Have you ever started your car and the idle is noticeably rough? Or you may have been driving down the road and the engine starts to cut out or sputter. When the engine in your car is running smoothly, it will fire on all of its cylinders. However, when one or more cylinders misfire, the engine’s performance can become rough due to an imbalance of power between the working and non-working cylinders. You will need to find out which cylinder is misfiring and what is causing the misfire to get your vehicle back to peak performance.
An engine requires four basic elements to run as it should: air, fuel, compression, and spark. There are three common reasons an engine misfire will happen. A misfire can occur if a cylinder experiences a loss of air, fuel, or spark from a spark plug. Normally, an engine will not start if there is not enough air or fuel, as this affects all of the cylinders at once. So, the most common cause of an engine misfire is when a spark plug does not create spark. Since there is more than one reason for a misfire, you will have to narrow down the cause.
If you are not familiar or comfortable with replacing any ignition parts or diagnosing the source of the misfire, we suggest you review a repair manual for your vehicle or take the vehicle to a professional repair shop. We can help you find a qualified repair shop near you here.
You can test for spark at each cylinder by using this method:
Unhook the spark plug wire from the spark plug and attach a spark tester. Some vehicles have coil packs attached directly to the spark plug instead of spark plug wires. If your vehicle has coil packs, you can put the spark tester between the coil pack and spark plug. Then start the engine, and the spark tester should spark internally. If you do see spark at the spark tester, the spark plug may not be firing and should be replaced. If you do not see a spark, replace the plug wire or coil pack and repeat the test.
After replacing the spark plug, plug wire, or both, remove the spark tester, and reattach the spark plug wire to the spark plug. Start the engine. If there is still a misfire, the ignition module may be worn out and need to be replaced. If your vehicle is equipped with a distributor instead of an ignition module, you may need to replace the distributor cap, rotor, or ignition coil. Replace the ignition module(s), or distributor parts and start the engine. If there is still a misfire, the problem is likely not a spark issue.
If you follow the steps above and are getting spark to the spark plug, the misfire may be caused by a lack of air or fuel in the cylinder. Testing certain parts of the air or fuel system require requires special tools like a voltmeter or fuel pressure tester to correctly diagnose an issue. Some of the needed tools are available through the Rental Tools program at your local O’Reilly Auto Parts.
Check the MAF (mass air flow) sensor. The MAF sensor works with the computer in your car to measure how much air flows into the engine. If the MAF sensor is failing, the air-to-fuel ratio may be incorrect or inappropriate for combustion. You will need to have a voltmeter to test the MAF sensor, but in some cases you may see that the MAF sensor’s screen or sensor element is visibly dirty. In this case, the MAF sensor may be able to be cleaned with specialized electronic parts cleaner or MAF sensor cleaner. The repair manual for your vehicle may be required to get the correct readings for the sensor to ensure that it is working as it should.
You may also need to test some areas of the fuel system.
Checking the fuel system will help make sure enough fuel is making it to the cylinder. Remember that gasoline is flammable and safety precautions need to be taken. Always have a fire extinguisher nearby when working on the fuel system of your vehicle.
- Change the fuel filter if possible. Many vehicles have a fuel filter in the fuel line between the fuel tank and engine. Some filters require special tools to be disconnected. You cannot see if it is dirty in most cases, but it may be a good idea to replace it at least as often as recommended in your owner’s manual. Some vehicles have a mesh filter around an in-tank fuel pump as well, but these cannot be replaced without taking the fuel pump out of the fuel tank.
- Test the fuel pressure. You will need a fuel pressure tester to attach to the port on the fuel injector rails on top of the engine and a voltmeter to test the fuel pump. Vehicles with a mechanical fuel pump can be tested with an in-line fuel pressure gauge.
- Test the fuel injectors. Use a voltmeter to test the electronics of the fuel injector. The repair manual for your vehicle may be required to determine the correct readings for the injector or injector wiring. Injectors may also be clogged, and you can use a fuel injector or fuel system cleaner in your fuel tank to attempt to clean the nozzles without removing the injectors.
- If you are not comfortable working on the fuel system of your vehicle or do not have access to the right tools, we suggest you take it to a professional shop for diagnosis and repair.
There are other less common reasons for an engine misfire which may also need to be diagnosed at a professional shop. These reasons can include electronic sensor failure, vacuum leaks, timing issues, piston ring failure, lack of compression, and more.
We hope this has helped you understand and diagnose an engine misfire. If you are not comfortable performing any of the repairs or diagnoses listed, we would be happy to provide a local shop referral where your car can be properly diagnosed and repaired.
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