire.

If you’ve fixed the cause of your backfire and you need to reset your car’s ECU, check out this article.

Is Car Backfire Illegal?

Car backfires in and of itself are not illegal because in most cases, you didn’t do anything wrong and you’re just trying to get to work.

However, if your car is backfiring loudly or excessively because you want it to and you’re causing a “disturbance”, police might look a little more closely at you.

Furthermore, if your car is running too rich, has burnt up the catalytic converters and will no longer pass emissions, then you’ll have an illegal car.

See Also: Pros/Cons of a Muffler Delete

How To Make Your Car Backfire

Now, if you’ve made it to this point in the article, you probably are a car enthusiast that wants to make their car backfire because, let’s face it, it’s cool to shoot flames and make a bunch of noise when you’re driving. 

In an older car, you can get it to backfire by first letting the engine warm up for a few minutes, then turning it off. Now, with your foot on the accelerator and the brake, start up the engine and give it a good rev! You should hear a bit of popping as the car tries to perfect the air/fuel mixture. 

Another way is to take your car up to 60mph and decelerate using the engine. You can do this by switching into a gear that’s one lower than the gear you are currently in. Then, let the engine and transmission slow you down. If you listen close, you may hear some burbles and pops from your exhaust system. 

Now, the final and most-fun way to get your car to backfire is to get a proper performance exhaust system and get an ECU tune to allow for a leaner or richer fuel/air mixture. This way, you can control the amount of backfire you get from driving your car!