Backfiring engines are the result of explosions behaving badly
My four-year-old loves all things on wheels, but he hates loud noises more. So when a custom pickup truck passing us on the street started backfiring as it sat at a stoplight, his admiration quickly turned into dread. He couldn’t see why, but this was clearly an offensive, if slightly scary, truck. Understanding that this was a mechanical, not moral, problem with the truck’s engine helped make sense of things though. “That truck needs fixing!”
To understand what’s going wrong during a backfire, it’s handy to keep in mind what’s supposed to happen in an internal combustion engine. In each cylinder, a small amount of gasoline and air is introduced to the cylinder, at which point an electric spark from a spark plug ignites the fuel, resulting in a small, controlled explosion. That quickly builds up an enormous amount of pressure, pushing the piston at the bottom of the cylinder down to provide power to your crankshaft. As the piston rises, the exhaust materials from the burnt fuel and air are vented out, making room to start the cycle over again.
As with anything explosive, the steps and amounts of each ingredient don’t allow for a lot of wiggle room before you get unintended results. For instance, if you have air filter problems, you’re likely to end up with too much fuel entering the cylinder. Without the correct proportion of oxygen, some of the fuel will be left over, slowly burning until the exhaust valve opens. At that point, more O2 will enter the cylinder, and the excess fuel will quickly burn off, resulting in a second explosion that can be heard as a loud pop. If the timing is off on your engine, a similar scenario can happen where the exhaust valve is opening before the fuel has burned off completely, again resulting in some of your energy being blasted out the exhaust pipe rather than utilized by your car.
The spark that lights the fuel can do their own version of this kind of leaky, mistimed operation as well. A cracked distributor cap can allow for moisture to make a sort of crude circuit between two wires, and power in one will cause a neighbor to spark at the wrong moment. Burnt carbon between spark plugs or ignition coils can do the same thing, where the carbon track acts a conduit for the electricity, making the wrong cylinder spark before it’s optimal to do so.
Potentially worse than loud pops
Aside from scaring small children, there can be negative consequences to frequent backfiring. It makes your car less efficient at a minimum, as fuel is being burnt or exhausted without providing power to your crankshaft. Over time it can cause damage to various components thanks to unintended vibrations and friction. Finally, some fuel components might end up producing black or blue smoke, as not everything is being burned in the cylinder as it should. With any luck, the pickup truck my four-year-old and I heard will just need a new air filter to quiet down, since some of the above can be a bit pricier to deal with.
Source: What Causes a Car to Backfire? by Jason Unrau, Your Mechanic