What are spark plugs? Maintenance and replacement guide

For vehicles powered by petrol and diesel, keeping them in good condition through regular car maintenance is important – but knowing how and when to replace this is something all owners should know.

This guide looks at why spark plus are vital to an engine, how to find them, and how much it will cost to replace them.

Spark plugs are responsible for initiating the combustion process within the engine of a car.

They create a spark across a small gap, which ignites the air-fuel mixture in the engine's combustion chamber in petrol and diesel vehicles.

This ignition creates a small explosion, which forces the piston(s) to move then generates the power for the vehicle to move.

Spark plugs are also designed to dissipate heat from the combustion chamber as well as removing combustion byproduct.

Both high temperatures and waste in the engine can cause the starter motor and other components to break or suffer damage.

In fact, some spark plugs are designed to self-clean during the combustion process.

Spark plugs should be checked during a car service.

Speak to your local garage or call a mobile mechanic if you are concerned about your spark plugs – or any other part of the engine.

Regularly checking your spark plugs is an important part of owning a vehicle.

For the majority of cars, they will have four spark plugs in the engine.

However, in some sports cars, there will be more.

For example, in a V6 engine, there will typically be six spark plugs. And in V8s, there will be eight spark plugs.

In boxer engines, there will also be four spark plugs and in some older vehicles, they will use a rotary engine that has one spark plug in a triangular unit – meaning there should be three spark plugs.

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In most cars, the spark plugs can be found at the top of each cylinder head, on the driver’s side of the vehicle.

For in-line vehicles, there will be a straight line of spark plugs.

And in V engine, they are located in the centre of the V-shape, along the sides of the cylinders.

In boxer and rotary engines, the spark plugs can be found in different places according to the make, model and age of the vehicle.

Spark plugs can last anywhere from 20,000 to 150,000 miles – and in some cases the lifetime of the vehicle.

However, this is unlikely unless the vehicle is regularly serviced, and the owner carries out maintenance on the spark plugs and the surrounding components.

There are three main types of spark plugs that differ in quality and length they can be in use.

• Copper – shortest lifespan
• Platinum – medium lifespan
• Iridium - longest lifespan

However, like most components of a vehicle, the real lifespan of them will depend on many factors, including driving style, how often it is serviced, quality of the item and work carried out, driving conditions, where the car is kept for lengths of time when the engine is cold and the standard of fuel that is used.

Depending on the quality of the spark plugs, and how often they are serviced, they should be replaced every 20,000 to 150,000 miles.

For newer vehicles, there should be a long life left on your spark plugs. However, if you have bought a second-hand car, then you may need to replace them sooner than expected.

Your spark plugs will be checked by a mechanic during an MOT – and you will be told if they need to be replaced.

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Changing spark plugs is a relatively easy task for an experienced mechanic. So, if you are unsure of what to do, take your car to a local mechanic or call for a Mobile Mechanic.

Before you start, make sure that the engine is cold, and the vehicle is parked on a level surface with the hand brake on.

Open the bonnet and locate the spark plugs. To remove the spark plugs, twist and pull each wire from the spark plug and place them somewhere safe. They can be blown away, so if you re working outside, be prepared.

The next step is to disconnect the electrical connectors and any bolts securing the engine coils.

The spark plugs are underneath this and can be unscrewed and then pulled out of their housing.

You may need to attach a spark plug socket to a socket spanner to do this.

In order to successfully install the new spark plugs, you’ll need to carefully thread each item into the cylinder head by hand to avoid cross-threading.

Use the spark plug socket and spanner to tighten the spark plugs into place.

You’ll then need to reverse the initial process by reattaching wires, coils, and any electrical coupling.

Following this, add a small layer of motor grease to each plug and the surrounding area and close the bonnet.

Try to start the engine and listen for anything that doesn’t sound right.

If you think there are some more serious problems, then take your vehicle to a garage as soon as possible.

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If you are looking to keep your current spark plugs in the best condition possible, it is important to clean them as part of your regular car maintenance.

After you have removed the spark plugs from the engine, cleaning them is a simple process.

First of all, wipe any grease or residue off the plug, and then use 220-griy sandpaper to clean the electrode (the small metal piece that extends out of the housing).

If it is dirty, it will be black. Keep using the sandpaper until it has a metallic colour to it.

You may also need to use a file if the carbon build-up has hardened.

Other tools that can help with this are wire brushes and brake cleaner.

These items can be found on RAC Shop.

If you are looking to buy new spark plugs for your car, then a complete set will cost anywhere from £50 to well over £250.

This is based on the make and model of the vehicle, quality of the spark plugs, and the cost for any work carried out by a trusted local garage.

If you have any questions about spark plugs, please leave them in the comments section below.