What Are The Symptoms Of Bad Spark Plugs?

Spark Plug

When is it necessary to change spark plugs? How difficult is it to change spark plugs for someone with no experience?

In this article, I’ll address all you need to know about your engine spark plugs.

What is a spark plug?

The spark plug is what ignites the air-fuel mixture in the combustion chamber of your engine.

It creates a spark that jumps from the center of the electrode to the ground electrode.

How does a spark plug work?

The spark plug has two primary functions:

  • The first one is to ignite the air-fuel mixture.
  • The second one is to remove the heat from the combustion chamber

If either of these functions is not being performed properly, it can lead to a reduction in power and acceleration.

Will your spark plug gap cause a misfire?

The short answer is “YES”.

If your car is experiencing any of the following symptoms, then it may be time to replace the spark plugs.

  1. Decreased fuel economy – Bad spark plugs can cause your car to burn more fuel than it should. This will lead to decreased fuel economy and higher fuel bills.
  2. Reduced power and acceleration
  3. A lot of vibration when accelerated
  4. The engine is running rough

You can check if your spark plugs are bad by removing them and taking a look at them. If they are black and sooty, it means that they are not firing correctly.

If your spark plugs are white or light brown, it means that they are firing correctly.

When to replace spark plugs?

Replace the spark plug at the proper interval recommended in the owner’s manual. If you replace your spark plug on time it will easily pay by itself by saving on the bad fuel economy.

So replacing will preserve your engine and save you money in the long run.

Also if you notice a crack in the ceramic of the spark plug, otherwise the spark can jump from the center electrode to the ground electrode through the broken ceramic and create misfiring issues. So, inspect the ceramic at the tip and look for cracks.

Check the electrode, it should not be dirty:

What tools do you need to change your spark plugs?

  • Ratchet (and some extensions for hard-to-reach spark plugs)
  • Spark plug socket (check the size of the socket from the spark plug manufacturer specification)
  • A gaping tool to measure and adjust the gap of your spark plug
  • A set of new spark plugs
  • Some spark plugs require anti-seize but you need to check before applying it to your new spark plugs.

What spark plug do you need for your car?

The best spark plug for your car is what is recommended by the car manufacturer.


Because the car manufacturer thoroughly tested the spark plug with the engine. If you start using different spark plugs you may encounter some issues.

So just stick to what is mentioned in your owner’s manual.

How to replace your spark plug?

Open the hood and look for the ignition coils.

Blow around the area to remove any dirt and debris that could fall into the spark plug area.

Remove the ignition coils (safely pull by the boot). You have then access to the spark plug which is deep down in the well.

Grab your spark plug socket. It’s a socket designed with a rubber boot to grab your spark plug and pull it out safely of the well. You don’t your plug to fall and break during this process.

Connect the socket to an extension to reach the bottom of the spark plug well. Push it down onto the plug.

Once you’re on the plug, break it loose. Then loosen the spark plug and pull slowly the extension and the socket to extract the spark plug.

Before installing a new spark plug, you need to make sure the gap between the center electrode and ground electrode is correct…

What spark plug gap do I need?

If the gap between the center electrode and ground electrode is too wide, the plug might struggle to spark and create a misfire in your engine.

If the gap is too small, you might get a small and weak spark that will not ignite the air-fuel mixture efficiently.

So proper gapping is essential.

Most of the plugs are pre-gapped but it is always good to check with a filler gauge.

For the gap specification, you can check the owner’s manual, sometime there would be the sticker under the hood or you can check the spark plug manufacturer specification.

To close the gap, find a hard surface and push the center electrode to close the gap.

To open the gap, use a gapping tool to open the gap as follow:

Now you’re almost ready to install your new plugs in the engine.

Before installing your new spark plugs, you need to check if the thread requires anti-seize. If it does, lightly coat the thread. It is used so that the thread won’t get stuck inside the engine. Most modern spark plugs do not require anti-seize. This will be specified by the spark plug manufacturer, so don’t forget to check.

To install the new plug, use the socket to gently insert the plug into the well. Don’t let it drop or it will get damaged!

When you tighten be sure to follow the torque requirement from the owner’s manual. If you tighten too much, you could damage the thread, if you don’t tighten enough, the plug could become loose which can be dangerous for the engine.

If you don’t have a torque wrench…

Use a regular ratchet and from where it is snug, just turn a sixteenth of a turn (that’s it!)

After installing the new spark plugs, start your car to make sure it is running well!

Alright, you’re now all set with brand new spark plugs!

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