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How to replace spark plugs

Abby Wingett | 18th July 2022 | 8 minutes to read

Worn spark plugs can be the source of a number of vehicle issues, from not starting to engine misfiring and sluggishness compromising your vehicle's fuel economy. With the right products, tools, and processes - it's a job you can do yourself. In this know-how guide, we'll show you how to gap spark plugs correctly, how to read a spark plug and know when to replace spark plugs.

If your engine runs rich, misfires, is sluggish or runs at high temperatures, you should take a look at your spark plugs.

  1. How to read your spark plugs to understand what's happening in your engine
  2. How do I know when it is time to replace my spark plugs?
  3. What you will need:
  4. Steps to changing your spark plugs

How to read your spark plugs to understand what's happening in your engine:

  • Covered in oil - Check your valve guides and piston rings
  • Light brown dust - Caused from oil leaking into the combustion chamber or the use of fuel additives
  • White ash on the insulator or melted electrode - This is caused from overheating or pre-ignition and should look at the timing system or pick a plug suited to the correct heat range
  • Damaged or cracked insulator - Indicates detonation and could mean you're using the wrong octane fuel or incorrect timing

How do I know when it is time to replace my spark plugs?

Replacing your spark plugs is a crucial part of your servicing and routine maintenance for your engine. Your spark plugs play a significant part in your vehicle's performance; your spark plugs primary function is to ignite the air-fuel mixture within your combustion chambers.

Without a complete set of functioning spark plugs, you may notice that your vehicle is performing poorly. Some of these symptoms include:

  • Poor fuel economy - when your spark plugs aren't working properly, your car works in overdrive, which means in the combustion chambers, it cannot burn the fuel sufficiently; therefore, your vehicle will chew through more fuel.
  • Lack of acceleration – if one or more of your spark plugs are worn, your plugs can no longer sufficiently ignite the air-fuel mixture. Therefore, your engine will have a lack of power, causing a lack of acceleration.
  • Poor idle condition and engine is noisy – if your engine is noisy, possibly knocking or rattling, this can be a sign that you need to replace your spark plugs.
  • Cylinder misfires – this can be caused by other things, too, like a faulty coil pack; however, misfiring can be caused if one or more of your spark plugs are not functioning correctly. If you notice your vehicle is misfiring, it might be worthwhile checking your spark plugs to eliminate what is causing the vehicle to misfire.
  • Hard staring or even the engine won't run at all – this can also be caused by a variety of things like your battery is flat; however, spark plugs can also cause an issue with starting your car. This is because your vehicle has to work harder to ignite the air-fuel mixture.

What will you need:

Step to changing your spark plugs

step 1 clean debris away from spark plugs/spark plug caps

Step 1: Clean debris away from spark plugs/spark plug caps

Before you change your spark plugs, you will need to clean away any debris that is around the spark plug caps because you do not want anything to fall down into the spark plug hole.

Step 2: Remove the old spark plug leads one at a time

Step 2: Remove the old spark plug leads one at a time

Now you have cleaned the area, you can remove the spark plug leads. The best practice is to label each lead using masking tape and marking the number on the tape so you know which ignition lead is for what spark plug. Putting leads into the different holes can cause your engine to run rough.

You will need to turn them 90 to 180 degrees to break the seal and only grab them by the boot; otherwise, you can damage the leads. Do this one by one until you have removed all the ignition leads.

step 3: remove the old spark plugs

Step 3: Remove the old spark plugs

Now you will need to remove the spark plugs, starting from number one and working your way along. You will need to choose the right extension bar to provide you with leverage as well as being able to reach the spark plug. Turn the rachet anti-clockwise and spin around the spark plug to remove it. Keep them in order once you have removed them as you are going to inspect them, carefully look for defects.

step 3: remove the old spark plugs

Step 4: Preparing the new spark plugs

Check the condition of the new spark plug and compare them against the old ones to ensure you have purchased the correct spark plugs. You can also do this by checking the number on the spark plug; however, this can vary slightly depending on which branded spark plug you have purchased.

Before fitting the new spark plugs, you need to check the gap between the earth electrode (the top of the spark plugs) and the centre electrode. You will need to check your vehicle's manual to check what the correct gap should be for your vehicle.

To check the spark plug gap, you can use a feeler gauge. Once you know your vehicle specification of the gap using a feeler gauge, place it between the two electrodes, and it should fit snug.

If it doesn't fit snug, you will need to adjust it. To adjust it, you can just rotate the feeler gauges, and this will open the earth electrode a little. However, if it is way to lose, then you can also make the gap smaller by tapping the earth electrode on a piece of timber; this will not damage the spark plug (refer to the know-how video above or the know-how article understanding spark plugs for further information on the electrode gap.)

Once you have ensured you have the correct gap, we recommend placing some anti-seize onto the spark plug thread, which will help reduce friction and wear, protect against seizing and corrosion, and reduces torque during disassembly.


Never use solid surfaces like concrete to adjust the gap.

step 5: tension the new spark plugs

Step 5: Refit your spark plugs, tighten them, and tension the new spark plugs

Fit your new spark plugs to your vehicle using a rachet to wind them in, so you put a little bit of tension on them, but not too much. It is easy to over-tighten them, especially on alloy cylinder heads with spark plugs which can cause thread damage to the cylinder head.

With your tension wrench, tighten the spark plugs up to the tension that is specific to your vehicle; this will be found in your vehicle manual.


Don’t over tension. Stop when you hear a click.

step 6: reinstall the ignition leads

Step 6: Reinstall the ignition leads

Now you will reinstall the leads back into the engine, but first of all, you have to check and ensure that there is no damage to these lead ends or the caps. Make sure there is no corrosion or missing components down inside that actually grab and hold the top of the spark plug.

You need to have a really careful look around the ceramic portion of the actual spark plug cap. Then you need to look at the boots for any tears or any signs that there may be water ingested in the leads, which could have caused them to split on the inside. Make sure they are sturdy and feel like they are attached correctly, and then you need to check the leads. Just look down the leads and look for any signs of white; white is normally a dead giveaway that it has been leaking.

In general, just check them completely over before installing them.

Before installing them, put a bit of di-electric grease onto the insulators at the end of the boots, where they seal onto the spark plug. Just a small amount; you don’t need a lot. The di-electric grease will seal, protects, insulates electrical connections, and prevents rust and corrosion.

Put all the leads back in place; you should hear them/ feel them click into place.

step 7: start the vehicle

Step 7: Start the vehicle

Start your vehicle like you would usually do. This is to ensure that the spark plugs and leads are working as they are supposed to and that there isn't any misfiring. If the car wasn't functioning properly, you can get out of the vehicle and check, push the leads in, making sure they are on properly.

Changing your spark plugs is quite a simple job, and we encourage you all to give it a go. However, if you prefer to get it done by a professional, contact your local repair shops, they will be able to assist you in installing new spark plugs

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