What are glow plugs? Maintenance and replacement guide

An engine is made up of many complicated components that made it all work in harmony – so ensuring that your glow plugs are fitted correctly and in full working order is something that you should be monitoring.

This guide looks at everything you will need to know about glow plugs and keeping your car in the safest condition possible.

Glow plugs are electrical heating devices used in diesel engines to help start a vehicle.

They are very common in diesel engines, and work by compressing air in the cylinder and then injecting diesel fuel into the engine.

These are the diesel equivalents to spark plugs for petrol engines.

One of the main differences is that diesel engines rely on the high temperature created by the compressed air to ignite the fuel.

However, in cold weather, the air temperature may not be high enough to ignite the diesel fuel effectively. This is where glow plugs are important for drivers of diesel cars.

They are used as heating tools that are installed in the combustion chamber of each cylinder of the engine. When the engine is started, they preheat the air so that the start-up is smooth and reliable.

After the engine has been running, and the temperature is regulated, the plugs turn themselves off.

Due to their importance for winter and colder weather – it is vital that they are working during these conditions.

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Glow plugs are primarily used in diesel engines.

Diesel fuel requires higher temperatures to ignite compared to petrol-powered vehicles.

This is why glow plugs are used to preheat the air in the cylinders to ensure that the engine can start and continue at the correct temperature.

In petrol engines, spark plugs are used for ignition instead.

Spark plugs create an electric spark to ignite the air-fuel mixture in the engine cylinders. This is why there is no need for glow plugs in petrol cars as they rely on spark plugs to initiate the combustion process.

In most diesel-powered vehicles, there is typically one glow plug per cylinder. This would mean that in a six cylinder engine, there are six glow plugs.

However, this can vary depending on engine design, manufacturer, and quality of the engine.

It can also depend on the engine size and vehicle model or modification.

The information will be outlined in the owner’s manual.

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Here is a step-by-step guide on the best way to remove and replace your glow plugs:

1. Open the bonnet, and locate the cylinders. Open them up and use a socket wrench to remove the old glow plugs.
2. Carefully insert the new plugs and tighten them using the same tool.
3. Make sure that they are properly tightened. Follow the torque specifications in your car owner’s manual.
4. You’ll then need to reconnect any wires or connectors that were removed when you initially disconnected the plugs.
5. Close up the cylinders and close the bonnet.
6. Start the engine and see if the engine is running smoothly.

If the engine has trouble starting or runs roughly, there may be an issue with the glow plugs.

In this situation, take your vehicle to a trusted local garage or call a mobile mechanic.

Glow plug replacement

If you are looking to buy some replacement glow plugs, then you can expect to pay around £10 per plug.

Of course, this can vary depending on the make, model, and specifications of the engine.

In many cases, if you buy multiple new glow plugs, then there is a discount on the price per plug.

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Checking the condition of glow plugs in a diesel engine involves a few steps – and something you should be doing as a part of regular car maintenance.

Before you get started, make sure the engine is turned off and that you're working in a well-ventilated area.

1. The glow plugs will be near or inside the cylinder head, with one glow plug per cylinder.
2. To get the glow plugs out safely – and without damaging them – use a socket wrench and carefully remove the glow plugs from the cylinder.
3. In some cases, you can wipe off the carbon deposits and reinstall the old plugs. Make sure you lubricate the plug before reinserting it.
4. Carefully tighten the new plugs and reconnect any wires or connectors that were removed.
5. Start the engine and away you go.

Please note, that in some higher spec models, you may be provided with a specific tool to remove the plugs.

Common issue with glow plugs are cracks, wearing, and carbon build-up. If you see any of these problems, replace them as soon as possible.

If you are confident in using a multimeter tool, you can check the resistance of each glow plug. To ensure that this is set properly, check the car manual and the pack of the new plugs.

Glow plugs normally last for around 80,000 to 100,000 miles - although this can depend on make, quality and driving style.

So, if you are wondering how long your new glow plugs will last, rest assured that they will be reliable for a while yet.

If you have any questions about glow plugs, leave them in the comments below.