Are you firing on all cylinders?

When it comes to using fuel efficiently, spark plugs are one of the most critical parts of a petrol car's engine. And they operate in the most hostile environment. Thousands of times per minute, they must spark strongly in each hot and highly pressurised engine cylinder. It's not surprising that they wear out. But they do so gradually - over tens of thousands of miles.

So, you may not notice the resulting deterioration in performance - the increased fuel consumption and decreased acceleration. Eventually, the signs become more noticeable - misfiring, erratic running and difficult starting. Long before that, the spark plugs should have been replaced. The replacement interval varies depending on the manufacturer; anything from every 10,000 miles to 60,000 miles or more may be specified.

If this sounds like a job for a garage at £60 or more per hour, don't be put off. Replacing spark plugs is just one of the many DIY tasks covered by Haynes Manuals. Haynes is the world's leading publisher of automotive repair manuals and renowned for teaching millions of car owners how to carry out routine maintenance and repairs. Haynes publishes manuals for more than 500 cars and vans as well as useful glovebox guides such as Your Car and Driving Abroad

Notes to Editors:

The information on these pages is supplied courtesy of Haynes Publishing, please credit accordingly if you intend to use it. For further information, please contact Rebecca Nicholls on Tel: 01452 260063 or email: pr@haynes.co.uk

For more advice on model-specific procedures, please refer to the appropriate Haynes Service and Repair Manual. Haynes Manuals retail at £17.99 (hardback) and are available from all good car accessory retailers and bookshops including: Halfords, Motor World and WH Smith. A comprehensive stockist locator can be found at www.haynes.co.uk.

Here's how to replace those spark plugs yourself.  

The car shown has easily accessible plugs. Not all models will be as simple as this and not all will have HT leads. For details applicable to your car, see the relevant Haynes Manual.

You will need a set of spark plugs of the correct type for your car, a spark plug socket and a torque wrench, plus a set of feeler blades or a special spark plug gap setting tool. A short length of flexible hose and some copper grease will also come in handy.

Be warned - switch off the ignition before starting work. Do not 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 parts you need to in order to get at the spark plugs and HT (high tension - high voltage) leads. Identify the leads if necessary (they may already be numbered) and disconnect them from the spark plugs by pulling on the connector, not on the lead.
2. Brush or blow away any dirt from around the spark plugs, then unscrew them with the socket and remove them. Have a look at the firing end of each plug: it should be a light brown or grey colour. A spark plug which is black and sooty, or oily, may be a warning of a problem developing.
3. Use the feeler blades (3a) or the gap setting tool (3b) to check the electrode gap of the new plugs (see 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, in which case skip this step.
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 spark-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 setting specified for your engine.
6. Reconnect the HT leads, refit any other components and start the engine. If it won't start, the chances are you've connected the HT leads in the wrong order.

Now here's a tip, similar to the many useful Haynes Hints in the Haynes Service and Repair Manuals. Cleaning spark plugs with a wire brush or an abrasive cleaner is no longer recommended. It can do more harm than good by leaving conductive deposits on the plug's insulating nose. Modern ignition systems work at very high voltages and the HT current can leak away via deposits left by cleaning.