What Is a Misfire and What Causes It?

A misfire means that a cylinder in your engine isn’t producing the power it should because the air-fuel mixture in it didn’t properly ignite and burn. It could be an intermittent problem — meaning it doesn’t happen all the time — and it could apply to more than one cylinder. But what exactly causes a misfire? 

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What Are the Symptoms of a Misfire?

A cylinder misfire usually results in a rough idle and/or stutter when accelerating — both of which can often be heard as well as felt — along with the illumination of the Check Engine light. Modern engines can usually compensate for a misfire enough to allow the car to be driven fairly normally, but that shouldn’t be your long-term solution.

Besides lowering fuel economy and increasing exhaust emissions, a misfire may limit acceleration (which you might need in a pinch) and worse, such as damage to the expensive catalytic converter by allowing unburned gasoline to flow through it. Furthermore, another problem could occur that triggers the Check Engine light, but you wouldn’t be aware of it because you’re already ignoring the light.

As such, it’s important to get a misfire diagnosed and fixed, as it may well end up being the cheaper option in the long run.

How Does an Engine Produce Power?

Three things are required for a cylinder to produce power:

• The correct air-fuel ratio needed for that mixture to burn
• A sufficient amount of compression
• A spark occurring at the proper time 

Knowing these three things can help you with diagnosing a misfire.

Correct Air-Fuel Ratio

For fuel to burn in the cylinder, it has to be mixed with the proper amount of air, which contains the oxygen needed for combustion. That mixture is roughly 15 parts of air (by weight) to every one part of gasoline (written as 15:1), though it can vary a bit and still ignite; in fact, producing peak power or starting when cold requires about a 12.5:1 ratio, while the best fuel economy results from about a 16:1 ratio.

The required ratio can be thrown off by either the wrong amount of air being drawn into the cylinder or the wrong amount of fuel being mixed with the air.

Compression

In order to ignite and produce power, the air-fuel mixture needs to be compressed. This is accomplished by the piston moving from the bottom of its stroke to the top of its stroke inside the cylinder. Compression is expressed as a ratio; for example, in modern cars, it’s typically about 10:1, meaning that what originally took up 10 cubic inches of space now takes up only 1 cubic inch.

If you don’t have high enough compression, the air-fuel mixture won’t ignite properly.

Spark

Once the proper air-fuel mixture is compressed to the proper degree, a spark from the spark plug ignites the mixture. The resulting explosion forces the piston down, which in turn rotates the crankshaft to produce power.

If the spark doesn’t occur or doesn’t come at the right time, the cylinder won’t produce any power — or at least, not its intended amount.

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Finding the Cause of a Misfire

Assuming the misfire has caused the Check Engine light to go on (which is typically the case) and the vehicle has an onboard diagnostics system (or OBD II), the first step is to check what codes are being generated. Besides potentially leading you to the cause, it can usually indicate which cylinder or cylinders are affected. For instance, it may tell you the misfire is on cylinder No. 3, which could lead you to check the No. 3 spark plug, spark-plug wire, the ignition coil that fires that plug or the fuel injector for that cylinder. This is important because many problems affect the overall running of the engine, but significantly fewer affect only one cylinder. Lacking this, you’re pretty much just shooting in the dark. 

Yet even if you can narrow the problem down to one cylinder, there are plenty of potential causes for a misfire.

The proper air-fuel mixture could be off due to a clogged or malfunctioning fuel injector. This is a common problem on port- and direct-injected engines — which have one injector per cylinder — but isn’t likely to cause a single-cylinder misfire on older engines with throttle-body or central fuel injection, where each injector feeds multiple cylinders. While there are many problems that can cause fuel-related issues, fewer would affect only one cylinder.

Likewise on the air side. Many faults could affect overall running of the engine, but something that might cause a misfire on only one cylinder would be a leaking manifold gasket that allows more air to flow into an individual cylinder than expected, causing a lean mixture that doesn’t ignite. Additionally, a bent pushrod or collapsed lifter that doesn’t allow the intake or exhaust valve to open would prevent air from being sucked into a cylinder or the exhaust from being forced out.

The same pushrod/lifter problem could affect compression, as well, which also hinges on sealing the cylinder. Leaks could occur on only one cylinder through worn valves or piston rings that allow pressure to escape, effectively lowering the compression ratio.

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