What Causes a Car to Backfire? (How to Fix)

A car backfiring can be a startling (not to mention taxing) event to deal with. Not only does it surprise, but its occurrence also indicates a potentially more serious engine problem.

Thankfully, most modern cars are now computer-controlled, reducing the likelihood of a backfire. Still, it pays to know how to address the problem.

Determining what triggers car backfiring simplifies diagnosing and resolving the problem. Here is a list of the most prevalent reasons why cars backfire:

1. Out-of-spec air-fuel ratio
2. Engine timing inaccuracy
3. Damaged distributor cap
4. Carbon tracking
5. Dilapidated air gulp valve
6. A weak or failing battery
7. Contaminated or malfunctioning O2 sensor
8. Defective mass air flow sensor
9. Misshapen or bent valves
10. Older engines

As detailed above, some reasons behind a car backfire include warped valves, defective sensors, a failing battery, and an engine running lean/rich. In other cases, it could be due to dirty components.

While most causes may only warrant proper upkeep, others may require the expertise of a professional.

What Causes a Car to Backfire?

1. Out-Of-Spec Air-Fuel Ratio

More commonly known as “running rich,” this condition occurs when the power mill develops an issue with its combustion process and sometimes happens during acceleration. In this situation, the supply of fuel the engine gets is more than it can burn, resulting in the exhaust valve spitting out unburnt fuel.

There are many causes for an engine running rich. But as experienced car owners have observed, a dirty or plugged filter is the most common of these culprits.

The accumulated filth in the air filter prevents the combustion chamber from burning all the fuel before the exhaust valves open, causing unburnt fuel to come in contact with red-hot exhaust headers (view on Amazon).

The other side to this scenario is “running lean.” And between the two, this is considered the main cause of a car backfire. When an engine has inadequate fuel but too much air, slow fuel burning and excess air/unburnt fuel are the outcomes.

A lean mixture can be traced back to several factors like clogged fuel injectors, a plugged fuel filter, or a failing fuel pump (view on Amazon) — all of which should be replaced the moment they wear out.

2. Engine Timing Inaccuracy

Engine timing inaccuracy simply means that the fuel-compression-ignition-exhaust cycle in an engine — which is supposed to be simultaneous between the cylinder head (view on Amazon) and the bottom of the cylinder — is out of sync.

This causes the ignition cycle to begin late in the combustion chamber, leading to unburnt fuel being spit out of open exhaust valves.

3. Damaged Distributor Cap

If your vehicle has a distributor cap instead of ignition coils on the spark plugs, you may want to visually inspect the former for any cracks or damage. The reason is that distributor caps play a crucial role in the distribution of electrical pulses across spark plugs — on top of keeping moisture out.

If distributor caps get warped or broken, chances are spark ignition (which ignites the fuel) will not take place since moisture leak will disrupt spark synchronization and, ultimately, car backfiring.

4. Carbon Tracking

Carbon tracking is carbon inside the distributor cap that leads away some electricity. It is usually associated more with misfiring than car backfiring in older cars. Nonetheless, it can happen to all types of vehicles — whether they utilize distributor caps or ignition coils for the spark plugs.

Unlike conventional ignition, carbon tracking causes the ignition spark to travel around the outside of the spark plug (as opposed to jumping the gap in the electrodes).

In the case of distributor caps, traces of carbon cause the spark to “go crossways from one wire to another.” If this ‘spark shortcut’ occurs too often, the resulting spark will be why a car is backfiring.

Symptoms are almost similar for cars with ignition coils. The spark also travels uncharacteristically, going through the wrong route and leaving the cylinder spark insufficient for burning fuel. This mishap creates a domino effect, eventually leading to a rich fuel mixture and a car backfire.

5. Dilapidated Air Gulp Valve

Cars with air injection systems would automatically have an air gulp valve.

The important thing about this component is that it remains closed while directing air from a vehicle’s air pump to its exhaust system. It should only let air into the intake manifold during deceleration. Otherwise, it cannot do its objective of creating a lean fuel mixture that helps prevent backfiring.

6. A Weak or Failing Battery

Another answer to the question, “What causes a car to backfire?” is a battery running low on voltage or with corroded terminals.

Out-of-shape batteries are highly likely to cause intermittent sparks, eventually leading to fuel buildup on the cylinders. The latter reacts with force when ignited, causing your vehicle to backfire.

Thankfully, keeping your battery condition and levels up to spec is easy. Use a voltmeter like Power Probe III w/Case & Acc-Fire PP319FIRE (view on Amazon) to get a reading of your battery’s running voltage.

You may have to jolt your battery to life or replace it entirely if the readout is less than 12.6V (or whatever is OEM-specified).

7. Contaminated or Malfunctioning O2 Sensor

This input sensor measures the amount of unburnt oxygen in the exhaust. A vehicle’s ECU then determines the right air-fuel ratio for the engine based on the data it provides.

In working condition, the O2 sensor gives pretty accurate data. Conversely, a damaged or bad O2 sensor will lead to the engine running lean or rich (resulting in car backfiring).

8. Defective Mass Air Flow Sensor

Functionally, the MAF sensor is similar to the O2 sensor, except it measures air entering the engine. However, the effect is the same — a faulty or contaminated MAF sensor will cause incorrect air-fuel ratio readings, leading to a lean or rich mixture condition and a car backfire.

9. Misshapen or Bent Valves

Car owners know all too well how vital intake and exhaust valves are to optimal vehicle performance. These valves allow air and fuel into the cylinders, lock them in during combustion, and let the fumes out the tailpipe once air and fuel have been properly combusted.

Simply put, these valves should always be in good condition to carry out their sealing duties. The moment they become bent, the closed nature of the combustion process is compromised.

Unfortunately, the outcome is not only car backfiring but also sky-high expenses. The cost of replacing deformed valves is not something to joke about, as it warrants dismantling a vehicle’s engine.

10. Older Engines

Lastly, we have vintage power mills to blame for a car backfire. This is especially true for owners of classic rides — those four-wheelers with old-school carburetors always starving for adjustments and tune-ups.

The powerplant responsible for bringing these icons to life are from days ago, and so is their configuration.

This is not to say that all vehicles with older tech automatically backfire. However, they are more prone than modern vehicles. Ignition, fuel system, and a controller module are a few areas to look into.

These already show significant dissimilarity in technology and design efficiency affecting the respective vehicles’ combustion process.

Factors like a problematic control module and mechanical failures may also cause a car to backfire. However, I did not include them in the list as they are pretty uncommon, if not intertwined with other car dilemmas.

Seek assistance from a professional mechanic if you suspect any other cause not detailed in this section.

How to Fix a Car Backfire

On top of the fixes discussed in the previous section, here are other ways how you can turn off the constant wailing of your vehicle:

Step 1

Examine the Engine Warning light or CEL, as most culprits to a car backfiring set off these warning lights.

Sometimes, the onboard computer may even engage “Limp Home Mode” to reduce engine power and prevent further drivetrain damage (source: Family Handyman). This step should make diagnosing the problem a tad easier for you.

Step 2

If you hear a low rumble or muffled pop, let the engine cool down before inspecting under the hood for loose/damaged hoses and frayed wires. Do not worry if you find either, as wire and hose repairs are DIY fixes.

Step 3

Next, check for a leak in the upstream exhaust. If the leak is not readily visible, keep an ear out for a hissing or sucking noise (indicative of a vacuum leak) and follow the sound to its source.

Sealing all visible exhaust leaks is crucial to ensuring no pressure pulse anomalies occur.

Step 4

Remember to inspect the air gulp valve. Most cars have 1—2 gulp valves per exhaust manifold (view on Amazon). The most telling indicator of a bad gulp valve is if air flows both ways through it.

Step 5

Check the spark plugs for fouling or discoloration (among other indicators). If it does not appear in poor condition on the outside, check for battery resistance (should be zero).

Replace affected plugs immediately if bad spark plug symptoms surface.

Step 6

See if you have defective O2 or MAF sensors. Your vehicle’s onboard computer heavily relies on these sensors when collecting data and monitoring different vehicle functions.

If these sensors become defective or worn, they send inaccurate data to the onboard computer, resulting in lean fuel mixtures and a higher propensity for car backfiring.

Typically, these sensors are just plugged and need a thorough cleaning. However, part replacement is advised should they turn out damaged.

Conclusion — Why Do Cars Backfire?

In conclusion, here are the top 10 answers to the question, “What causes a car to backfire?”:

1. Out-of-spec air-fuel ratio
2. Engine timing inaccuracy
3. Damaged distributor cap
4. Carbon tracking
5. Dilapidated air gulp valve
6. A weak or failing battery
7. Contaminated or malfunctioning O2 sensor
8. Defective mass air flow sensor
9. Misshapen or bent valves
10. Older engines

Most of the expert advice available in online forums is already embedded in the above steps detailing how to fix a car backfire.

Ultimately, you will have to be religious with visual inspection and scheduled maintenance and replace damaged/wear-prone components before they give out. Of course, this pre-emptive action is not always feasible — which is why today’s guide is here to help you.