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.
This article originally appeared on YourMechanic.com as What Causes a Car to Backfire? and was authored by Jason Unrau.