What Causes a Car to Backfire?

If you’re into any form of auto racing, you probably think loud cars with flames shooting out the exhaust are awesome. That’s less awesome when it’s your 1999 Toyota Corolla and you’re just trying to get to work. What you are experiencing is a backfire. What causes an engine to backfire, and how do you fix it? Here are some answers to help you resolve this loud and potentially damaging issue.

What is backfire?

A car backfire is when unburned fuel reaches the exhaust system and ignites, causing a pop or bang sound. Backfiring at night allows you to see flames lighting up the exhaust pipe as the fuel explodes in the wrong place. Some of the symptoms of a backfire are annoying, but they may be a sign of bigger problems. Before we get into why cars backfire, let’s look at how an engine is supposed to work.

Gasoline engines are designed to burn a combination of fuel and air in the combustion chamber. This process starts when the engine draws air through the intake manifold while the fuel injectors spray fuel into the cylinders. The piston compresses the two, and the spark plug ignites the mix.

The explosion pushes the piston down, sending power to the crankshaft, creating horsepower. Then, the exhaust valves open and allow the spent gasses to escape. Passing through the exhaust manifold, the gasses are cleaned by the catalytic converter before exiting the tailpipe.

A backfire means something went wrong in that process. Several issues could cause the fuel to not ignite properly, and when unburned, the fuel flows past the exhaust valve and ignites on the hot exhaust parts. The random fuel ignition can happen in the exhaust ports, exhaust manifold or even the catalytic converter.

This excess fuel causes a loss of performance you feel when accelerating. If left uncorrected, the issue will cost you money in the long run with increased fuel use and eventual engine damage. Serious misfiring can crack the exhaust manifolds or damage the plastic intake manifold and associated gaskets. Those are all expensive to replace. The engine wasn’t designed to operate with backfires, so get any backfires checked out ASAP.

What causes an engine to backfire?

Just as your headache might be caused by dehydration, lack of coffee or loud neighbors, there can be multiple answers to what causes backfires. Here are the most likely causes.

Bent valves

The intake and exhaust valves allow fresh air into the engine and let out spent gasses after the proper combustion process. If the valves are coated in sludge, damaged or bent, the valve doesn’t seal the combustion chamber. When the piston compresses the air and fuel mix, some of the mix escapes the damaged or bent valves. The fuel explodes somewhere else, causing the backfire. Bent valves are an expensive fix, but ignoring them would be even more costly.

Bad engine timing

Bad engine timing can cause misfiring issues and bent valves. The piston and valves can both occupy the same space in the cylinder, but proper engine timing ensures only one is in that space at a time. On the exhaust stroke, the exhaust valve drops into the cylinder to create an exit while the piston rises to push out the gasses. The valve withdraws into the cylinder head, closing the exhaust port, as the piston rises and reaches its peak. Normally, this happens thousands of times per minute.

With improper timing, for example, when a timing chain jumps the cam sprocket, the critical separation of components by time goes out the window. Parts start trying to occupy the same place simultaneously, which makes a lot of expensive noises.

There is also the slightly less serious issue when engine parts aren’t contacting, but the timing is way off. In this case, the intake or exhaust valves open at the wrong time, allowing a similar effect as bent valves. The unburned fuel leaves the combustion chamber to cause a backfire somewhere else.

Running too lean

When there is more air than necessary in the engine, the mix of air and fuel is “lean” and causes problems. This can be caused by a failing fuel pump, clogged fuel filter, stuck regulator and malfunctioning or clogged fuel injectors. With the decreased fuel, the air in the mix fills the combustion chambers, and while the fuel still ignites, it burns slowly and can still be combusting as the exhaust port opens.

Running too rich

This is the opposite problem. Rather than too little fuel, a rich air/fuel mix has too much gas. Running rich creates a problem in the engine when it tries to burn the fuel at the right time. Because there is too much gas, the excess fuel remains unburned and flows with the spent gasses out of the engine. Running too rich is also another way to potentially flood the engine.

Ignition system problems

There are many other reasons why cars backfire than just the above. The ignition system is particularly suspect. Spark plugs can cause backfires if they are worn, damaged, covered in carbon buildup or gapped too wide. The result is the same, allowing fuel to pass unburned. The same applies to other ignition components, with spark plug wires, ignition coils and distributors all potential suspects if you are experiencing backfires.

What to do if your engine backfires

The key takeaway is there is an underlying issue that needs to be corrected. You aren’t just trying to prevent backfires but also fixing the issue causing the backfire symptom.

If your Check Engine Light is on, take your vehicle to a local auto parts store or mechanic to have the stored diagnostic code read, usually for free. You can also read the code yourself with an inexpensive and DIY-friendly code reader. The code(s) can usually narrow down what specific component is causing the backfire.

From there, your best bet is a mechanic or dealership, the latter if the vehicle is still under warranty. Yes, failing spark plugs could potentially cause backfiring, but changing out the plugs when the issue is really the coil packs is just throwing money away. Professional diagnosis can potentially save you money and get you back on the road quickly.