What Causes a Car to Backfire?

You’ve got an image of it in your head right now. An old car drives off jerkily from a stop sign, tires screeching as it lurches and bucks, with shotgun bangs coming from the tailpipe. The exhaust is puffing blue-black from the tailpipe and the ancient sedan shudders to a stop. A final pop from the exhaust closes this familiar scene.

It’s an exaggeration of how a backfire actually occurs, but it’s not totally inaccurate. The smoky exhaust indicates an inefficiently performing engine. The jerky driving spells out that it’s out of tune. And that final detonation at the end is actually fairly common in this situation.

What Makes a Car Backfire?

Backfiring is the process of a spark plug, or multiple spark plugs, igniting the fuel in their cylinder out of turn, in a part of the combustion process where the exhaust valve is open on that cylinder. Here are some causes of that delayed detonation that may clear it up a bit more.

Running too rich

If your engine is being supplied more fuel than it needs to burn efficiently, it’s called a rich fuel to air mixture. It can be caused by a multitude of issues including some as straightforward as a dirty air filter. When an engine runs too rich, there is too much fuel to create an explosive, fast-burning flame. When that happens, the fuel burns slower, and isn’t complete before the exhaust part 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.

Engine timing is incorrect

Specifically, delayed timing causes a backfire. This is better known as retarded timing. What that means is the engine cycle of fuel-compression-ignition-exhaust in the top end (cylinder head) isn’t in sync with the bottom end(cylinder block). This causes the ignition cycle to begin late in the combustion chamber and ignite the fuel as the exhaust valve is opening.

Cracked distributor cap

In vehicles that don’t have ignition coils on the spark plugs, a distributor cap and wire set are used to disperse the electrical pulse to the spark plugs. This electrical pulse is what causes the spark plug to spark and ignite the fuel in its cylinder. If a distributor cap is cracked, moisture can get in and cause the spark from one cylinder to track to another, incorrect cylinder. When the incorrect cylinder fires out of time when the exhaust valve is open, you will experience a backfire.

If your vehicle is equipped with a distributor cap, have it replaced when a tune-up is performed as part of your regular preventative maintenance.

Carbon tracking on spark plug wires

There are a couple different scenarios where carbon tracking can come into play. In a design that incorporates a distributor cap, all the spark plug wires are attached to the top of the distributor cap. Over the course of time, the environmental elements can cause the spark to cross over from one spark plug wire to another in close proximity. When that happens frequently a carbon track forms, which is like a shortcut for the spark. It causes a misfire very similar to that of a cracked distributor cap.

Carbon tracking can also form on spark plug wires or ignition coils that are mounted directly onto the spark plug. In the same way, part of the spark takes an incorrect path, and the remaining spark isn’t enough to ignite the fuel, leaving some in the cylinder. The next ignition may be enough to fire the spark plug, this time with extra fuel in the cylinder. The flame doesn’t burn as explosively and isn’t complete before the exhaust valve opens. The rapid burn that occurs while the exhaust valve is open causes a backfire.

Almost all backfire situations will have other symptoms attached to them such as a Check Engine Light illuminated. A backfire is a sign that your car isn’t running efficiently and needs to be addressed in short order.


Next Step

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Clarissa

27 years of experience
234 reviews
Clarissa
27 years of experience
Oldsmobile Cutlass Ciera V6-3.1L - Engine is backfiring - Wayne, Pennsylvania
Clarissa was extremely knowledgeable about the car’s engine, and was able to tell me what needed to be repaired in a way that I easily understood. She seemed to be very honest and trustworthy, and left me with the feeling that there would be no “paraded” charges unlike some car repair places. She was respectful and empathetic to my desire to save money.

Javier

11 years of experience
59 reviews
Javier
11 years of experience
Mazda B2600 L4-2.6L - Engine is backfiring - Tomball, Texas
Javier contacted me because previous bookings ended earlier than scheduled and I was able to have him service my car before my book time which was fantastic! He was quick and knowledgeable and showed me what was wrong and answered all of my questions. I would definitely recommend him and we'll have him come service my vehicle once the parts are in.

Peter

43 years of experience
1321 reviews
Peter
43 years of experience
Ford Ranger V6-4.0L - Engine is backfiring - Phoenix, Arizona
Very professional, showed up ,took the time to explain all matters concerning his visit.very likable man.honest and caring. Thank you Peter for your help.. .Greg Visnich

Scott

36 years of experience
898 reviews
Scott
36 years of experience
Chevrolet Corvair H6-2.4L - Engine is backfiring - Mesa, Arizona
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