All petrol engines use spark plugs
to ignite the fuel-air mixture in the cylinders. These plugs gradually wear out and have to be replaced periodically. The replacement interval varies depending on the manufacturer; anything from every 10,000 miles to 60,000 miles or more may be specified.
Worn spark plugs can cause difficult starting, misfiring, erratic running and poor fuel consumption. A persistent misfire can also damage the catalytic converter (where applicable).
The car shown here has easily accessible spark plugs. Not all models will be as simple as this; not all will have HT leads. For details applicable to your car, see the relevant Haynes Service and Repair Manual.
You will need a set of spark plugs of the correct type for your car, a spark plug socket spanner and a torque wrench, and a set of feeler blades or a special spark plug gap setting tool. A short length of flexible hose and some copper grease will come in handy.
Switch off the ignition before starting work. Don't attempt to remove spark plugs while the engine is hot - not only may you burn yourself, you could damage the cylinder head.
1 Open the bonnet and remove whatever you need to in order to get at the spark plugs and HT leads. Identify the leads if necessary - they may already be numbered - and disconnect them from the spark plugs. Pull on the connector, not on the lead.
2 Brush or blow away any débris from around the spark plugs, then unscrew and remove them. Have a look at the firing end of each plug: it should be a light brown or grey colour. A plug which is black and sooty, or oily, may be an early symptom of a problem developing.
3 Use the feeler blades or the gap setting tool to check the electrode gap of the new plugs (see your car's handbook, or your Haynes manual, to find out what the gap should be). Adjust if necessary by carefully bending the side electrode. Some plugs don't have an adjustable gap.
4 Put a smear of copper grease on the threads of each new plug and screw them into their holes. Use a length of flexible hose to start them off - if you get a plug cross-threaded, the hose will slip before you cause any damage.
5 Final tightening of the spark plugs should be done using a torque wrench, otherwise there's a risk of doing them up either too tight or not tight enough. See your Haynes manual for the torque specified for your engine.
6 Reconnect the HT leads, refit any other disturbed components and start the engine. If it won't start, chances are you've connected the HT leads in the wrong order.
- Fit new HT leads if the old ones are cracked, oily or otherwise damaged.
- Cleaning spark plugs with a wire brush or an abrasive cleaner is no longer recommended because of the risk of damaging them.
- Black, sooty spark plugs can be a symptom of a clogged air filter element.