Technically, a misfire is the result of incomplete combustion (or zero combustion) inside one or more of an engine’s cylinders. But to you, the driver, the problem will usually feel like hesitation or shaking when the car is running. On modern vehicles, the check engine light will also pop on when there’s a misfire.
When the check engine light illuminates, your car’s primary computer, which is often referred to as the powertrain control module (PCM), will store a diagnostic trouble code (DTC) in its memory. Codes P0300 to P0312 are the primary DTCs associated with an engine misfire.
Most Common Causes of an Engine Misfire
Okay – let’s say your car has an engine misfire. The question is: Why is the problem happening? Unfortunately, there are many potential causes, so answering that question isn’t always easy. If your car is suffering from a misfire, it’s best to let a professional figure out why.
Here is what a mechanic might find when diagnosing the issue:
1. Ignition system problems
When most people hear the term misfire, they think of worn out spark plugs. What they don’t realize is the spark plugs are just one part of the ignition system. A typical modern ignition system contains a variety of components, including the control module, crankshaft position sensor, coil packs, wiring and, of course, the spark plugs. Issues with any of these parts can result in an engine misfire.
2. Air and fuel delivery problems
Air and fuel mix together inside the engine, then the mixture is ignited by the spark plug. The explosion sets the engine in motion, creating the rotational force needed to propel your car down the road. Any issue that throws off the air/fuel mixture – ranging from a failed fuel injector to a vacuum leak – can cause a misfire.
3. Emissions equipment problems
Late-model cars have an array of emissions equipment that helps minimize the amount of pollution released into the atmosphere. A couple of examples include the exhaust gas recirculation (EGR) system and the positive crankcase ventilation (PCV) system. In some cases, issues with emissions equipment can alter the engine’s air/fuel mixture enough to create a misfire.
4. Engine mechanical problems
Many people also don’t realize that an engine mechanical problem can cause a misfire. Each cylinder inside the engine contains a piston that must compress the air/fuel mixture for complete combustion. And when the piston is moving upward, the cylinder must remain completely sealed off to create adequate compression. Internal engine problems can prevent the cylinder from sealing properly, leading to a loss of compression and an engine misfire.
5. Sensor and module problems
Today’s vehicles contain a plethora of sensors, many of which the PCM uses to determine control of critical functions, such as fuel delivery and spark timing. A such, sensor problems can easily contribute to an engine misfire. Although relatively rare, a problem with the PCM itself can also cause a misfire.
6. Control circuit problems
All of the input and output engine management devices (i.e., sensors, ignition coil packs, etc.) are connected where needed via electrical circuits. Problems within these circuits, such as damaged wiring or a loose connection, can cause an engine misfire.
Is It Safe to Continue Driving with an Engine Misfire?
You should not continue driving with an engine misfire. Although the car may run well enough to get you where you’re going, you risk potentially damaging costly components, such as the catalytic converter and the engine itself. That’s why you should have a professional diagnose and repair a misfire right away.