What Happens When a Car Misfires?

A misfire indicates there’s something wrong with what keeps a car’s combustion process going. Here’s what it could be, and how to avoid it.

Every internal combustion engine, like those in a car, needs three things to function: fuel, air, and spark. Having the correct amount of all three every time the engine is turned on—or each time the engine’s cylinders fire to maintain continual operation—ensures the engine will be able to perform its job without issue for many thousands of miles, assuming the vehicle’s owner performs proper maintenance at regular recommended intervals.

However, when the engine is deficient or has too much of one or more of the three essentials to run, that imbalance may cause what’s called a misfire, preventing the engine from running properly. Here are some common causes of misfires.

When Something Is Off

A misfire occurs when the correct amount of fuel, air, and spark aren’t present at the correct time. This could be caused by one or several variables.

• Spark-related: Either the spark plug—which ignites the air/fuel mixture to begin combustion—isn’t firing enough, or not at all. A bad spark plug, spark plug wire, coil pack, coil, or ignition control module are all potential causes; it depends on what the car’s engine uses for ignition. This requires some troubleshooting to pinpoint and remedy and is often best left to a professional repair shop to figure out.
• Fuel-related: If the fuel pump isn’t functioning correctly, or the fuel filter is clogged, not enough fuel is being injected into the cylinder to create healthy combustion. Additionally, a stuck-open or clogged fuel injector could cause a misfire as well.
• Mechanical misfire: This is when the engine has worn out due to a slipped timing belt or chain, a tattered belt or chain tensioner, or a damaged valve. Mechanical misfires are very serious because the timing between the piston and valves is off, and the two are at risk of colliding and causing major damage inside the engine.
• Vacuum leak: A vacuum powers a lot of inside-the-engine processes. For instance, if not enough vacuum is actuating certain components associated with a turbocharger, either too much or too little air might get sent to the combustion chamber.

What a Misfire Feels Like

From the driver’s seat, a misfire feels like a coughing or stumbling happening under the hood. The methodical, rhythmic operation of the engine is interrupted, and those in the cabin feel it in real time.

A misfire could happen as the car sits at idle, is driving down the road at a constant speed, or during acceleration or braking. It most commonly happens under what’s called load, meaning the car is accelerating down the road under its own power. Depending upon the amount of acceleration needed, this is when fuel, air, and spark are needed the most. So if there isn’t enough of one or more in the equation, it’ll be most apparent during acceleration.

Misfires under load may lead to poor or no acceleration too, which could become a major safety issue on the road. Misfires ought to be remedied as soon as they occur.

What Should One Do if Their Car Is Misfiring?

Besides being a potential safety risk, there’s also a high likelihood that an unresolved misfire could cause major damage to the engine, resulting in an expensive repair bill. Therefore, it’s best to have it fixed as soon as possible.

A misfire could be followed by what feels like a large jolt or bang; this is when unburnt fuel is ignited when it wasn’t supposed to be. This incorrect fuel ignition not only puts a lot of strain on the engine, but also potentially damages the car’s catalytic converter, which is an expensive emissions-related component to replace. 

On the less-expensive end, a misfire will cause poor drivability and negatively affect fuel economy. However, the cost to remedy goes up exponentially if it leads to a damaged catalytic converter or oxygen sensor. For example, the replacement cost alone for a catalytic converter ranges between $300 and $2,500 on average, not including the labor hours required to perform this repair.

In general, fixing a misfire isn’t an expensive proposition, as most are caused by worn out ignition components that are generally easy to replace. However, if not remedied sooner than later, it could cause these much larger problems to arise. Therefore, it’s best to remedy them as soon as possible.

Luckily, most misfires cause the check-engine light to illuminate, giving the driver a clear message that something is wrong with the engine. A trained mechanic is able to use diagnostic equipment to determine the specific issue and how to repair it. It is also important for the car owner to be aware of the overall condition of the vehicle and follow the manufacturer’s recommended service schedule closely to help prevent these kinds of issues.

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