Why Car Runs Worse After Changing Spark Plugs?

When it comes to maintaining a car’s performance, spark plugs play a crucial role. These small yet essential components provide the necessary spark to ignite the air-fuel mixture in the engine cylinders. However, it can be frustrating when a car runs worse after changing spark plugs.

In this article, we will explore the potential causes behind this issue and provide troubleshooting steps to resolve it effectively.

Spark plugs are responsible for igniting the air-fuel mixture that powers the combustion process. They generate a spark across a small gap, igniting the compressed mixture and creating the necessary force to move the pistons.

Possible Causes for a Car Running Worse After Changing Spark Plugs

Incorrect Spark Plug Gap

One common reason for a car running worse after changing spark plugs is an incorrect spark plug gap. Each vehicle has specific gap requirements that ensure optimal performance. If the gap is too wide or too narrow, it can lead to misfires, rough idling, and decreased power. Double-checking and adjusting the gap according to the manufacturer’s specifications is essential.

Wrong Spark Plug Heat Range

The heat range of a spark plug refers to its ability to dissipate heat. Using the wrong heat range can lead to poor combustion, engine knocking, and decreased performance. It’s crucial to choose spark plugs with the correct heat range for your vehicle. Refer to the owner’s manual or consult a trusted mechanic to ensure the right spark plug selection.

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Faulty Spark Plug Installation

Improper installation of spark plugs can also contribute to a car running worse. Issues like loose plugs, incorrect torque, or damaged threads can cause misfires and reduced engine efficiency. During installation, it is vital to follow the recommended tightening specifications and ensure the spark plugs are securely fitted.

Ignition Coil Issues

In some cases, changing spark plugs can reveal underlying ignition coil problems. If the ignition coils are faulty or damaged, they may not provide sufficient voltage to the spark plugs, resulting in misfires and rough engine performance. Inspecting the ignition coils and their connections can help identify potential issues that might be affecting the car’s performance.

Troubleshooting Steps

Verifying Correct Spark Plug Gap

To ensure the correct spark plug gap, follow these steps:

  1. Refer to the vehicle’s manual or consult a trusted source for the recommended gap.
  2. Use a feeler gauge to measure the gap between the center and ground electrode of each spark plug.
  3. Adjust the gap if necessary, carefully bending the ground electrode.

Ensuring Proper Spark Plug Heat Range

To select the right spark plug heat range:

  1. Consult the owner’s manual or a reputable source to determine the recommended heat range for your vehicle.
  2. Purchase spark plugs that match the required heat range.

Checking for Loose or Damaged Spark Plugs

Inspect the spark plugs for the following issues:

  1. Ensure that the spark plugs are tightly and correctly installed.
  2. Examine the spark plug threads for any signs of damage.
  3. Replace any damaged or worn-out spark plugs.

Testing the Ignition Coil

To check the ignition coils:

  1. Disconnect the ignition coil connectors.
  2. Use a multimeter to measure the resistance across the primary and secondary terminals of each ignition coil.
  3. Compare the readings with the manufacturer’s specifications.
  4. If the readings are outside the recommended range, consider replacing the faulty ignition coil.
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Additional Factors to Consider

Apart from spark plugs, several other factors can impact a car’s performance. Consider the following:

  • Fuel Quality and Octane Rating: Poor-quality fuel or using a lower octane rating than recommended can result in engine knocking and reduced power. Use high-quality fuel with the appropriate octane rating for your vehicle.
  • Air Filter Condition: A clogged or dirty air filter restricts airflow to the engine, affecting combustion and performance. Regularly inspect and replace the air filter as needed.
  • Other Engine Components: Issues with the fuel injectors, oxygen sensors, or other engine components can also contribute to a car running worse. If troubleshooting the spark plugs and ignition system does not resolve the problem, it is advisable to seek professional assistance.


Experiencing a decline in performance after changing spark plugs can be frustrating. However, by understanding the potential causes and following the troubleshooting steps mentioned in this article, you can identify and resolve the issues effectively. Remember to pay attention to the spark plug gap, heat range, installation, and inspect the ignition coil. By addressing these factors and considering additional variables, you can restore your car’s performance and enjoy a smoother ride.


Q: Can using the wrong spark plug brand affect a car’s performance?

A: While it is generally recommended to use spark plugs of the same brand as the original, using a different brand that meets the required specifications should not significantly affect performance.

Q: How often should spark plugs be changed?

A: The recommended interval for spark plug replacement varies depending on the vehicle and the type of spark plugs used. Typically, it is advisable to replace spark plugs every 30,000 to 100,000 miles or as per the manufacturer’s guidelines.

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Q: Can bad spark plugs cause engine misfires?

A: Yes, worn-out or faulty spark plugs can lead to engine misfires, rough idling, decreased fuel efficiency, and overall poor performance.

Q: Are all spark plugs the same size?

A: No, spark plugs come in various sizes and configurations. It is crucial to use the correct spark plug size and type specified for your vehicle.

Q: Can I change spark plugs myself, or should I seek professional help?

A: Changing spark plugs can be a relatively simple task for those with basic automotive knowledge and experience. However, if you are unsure or uncomfortable, it is advisable to consult a professional mechanic for assistance.

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