How spark plugs work
Spark plugs are used in petrol (gasoline) engines as a source of ignition at the end of the compression cycle. The spark ignites the petrol, which forces the piston along the cylinder, which in turn forces the crankshaft to rotate. Without the spark plug the engine wouldn’t be able to run. Diesel engines use glow plugs.
Most modern spark plugs are made from ceramic and iridium or platinum and should last for 60,000 miles plus. However they should be inspected every 30,000 miles or so, which will give you an idea of the health of the engine.
This task requires little experience, uses basic tools, and will take up to an hour, depending on your model.
When to change your spark plugs
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 spark plugs, use our before you begin checklist, and find your car for specific instructions.
How to change your spark plugs
Here's an example of how it's done
A very brief summary of the task:
- 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
- Work on one spark plug at a time. Remove the ‘boot’ from the top of the plugs with a twisting motion
- Use a spark plug socket and ratchet to remove the plug
- Adjust the new spark plug’s gap (if necessary/possible), coat the threads with anti-seize compound and install the plug, being careful not to cross-thread it or over-tighten
Why you should change your spark plugs
Spark plugs need to be maintained properly if the engine is to operate smoothly. Neglect them and you could experience several issues.
First, you may find it difficult to start the engine. You’ll have to turn it over several times before it fires and this may flatten the battery over time. Once the engine is running, it may have a rough idle and feel ‘lumpy’ or it may misfire, 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 cause increased fuel consumption and a reduction in engine power, and should be investigated at your earliest opportunity.
Tools you will need
Only basic tools are required for this job, although you may need to remove other components to access the plugs.
- Spark plug socket
- Torque wrench
- Spark plug gap gauge
Parts you may need
- Spark plugs - Refer to your handbook for the correct specification
- Spark plug wires/HT leads - You may need these if the old ones are in poor condition
How much do new spark plugs cost?
|Spark plugs||£3-£25 each|
|Spark plug wires/HT leads||£20-£50|
|Garage fee savings||£75-£150|