Haynes How-To: change a spark plug

Sections

• What are spark plugs?
• Why do spark plugs to be changed?
• How can you tell when your spark plugs need changing?
• How do you replace spark plugs?

What are spark plugs?

Spark plugs are used in petrol engines as a source of ignition. An electrical spark ignites the fuel, which forces the piston along the cylinder, which in turn causes the crankshaft to rotate. Without the spark plug the petrol engine wouldn’t be able to run.

Spark plugs are not used in diesel engines, which fire instead by simply compressing the fuel in the cylinders; no additional spark is necessary.

Why do spark plugs need to be changed?

Spark plugs need to be maintained properly if the engine is to operate smoothly. Neglect them and you could experience a number of problems.

For example, starting the engine may become a problem; you might have to turn it over several times before it fires, and this may flatten the battery over time.

In addition, once the engine is running it may have a rough sound when idling and could feel ‘lumpy’. The engine might even misfire (a distinct banging sound), which indicates that not all cylinders are working as they should. Misfiring should be sorted immediately, because you risk damage to the catalytic converter, which is expensive to replace.

A faulty spark plug may also increase fuel consumption and reduce engine power, and should be investigated at your earliest opportunity.

How can you tell when your spark plugs need changing?

Most modern spark plugs are made from ceramic and iridium or platinum, and should last for 60,000 miles or more. However they should be inspected every 30,000 miles or so, which will give you an idea of the health of the engine.

Consult your car’s manual, which should list the recommended intervals for changing your spark plugs. If you can’t find a recommendation, it’s worth removing them and checking their condition. It doesn’t take long and also gives you a clue as to how the engine is performing.
For example, a healthy spark plug will have light brown electrodes and insulator and no sign of melting, wear or deposits. However if you see oil deposits the piston rings may be worn. White deposits could mean that the plug isn’t of the right heat grade and carbon deposits show a rich mixture or weak ignition.

All cars are slightly different, so if it is time to change your thermostat, use our before you begin checklist, and find your car for specific instructions.

How do you replace spark plugs?

Changing a spark plug requires little experience, using basic tools, although you may need to remove other components to access the plugs. It will take up to an hour, depending on your model and could save you £75-£150 in garage fees.

Tools you will need (click to view at Halfords.com)

• Spark plug socket
• Torque wrench
• Ratchet
• Socket extension
• Spark plug gap gauge
• Spark plugs (£3-£25 each; see car’s manual for correct specification)

You may also need

• Anti-seize compound
• Spark plug wires/ HT leads (£20-£50; if the old ones are in poor condition)

Below is an example video but look for a full step-by-step guide for your specific car at Haynes OnDemand, here.

A quick guide to changing a spark plug

1. Make sure the engine is cold that you have the correct tools for the job. You may need to remove parts to gain access to the plugs
2. Work on one spark plug at a time. Remove the ‘boot’ from the top of the plugs with a twisting motion
3. Use a spark plug socket and ratchet to remove the plug
4. Adjust the new spark plug’s gap (if necessary), coat the threads with anti-seize compound and install the plug, being careful not to cross-thread it or over-tighten