Think your car may be misfiring? Watch this video for expert mechanic help about what to expect and how to diagnose an engine misfire, and keep reading for more details:
What Does a Misfire Mean?
What is a misfire?
A misfire is when a part of the combustion process goes wrong in one of your car’s cylinders.
What is that combustion process?
In a four-stroke internal combustion engine, this process involves these four repeating steps, or strokes:
- Intake stroke: An air-fuel mixture is drawn into a cylinder through the intake valve by the downward moving piston.
- Compression stroke: The air-fuel mixture is compressed by the upward moving piston.
- Power stroke: A spark plug fires (or “sparks”), igniting the air-fuel mixture. The expanding heat and pressure drive the piston back downward in its bore.
- Exhaust stroke: Once the fuel has burned, the piston starts to move back up to its original position, opening the exhaust valve, through which exhaust is released.
The combustion process is monitored by the knock sensor, crankshaft, and camshaft sensors, which send signals to the engine or powertrain control module (ECM or PCM) if a cylinder misfires. When you have an engine misfire, you may notice your vehicle is hesitating, has a loss of power, or is even “sputtering” when you press down on the accelerator.
‘P0300’ OBD2 fault codes indicate cylinder misfires
You’ll be able to tell which cylinder misfired based on the OBD2 trouble code sent to the ECM or PCM. Misfire codes start with “P0300,” and end with the number of the misfiring cylinder. For example, code P3004 indicates a misfire in cylinder 4.
The most common engine misfire-related OBD2 trouble codes are:
P0300 – Random or Multiple Misfire Detected
P0301 – Cylinder 1 Misfire Detected
P0302 – Cylinder 2 Misfire Detected
P0303 – Cylinder 3 Misfire Detected
P0304 – Cylinder 4 Misfire Detected
P0305 – Cylinder 5 Misfire Detected
P0306 – Cylinder 6 Misfire Detected
P0307 – Cylinder 7 Misfire Detected
P0308 – Cylinder 8 Misfire Detected
How are engine cylinders numbered?
Each cylinder of an engine is numbered from the front-most cylinder to the rear, which means the front-most cylinder is number 1.
On inline engines, the cylinders are numbered in numerical order from the front (belt side) to the back (transmission side).
On V-shaped, or Flat, engines, the even cylinders are on one side of the engine, and odd cylinders are on the other. Each side of a V-shaped or flat engine is known as a “cylinder bank,” and the cylinder bank that contains cylinder number 1, is also known as “bank 1.”
Can I drive with a misfire?
If you know you have a misfire, and your Check Engine light is flashing, it’s important to know that you shouldn’t drive your car. You could damage the catalytic converter, which is not an inexpensive fix. At the first sign of an engine misfire, it is important to note the symptoms and diagnose the problem quickly. At the first sign of an engine misfire, it is important to note the symptoms and diagnose the problem quickly.
What does a misfire feel like?
- Jerky or slow-to-accelerate: An engine misfire can result in jerking or difficulty accelerating. One reason for poor acceleration is if your car goes into reduced power mode, known as “limp” or “limp-home” mode.
- Rough, shaky, or louder than usual: Your car doesn’t feel as still or your engine is noisier while idling. Your car may also feel rougher or shakier than usual while driving. You could also experience stalling or stalling out after idling.
- Noisy: Misfire noises can sound like a bang, pop, or like your car is struggling.
- Possibly smelly: Depending on what caused the engine misfire, you could smell fuel, exhaust, or something else unusual that could be from a misfire-related leak. Your exhaust could also look darker or thicker than usual.
Another misfire symptom you’ll likely notice is that your Check Engine light’s on.
What causes an engine to misfire?
- Faulty spark plugs or ignition coils: Either could be the cause of a misfire if a spark plug doesn’t fire. Too much fuel could also prevent a spark plug from firing.
- Plugged or stuck-open fuel injector: This could prevent the right amount of fuel necessary for the air-fuel mixture.
- Faulty intake valve: If the intake valve won’t open, fuel won’t be able to enter the cylinder.
- Faulty exhaust valve: If the exhaust valve won’t open, exhaust won’t be released, resulting in burned and fresh fuel mixing.
How do I diagnose a misfire?
Use an OBD2 scan tool that graphs data to determine which cylinder is misfiring
If you have a scan tool that can graph data, you should be able to figure out which cylinder is misfiring. To confirm the misfiring cylinder after graphing the data, turn off your car, swap the ignition coil you think is faulty with a good one, turn your car back on, and graph to see if the data from the cylinder where you moved the bad coil spikes.
Test each cylinder
If you don’t have a scan tool that graphs, you can test each cylinder by swapping the coils individually while the engine is off.
Disconnect and test each fuel injector
With the engine off, disconnect a fuel injector, turn on the car, and if you notice that the revolutions per minute (RPM) or engine vibration doesn’t change, the cylinder that fuel injector was connected to is likely the one misfiring. In this situation, the fuel injector isn’t necessarily what’s causing the problem—the misfire could be caused by a bad ignition coil or spark plug.
Test the compression and for cylinder leaks
If you’ve tested the ignition coils, fuel injectors, and spark plugs and haven’t found anything that seems to be wrong electrically, a mechanical problem could be causing the engine misfire. In this situation, you should conduct compression and cylinder leak tests, which require special tools to complete. With these tools, you should be able to successfully diagnose your engine misfire and begin the process for repairs.
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