Why Is My Car Misfiring After Changing Spark Plugs?

Changing the spark plugs in your vehicle is a routine maintenance task intended to keep your engine running smoothly. However, you might find that after doing this simple procedure, your car starts misfiring. This can be puzzling and even frustrating, especially if the vehicle was running fine before the change. This article will delve into why is my car misfiring after changing spark plugs and offer solutions to rectify the problem.

misfire after spark plug change

Why Is My Car Misfiring After Changing Spark Plugs?

If your car begins misfiring after changing spark plugs, it’s essential to consider multiple factors that could be contributing to this issue. Here are some primary causes:

1. Faulty or Incorrect Spark Plugs

The first and most obvious suspect when a car misfires after changing its spark plugs is the new spark plugs themselves.

  • Incorrect Type: Not all spark plugs are made equal. Using a new spark plug that’s not meant for your vehicle’s make or model can lead to misfiring. Always check the owner’s manual or consult a trusted mechanic about the right type of spark plugs for your vehicle.
  • Defective Plugs: Like with any other manufacturing process, sometimes there can be quality control issues, and you may end up with a faulty plug. If a plug isn’t firing correctly, the engine won’t combust the fuel-air mixture as it should.

2. Improper Gap Size

The gap between the center and side electrode of a spark plug is critical. If this gap is too wide or narrow, it can affect the spark’s ability to ignite the fuel and air mixture in the cylinder, leading to misfires.

  • Check the Gap: A simple feeler gauge tool can help you measure and set the correct gap. Ensure you cross-reference your vehicle’s specifications to get the right gap size.

3. Improper Installation

A simple oversight during installation can cause misfires.

  • Over-tightening: It might damage the spark plug or the cylinder head threads, leading to misfiring.
  • Under-tightening: It might cause a poor connection or even allow the spark plug to eject while the engine is running.

Always follow the manufacturer’s torque specifications and use a torque wrench if possible.

4. Damaged Ignition Coils or Wires

While changing the spark plugs, it’s possible to inadvertently damage the ignition coils or wires, especially if they are old or brittle.

  • Physical Inspection: Check for any visible cracks, burns, or signs of wear. If you find any, replace them.
  • Electrical Inspection: Use a multimeter to ensure the wires carry the correct current. If not, it might be time to replace them.

5. Compromised Seals or Gaskets

When removing spark plugs, especially in overhead cam engines, it’s easy to accidentally dislodge or damage the tube seals or valve cover gaskets. This can allow oil or other contaminants to seep into the spark plug tubes, leading to misfires.

  • Regular Inspection: After changing your spark plugs, periodically inspect the area around the spark plugs for any signs of oil or other contaminants.

6. Undetected Issues That Coincide

Sometimes, the misfire may not be directly related to the spark plug change but merely coincidental.

  • Fuel System Issues: A clogged fuel injector or a malfunctioning fuel pump might not deliver the required fuel, leading to misfires.
  • Sensor Malfunctions: Components like the mass air flow sensor or oxygen sensors, when malfunctioning, can alter the engine’s air-to-fuel ratio, leading to misfires.

How to Tell if a Spark Plug is Misfiring?

Identifying a misfiring spark plug is crucial for the efficient operation and longevity of your vehicle’s engine. Here are some key signs and methods to determine if a spark plug is misfiring:

  1. Engine Vibration: When a spark plug misfires, it causes the engine to run unevenly, leading to noticeable vibrations due to bad spark plug. This can especially be felt when the car is idling.
  2. Decreased Engine Performance: A misfiring spark plug reduces the engine’s power and efficiency. You might notice a sluggish response when accelerating.
  3. Check Engine Light: Modern cars have an onboard diagnostic system (OBD-II). A misfiring spark plug can trigger the “Check Engine” light.
  4. Increased Fuel Consumption: Misfires lead to inefficient combustion, which can increase fuel consumption.
  5. Audible Sounds: Misfiring often produces a repetitive, rhythmic stuttering sound or even a popping sound from the engine.
  6. Engine Code Reader/Scanner: If you can access an OBD-II scanner, it can read the error codes from your vehicle’s computer. Codes related to misfiring often start with “P030” followed by another digit (e.g., P0301 for cylinder 1 misfire after spark plug change).
  7. Physical Inspection: Remove the spark plugs and inspect them. A fouled plug, one covered in oil or soot, or one with a damaged electrode can indicate issues. Additionally, check the gap between the electrodes using a spark plug gap tool; an incorrect gap can lead to misfiring.
  8. Smell of Unburned Fuel: Since a misfiring cylinder doesn’t combust the fuel appropriately, you might notice the smell of gasoline from the exhaust.
  9. Rough Idling: The engine may run ragged or feel choppy when idling due to uneven combustion.

If you suspect a misfire, it’s essential to address it promptly. Continuous misfiring can damage the engine. Always consult with an expert technician if you’re unsure about the health of your spark plugs or the cause of a misfire.

Signs and Causes of Misfiring YouTube

How do You Know Which Spark Plug is Misfiring?

Identifying the specific spark plug or cylinder that is misfiring is crucial to efficiently addressing the issue. Here’s how you can pinpoint the problematic spark plug:

OBD-II Scanner/Code Reader:

Modern vehicles come equipped with an onboard diagnostic system (OBD-II). Connecting an OBD-II scanner or code reader to the vehicle’s diagnostic port can retrieve error codes related to misfires. A code like “P0301” indicates a misfire in cylinder 1, “P0302” indicates cylinder 2 misfire after spark plug change, and so on. This is the most straightforward way to determine which cylinder is misfiring.

Inspect Spark Plug

Physical Inspection:

  • Remove and inspect spark plugs one by one. A fouled or damaged spark plug can hint at which cylinder is misfiring.
  • Look for signs like oil, soot, or wear on the electrode. If one plug appears distinctly different from the others, that could be your culprit.

Engine Cylinder Compression Test:

A compression tester can help identify if a particular cylinder is low on compression. While this isn’t a direct indication of a misfiring spark plug, it can point to issues in a specific cylinder that might be related to or causing misfiring.

Listen to the Engine:

Sometimes, you can identify a rhythmic misfire by merely listening to the engine, especially while idling. You can sometimes hear differences between cylinders by using a mechanic’s stethoscope or even a long screwdriver (with your ear to the handle and the tip touching the engine).

Swap Components:

  • Move the suspected spark plug to another cylinder. If the misfire follows the spark plug, then that plug is likely the problem.
  • Similarly, you can swap ignition coils between cylinders (if your vehicle has coil-on-plug ignition). If the misfire moves with the coil, then it might be the issue, not the spark plug.

Use an Infrared Thermometer:

Point the thermometer at the exhaust manifold near each cylinder’s exhaust port. A colder reading indicates that the cylinder isn’t firing as efficiently as the others, which could mean a misfire.

Engine Misfire Detector:

Some specialized tools can detect misfires by monitoring engine vibrations. These tools can sometimes identify which cylinder is misfiring.

Will Changing Spark Plugs Fix Misfiring?

Changing spark plugs can often solve misfiring if the plugs are the root cause of the problem. Over time, spark plugs can become fouled, wear out, or get damaged, leading to inefficient combustion in the engine cylinders. This inefficiency manifests as a misfire. By replacing old or faulty spark plugs with new ones, the engine can return to its proper combustion rhythm.

However, it’s worth noting that misfiring can be caused by various issues, not just spark plugs. Other culprits could be ignition coils, fuel injectors, oxygen sensors, or even problems with the engine’s timing. Thus, while changing spark plugs is a good first step in addressing misfires, especially if they’re old or visibly damaged, it may not always resolve the issue. If misfires persist after changing the plugs, further diagnostic work will be needed to pinpoint and address the underlying problem.


If your car starts misfiring after a spark plug change, it’s essential not to panic. Start by retracing your steps, ensuring you’ve used the correct spark plugs, installed them correctly, and checked related components. If the problem persists, it’s always wise to consult a professional mechanic to identify and address the root cause, ensuring your vehicle is safe and running at optimal performance.


Mr. Shafiqule Islam is a graduated Mechanical Engineer and has more than 15 years experience of repairing and maintenance of different brand vehicles like Toyota, Mitsubishi, Ford, Mercedes, BMW etc. He is also giving training to Mechanics. He has started writing to share his practical knowledge to Vehicle Owners, Drivers and Mechanics to keep their cars at best fit.

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