What Causes a Car to Backfire?

We all have the image in our heads; a junky old car is clunking around on the road with what sounds like gunshots emitting from the tailpipe. While there are not literal gunshots originating from the vehicle, it’s not a gross embellishment of what is actually occurring. The loud pops and smoke-filled exhaust is a clear indication that the engine is performing inefficiently.

So what causes a car to backfire? Is it bad for my car? And how can I prevent my car from backfiring?

What Makes a Car Backfire?

A backfire is the result of a spark plug (or multiple spark plugs) igniting the fuel in their cylinders out of turn. This is a part of the combustion process in which the exhaust valve is open on that cylinder, which contributes to the potential of backfires. If you are curious as to what causes this delayed detonation, consider taking a look at the following things:

Is Your Distributor Cap Cracked?

For cars and trucks that don’t have ignition coils on the spark plugs, a distributor cap and accompanying wire set are used to disperse electrical pulses to the spark plugs. It’s this electrical pulse that causes the spark plug to actually spark and ignite the fuel within its cylinder. If your vehicle’s distributor cap is cracked, it can allow moisture to enter, which causes the spark from one cylinder to track to a separate, incorrect cylinder. When the erroneous cylinder fires out of time during a period when the exhaust valve is open, it can cause a vehicle to backfire.

For vehicles that are equipped with a distributor cap, it’s advised to have it replaced during your next tune-up or regularly scheduled maintenance.

Is your Car Running Too Rich?

A rich fuel to air mixture occurs when your engine is being supplied more fuel than it needs to efficiently burn. This can be caused by a number of different issues, from something as simple as a dirty air filter or faulty air flow sensor. But regardless of the cause, when an engine runs too rich, there is too much fuel to create an explosive, fast-burning flame that creates efficient combustion. When this happens, it causes the fuel to burn more slowly, and the combustion is incomplete before the exhaust portion of the engine cycle. When the exhaust valve opens on that cylinder, the extra air allows the unburned fuel to explosively burn and the popping noise of a backfire is heard.

Is There Carbon Tracking On The Spark Plug Wires?

Carbon tracking is when a contactor is de-energized and pulls away, creating an arc that creates a carbon bi-product from the hot electrical arc. Over time, carbon collects and builds on the contactor. There are a few scenarios in which carbon tracking can be in play.

For example, in engine designs that include a distributor cap, the spark plug wires are all attached to the top of the distributor cap. Over time, environmental elements can start to cause the spark to cross over from one spark plug wire to another within close proximity. If this begins to frequently occur, a carbon track begins to form. Carbon tracks act as a shortcut for the spark, disrupting its intended path. This causes a misfire much in the fashion that a cracked distributor cap does.

Spark plug wires or ignition coils that are mounted directly onto the spark plug can also produce carbon tracking. Much in the same way it occurs on top of the distributor cap, part of the spark takes an incorrect path and the remaining spark isn’t sufficient in igniting the fuel. This leaves some fuel remaining in the cylinder, and if the next ignition is enough to fire the spark plug, it may also ignite the extra fuel in the cylinder that remained from the previous fire.

In this case, the flame doesn’t burn as explosively as it should, and the explosion is incomplete prior to the exhaust valve opening. This rapid burn occurs while the exhaust valve is open, and that’s what causes the vehicle to backfire.

Is The Engine Timing Incorrect?

One of the most common causes of backfiring is delayed timing, or more commonly known as retarded timing. Delayed timing is when the engine cycle of fuel compression ignition exhaust in the cylinder head is out of sync with the cylinder block. This causes the ignition cycle to start late in the combustion chamber, which ignites the fuel as the exhaust valve is opening and causes a backfire.

Is a BackFire Bad For Your Car?

As we have outlined throughout this piece, a backfire is an indication that your engine is not properly functioning. Backfiring can cause damage to your vehicle's exhaust or intake when left unaddressed. It is also a sign that your vehicle’s engine isn’t creating as much power as it should, which leads to a severe decrease in fuel efficiency.

How To Prevent a Car from Backfiring

For driving enthusiasts and drag racers, backfiring is considered to be cool. Hearing those loud pops and watching the flames propel out of a race car exhaust can be exhilarating. But while it may be exciting in many ways, it’s not good for a car when it occurs with frequency. And it’s almost never a good thing when it occurs on a non-race car. While the modern engine control systems are designed to alleviate most of the issues that lead to backfiring, there are a couple of things you can do to help prevent your car from backfiring.

1. Try To Stop Air Leaks

In the assumption that components such as your idle air control valve and mass airflow sensor are in working order, look for frayed, unattached, or missing vacuum hoses in your engine bay. With a little time and a bit of tracing, they’re typically accessible and simple to remedy if found in non-working order. Stopping or preventing air leaks is an inexpensive, effective fix for backfiring if it's the root of the cause.

2. Freshen Up The Spark

Our forefathers were practical and observant vehicle owners who commonly had to check and clean their spark plugs just to keep their car in proper working order. Today’s generation has it far easier than our parents or grandparents did (outside of new car prices), but that doesn’t mean we can overlook changing the plugs and plug wires when needed. Refer to the owner’s manual for your vehicle to find out the proper replacement interval for your spark plugs, and be cognizant of anything other than the normal wear-and-tear on your plugs. Fresh spark plugs can often help to prevent backfiring.

3. Change Your Vehicle’s Oxygen Sensors

Though this is not a common component to change or replace on your vehicle, it can help to prevent backfiring when a fresh sensor or two is installed. These sensors detect the oxygen level in the fuel system and relay the information to the engine control unit. If your vehicle has over 75,000 miles on it, you should strongly consider replacing these sensors anyway, as they slowly wear over time. If changing these sensors alone fails to fix the backfiring, your engine will still run smoother and more efficiently, and your gas mileage will also increase as a result.

4. Maintain a Healthy Exhaust

Probably the most noticeable backfiring occurs in the exhaust system, where the catalytic converter must endure quite a bit of wear and tear even under normal conditions. Backfiring can often be an indication that the catalytic converter is no longer functioning properly, or could be nearing the end of its service life. Ensure that you are maintaining a healthy exhaust and monitor your catalytic converter, particularly if your vehicle is experiencing backfires.

5. Check engine belts

Depending on what type of vehicle you own, you could potentially have multiple belts in the front of the engine, or sometimes only a single serpentine belt to drive multiple components of the engine block. No matter which belt system your vehicle uses, the belts in your car wear over time and the tension loosens as you add on miles. Over time, this can eventually throw off your engine timing, and the control unit may not be able to fully compensate for it any longer. These damaged or ill-functioning belts can lead to backfires, so ensure your belts are in proper working order.