Why is my car misfiring?

Engine misfiring can be an unpleasant experience, not only for you and your passengers but also for nearby pedestrians. In addition to loud popping sounds, you might notice thick plumes of smoke coming from the exhaust of your car and a distinct smell of fuel. 

So, what actually causes car engines to misfire, and how do you stop it from happening? Let's take a closer look.

Contents:

What is an engine misfire?

What does an engine misfire feel like?

What does an engine misfire sound like?

Five reasons your car could be misfiring

What is an engine misfire?

To understand exactly what a misfire is, you need some basic knowledge of how car engines work. 

They operate in four steps:

• The piston goes down and fills the cylinder with a mixture of air and fuel

• The piston goes up again, compressing the mixture under high pressure

• The ignition from the spark plug ignites the mixture, and this explosion pushes the piston down while rotating the crankshaft

• Finally, the piston goes up, emptying the burned mixture through the exhaust

This is where the term “four-stroke engine” comes from.

A misfire happens when one or more of these crucial stages is either wrong or doesn’t occur at all.

What does an engine misfire feel like?

When you’re driving, you can’t exactly look at the back of the car to see if it's billowing with smoke, and even if you hear a popping sound that seems like a misfire, how can you be sure? 

Knowing what a misfire feels like can certainly help. Usually, your car will seem to momentarily stumble and lose power before regaining its speed.

Even if just one spark plug isn’t firing correctly, the effects can be drastic. You will likely notice excessive exhaust smoke (which you may be able to see in your rear-view mirror), a lack of acceleration and less power when you put your foot down.

What does an engine misfire sound like?

The sound of an engine misfiring is fairly distinctive in terms of car noises. It is usually extremely loud and can be likened to a kernel of popcorn popping in the pan. Chances are, you’ve probably heard an engine misfiring at some point before, even if you didn’t realise at the time. 

That being said, different engines produce slightly different sounds in the event of a misfire; some will make a chugging noise, for example.

Five reasons your car could be misfiring

Ignition system

A problem with the ignition system is the most likely cause of engine misfiring. Spark plugs are one of the most well-known parts of this system and are relatively affordable to replace, but the fault could be with any of the components, including the control module and crankshaft position sensor. 

In fact, another component which links to the spark plugs could be at fault: the ignition coil. Some cars have one coil with a sparking cable that goes to each spark plug, while others have a different coil for each spark plug. 

Either way, your ignition system is vital so it’s important to have it checked and repaired by a mechanic as soon as possible.

Intake manifold gasket leak

This problem is more common in older cars which have steel gaskets, but it can occasionally happen in newer cars as well. 

No matter the age of your car, it’s worth opening the bonnet and having a look for any leaks around the intake manifold gasket or broken vacuum hoses. In the event that you do find a leak, or even if you don’t feel confident checking yourself, a mechanic can help you get back on the road in no time. 

Low fuel pressure

A number of things could cause your car to have low fuel pressure, such as clogged fuel filters or a defective pressure regulator. Fuel filters can easily be replaced by a mechanic, and this is routinely done when you book your car in for a service.

One of the most common causes, however, is a weak or faulty fuel pump.

Even if your fuel pump provides slightly less than the required pressure, it can cause problems - most notably, your engine will run lean and misfire as a result. 

Running lean means that there isn’t enough fuel in the mixture to burn; in this situation, all four cylinders will misfire at once.

Sensor problems

Technology has come on in leaps and bounds over the last decade, but many of these advances come with the added risk of malfunctions. For example, modern cars are equipped with a multitude of sensors which gather information for the ECU. 

The ECU controls things such as spark timing and fuel delivery based on this information. When the information is incorrect, it can cause your engine to misfire.

Low compression

Low compression is most often caused by a defective engine timing belt (also known as a cambelt). This component plays an important role in synchronising the movement between the camshaft and the crankshaft. If you’re interested in learning more about timing belts, check out our guide here.

You can test the timing of your engine by purchasing a timing light, but this can be a difficult process to master and you might still need to take your vehicle to the garage for repairs. The best thing to do is have a trained mechanic diagnose and fix the problem for you.

Enjoyed reading this article? There's more where that came from! 👇

• How to find out if your car has a timing chain or a timing belt

• What is an ECU?

• Why does my car lose power?