Car Engine Misfiring? 7 Most-Common Issues & Fixes

When you get into your car, turn the key, and want to drive, you expect your car to run correctly. It shouldn’t shake, shimmy, or misfire. If it does, you need to diagnose the problem quickly.

Air and fuel should mix at a perfect blend to ensure good combustion, but if it is misfiring, you need to fix it sooner rather than later. We’ll give you a head start by covering the most common causes of misfiring and how to fix them.

Having problems with your motorcycle? We have an article about misfiring issues with motorcycles that may help diagnose why.

What Is Engine Misfiring?

Engine misfiring is what it sounds like. The engine uses a mixture of fuel and air to ignite and push the piston in the cylinder. If that combustion cycle doesn’t happen at the right time, or happen at all, that is considered a misfire.

These misfire events shouldn’t happen frequently, or at all. If they occur frequently, they can lead to poor engine performance or engine damage.

The most common symptoms of an engine misfire include:

  • A rough idle
  • Excessive vibrations from the engine
  • Rough and slower acceleration
  • Engine sound changes from smooth to choppy
  • A check engine light illuminates

What Causes An Engine Misfire?

The engine needs three main ingredients for combustion: fuel, air, and a spark.

These three must mix at the proper ratio and at the correct time in the combustion cycle for your engine to run. If that doesn’t occur, it misfires.

There are 7 main reasons why these misfires can occur, and they are related to your three ingredients and the timing to ignite them.

What Are The Common Issues That Cause An Engine Misfire?

#1 – You Have A Bad Ignition Coil Or Distributor

An ignition coil or the distributor supplies the spark to make the fuel and mixture “go boom”.

On a car with multiple cylinders, the distributor will have a wire coming in from the ignition coil that supplies the spark energy. That energy is then distributed to each cylinder and spark plug by another wire – this is why it’s called a distributor.

If the ignition coil is bad, or is starting to fail, it won’t supply the spark energy to the distributor to make the car run. If the distributor starts to fail, one or more cylinders won’t ignite the fuel mixture and the car’s engine may run poorly or stall.

Newer vehicles have moved away from the distributor type of ignition and onto either coil-on-plug or coil-near-plug ignitions.

The coil-near-plug type will have a small ignition coil for each cylinder, and it will be located close to the cylinder. The coil-on-plug will connect directly to the spark plug and will not have a separate wire to transmit the spark.

How to Fix It

No matter the type of ignition system on your car, each part is replaceable. You can buy a replacement distributor, the small components that make up the system, and each individual coil-on-plug or coil-near-plug component.

You should verify which component is the problem through testing and troubleshooting before buying new parts.

For cars that have multiple identical parts, such as ignition coils, you can change coils between cylinders and verify that the misfire problem does change with the suspect coil.

#2 – You Have A Spark Plug Issue

The second most common issue that causes an engine misfire is one or more bad spark plugs.

The spark plugs are responsible for igniting the air-fuel mixture in the combustion chamber. If they don’t fire correctly, the engine misfires and causes a rough idle and poor acceleration, and it may also cause the check engine light to illuminate.

How to Fix It

Spark plugs can be replaced if they have gone bad, but you should determine why they have gone bad.

  • Are they fouled with oil from a leak?
  • Are they wet with fuel because they are not igniting the air-fuel mixture?
  • Are they past their optimum replacement timing and are worn out?

You should determine why they have gone bad so you don’t continually replace plugs because of a separate issue.

For more information, read our article all about the “what and the where” of spark plugs. 

#3 – You Have An Intake Manifold Gasket Leak

Between your intake manifold and cylinder head, there is (or should be) a gasket that keeps the air-fuel mixture directed into the combustion cylinder.

If that gasket is missing or leaking, it can allow extra air to enter the combustion chamber. This extra air imbalances the air-fuel mixture, causing it to become lean. That means there is too much air to burn efficiently, and it can cause the engine to misfire.

You may also have vacuum lines that are connected to the intake manifold, and they can be damaged or degraded, allowing a vacuum leak. This can also cause a lean condition and an engine misfire.

How to Fix It

The intake manifold gasket can be replaced. It isn’t a simple job because the intake manifold, some of the fuel system, and possibly other components must be removed to change the gaskets.

To find out if the gasket is leaking, you can spray a flammable liquid around the edge of the intake manifold.

If you hear the engine speed up with the fluid being sprayed, it indicates that a portion of the flammable fluid is leaking into the combustion chamber and adding extra fuel to the air-fuel mixture.

If the gasket isn’t leaking, it may be a vacuum leak. There are smoke machines that you can connect to a vacuum line and fill the vacuum system with white smoke to watch for leaks.

Professional machines are expensive, and it may be a better choice to take your car to a mechanic who has a machine.

#4 – Your Car Has Low Fuel Pressure

Adding too much air can cause a lean misfire, and so can not having enough fuel.

Low fuel pressure means you have a problem with your fuel pump not pumping the required amount of fuel, the fuel pressure regulator causing a restriction, or your fuel filter is clogged.

How to Fix It

You can use a manual fuel pressure gauge to verify if you have a low fuel pressure condition. If the gauge indicates a problem, the next step is to find out why you’re having the problem.

If it’s a clogged fuel filter, it can be replaced. Some filters also have a built-in regulator, and that could also eliminate the regulator as a potential problem.

If the fuel pump is the issue, it can also be replaced. You may need to remove the fuel tank from your car, which can be a big job. It may be easier to pay a mechanic to replace it.

If you need to diagnose a bad fuel pump, check this article.

#5 – You Have A Bad Engine Sensor

Most new cars have many sensors to measure the engine performance and ignition timing. A failure or degradation of one of the engine sensors can cause an engine misfire.

The sensors that can be attributed to a lean air-fuel condition are the Mass Air Flow sensor (MAF), the Manifold Absolute Pressure sensor (MAP), the Coolant Temperature sensor, or an Oxygen sensor (O2).

How to Fix It

Modern cars have an onboard diagnostic system (abbreviated OBD) that can inform you of a problem. The system can illuminate the Check Engine light, which will indicate that a sensor has a problem.

The first step will be to use a trouble code scanner to know what code is being stored. Most auto parts stores have them available and can scan your car for you if you don’t have a trouble code scanner. Most mechanics will also have one.

Most OBD systems will register a trouble code of P0300 or P030X, where the X will define what cylinder is detecting a misfire. From there, you can start diagnosing other sensors, fuel pressure, and spark plugs to find the cause of the misfire condition.

#6 – You Have a Bad Fuel Injector or Carburetor

Carburetors have been used for decades, but they were phased out in the 1980s as fuel injection of various forms was adopted.

Throttle Body Injection (TBI) was adopted in the mid-1980s as a replacement for carburetors and in the late 1980s, individual cylinder fuel injectors replaced the TBI units.

The fuel injector adds the correct amount of fuel at the correct time into the combustion chamber. If the fuel amount is less than optimum, a lean condition occurs. This can cause a misfire.

How to Fix It

Fuel injectors can be purchased individually and replaced if they fail.

It’s more common to have a dirty fuel injector that can be fixed by adding a liquid fuel injector cleaner to the fuel tank. If, in fact, it has failed, you can replace it, or a mechanic can replace it.

For a more detailed look at fuel injection problems, read our article here.

#7 – You Have Low Compression In The Engine Cylinders

If you have perfect fuel pressure for your car and the correct spark, your misfire may be caused by low compression in the engine cylinders.

Low compression can occur because of a stuck cylinder head valve or worn piston rings.

To determine if you have low compression, remove the spark plug and thread a compression tester or a leak-down tester into the spark plug hole.

These gauges will indicate the compression level in each cylinder by completing the test, and then you can compare that to the car’s manufacturer specifications. If any are low (below 90% of the value), you can further inspect for worn components.

How to Fix It

If you have low compression in one or more cylinders, you will need to check for worn or damaged components.

Stuck valves, worn timing components, and damaged or worn piston rings are the most likely causes of low cylinder compression.

All of these components are expensive to replace, and this job may be best suited for a professional mechanic.

Was this article helpful? Like Dislike

Click to share...

Did you find wrong information or was something missing?
We would love to hear your thoughts! (PS: We read ALL feedback)