FixMyCar discusses six reasons why your car might be misfiring and how to fix them.
Book a car repair nowEllie Dyer-Brown
Monday August 11 2025
7 min read
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.
In a nutshell: Your car might be misfiring due to a problem with the ignition system, a leaking inlet manifold gasket, low fuel pressure, sensor problems, low compression or a faulty ECU.
To understand 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 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 process 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 happen at all.
A misfire happens when the engine isn’t firing on all cylinders. The most common causes are a weak spark from bad spark plugs, coil packs, or a faulty battery. Fuel issues, like a failing injector, can also cause misfires, and on the mechanical side, things like a blown head gasket or worn piston rings can lead to compression loss. If your car’s misfiring, it’s best to get it checked by a professional to pinpoint the exact cause.
Matt Wrankmore
Head of FixMyCar's Garage Network
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 undoubtedly help. Usually, your car will stumble and lose power before regaining its speed.
The effects can be drastic, even if just one spark plug isn’t firing correctly. 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.
The sound of an engine misfiring is distinctive in terms of car noises. It is usually 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.
A problem with the ignition system can cause your engine to misfire.
Spark plugs are one of the most well-known parts of this system and are relatively affordable to replace. They provide the spark needed to ignite fuel in the combustion chamber; even if just one has stopped working, your engine won't be able to fire on all cylinders.
Learn about the other symptoms you might experience when your spark plugs need replacing.
The crankshaft position sensor can also sometimes cause misfiring. Its purpose is to monitor the rotational speed or position of the crankshaft, and this information is used by the engine control unit (ECU) to determine the fuel injection and ignition system timing. A faulty sensor will mess up the timing, resulting in a misfire.
Signs of a bad crankshaft sensor include:
Misfiring engine
Hard starting
Another component that could be at fault is 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 plug. The symptoms of a faulty ignition coil are similar to worn spark plugs.
Whether your car is misfiring due to a worn spark plug, faulty sensor or something else entirely, the ignition system is vital, so it’s essential to have it checked by a mechanic as soon as possible.
The manifold gasket is located between the engine block and the inlet manifold; its purpose is to seal the engine, preventing fluids from mixing. However, it can sometimes start to leak, usually after many years of use. This problem is more common in older cars with steel gaskets, but it can occasionally happen in newer cars.
No matter your car's age, it’s worth opening the bonnet and looking for leaks around the intake manifold gasket or broken vacuum hoses. This guide provides advice about identifying and fixing leaks.
If you find a leak, or even if you don’t feel confident checking yourself, a mechanic can help you get back on the road quickly.
Several things could cause your car to have low fuel pressure.
One of the first things to check is whether your fuel filter is clogged. It usually needs replacing every two to three years - the correct maintenance interval should be supplied in your vehicle handbook - and may be included as part of an annual service.
A defective fuel pressure regulator could also be to blame. It works by altering the fuel flow to the injectors to maintain the correct pressure. When it stops working as it should, you'll likely experience performance issues, black smoke from the exhaust, an illuminated check engine light and misfiring.
A weak or faulty fuel pump is the most common cause of low fuel pressure. 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.
Running lean means there isn’t enough fuel in the mixture to burn; in this situation, all four cylinders will misfire.
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, allowing it to control things such as spark timing and fuel delivery. When the information is incorrect, it can cause your engine to misfire.
Low compression means the engine cylinders can't compress enough air, often due to a defective engine timing belt (also known as a cambelt). This component plays a vital role in synchronising the movement between the camshaft and the crankshaft. Check out this guide if you want to learn more about timing belts.
You can test your engine's timing by purchasing a timing light, but this can be a complicated process to master, and you might still need to take your vehicle to the garage for repairs.
Other things can cause low compression, including:
Worn piston rings
Leaking valves
Holes or cracks in pistons
Blown head gasket
Cracked cylinder walls
Since so many things can cause low compression, it may be better to book a diagnostic test rather than try to identify the problem yourself.
The engine control unit monitors and adjusts numerous systems in your car. One of the major things it controls is when the spark plugs fire; a faulty ECU may send incorrect signals, causing them to misfire.
When something goes wrong with the ECU, the check engine light will almost certainly appear on your dashboard, and there will be a noticeable drop in your vehicle's performance.
The average cost of an ECU repair on FixMyCar is £170.87.
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Ellie is FixMyCar's Content Manager. She has over three years of experience writing about cars and regularly collaborates with automotive experts to provide trustworthy advice for drivers that is easy understand. Her work has been featured in Yahoo! Finance, iNews, The Daily Express and The Sun. She has a BA in English literature and an MA in creative writing from Durham University. Outside of work, Ellie follows F1 and eagerly awaits Ferrari's next era of dominance in the sport. She drives a Suzuki Swift.
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