What are glow plugs?
Petrol and diesel vehicles both use “plugs” to begin the combustion process that starts the engine. However, petrol engines use spark plugs, while diesel engines use glow plugs.
What do glow plugs do?
So, what’s the difference between a glow plug and a spark plug? Since petrol is highly combustible, petrol engines rely on a spark to ignite the fuel and air mixture, which is provided by spark plugs.
Diesel burns at a much lower temperature compared to petrol, and so doesn’t require a spark to ignite the fuel. Instead, diesel engines use compression force to create the heat required to ignite the fuel and air mix. In cold start conditions, a diesel engine requires some help from a glow plug. These plugs don’t produce a spark, but instead heat up at one end to aid the combustion process and produce a cleaner burn with fewer emissions.
How long do glow plugs last?
Glow plugs do typically have a long lifespan, but over time they will naturally degrade due to wear and tear, and even faster in cold weather. On average, you can expect your glow plugs to last around 100,000 miles. So, you don’t need to worry about replacing them too often.
How to test glow plugs
It can be difficult to check your glow plugs without a specialised piece of equipment called a multimeter. This tool checks the glow plug’s electrical resistance and voltage, to determine how well it’s performing.
If you don’t have a multimeter, you should still keep an eye on how your engine starts to try and spot any potential issues with your glow plugs. There are a few signs that indicate your glow plugs might need replacing:
- The engine struggles to start when it’s cold
- There’s raw, uncombusted fuel in the exhaust soon after you start the engine
- Your vehicle is idling rough
- There are substantial amounts of smoke when you first start the engine
If your glow plugs are the part causing issues, then the engine should run normally after it warms up.
How to change glow plugs
Depending on your vehicle make and model, this process can be relatively simple, or more complicated. If you’re unsure about carrying out any vehicle maintenance, speak to your local Halfords garage where we’ll be happy to help.
Here’s a generalised guide of the process to give you an idea of what to expect:
- Make sure the ignition switch is in the off position.
- Access the glow plugs: Some cars have a cover that protects the glow plugs, which you’ll need to remove to access them.
- Disconnect the glow plug wires: Each glow plug will be attached to a wire, which you’ll need to remove. Depending on your vehicle, you may need to remove bolts, nuts, terminals, or other securing mechanisms.
- Remove the old glow plugs: Using a socket and ratchet, remove each of the glow plugs. You may need to use a deep well socket to reach properly. A releasing agent such as WD40 may help to support removal. Assess the condition of each glow plug to make sure that nothing has broken off and remains inside the engine.
- Fit the new glow plugs: You can now fit the new glow plugs, positioning them by hand then tightening with a ratchet. To avoid breakages, do not exceed the torque recommended by your manufacturer.
- Reattach the glow plug cables: reconnect the glow plug cables and secure them, then reattach the valve cover if you removed one. You can then start the engine to test the glow plugs.
How many glow plugs do I need?
The number of glow plugs you need depends on your vehicle make and model. Diesel vehicles require one glow plug per cylinder, but even if just one needs replacing, you should always replace glow plugs as a set for the best performance.
How much are glow plugs?
Replacing your glow plugs shouldn’t be an expensive job, but the exact cost will depend on your vehicle, and the brand of glow plugs you choose.
We have a range of glow plugs available online, and you can find the right fit for your vehicle by entering your registration number. Our car part finder tool will select only the plugs that fit your make and model, so you know you’ve got the right parts for the job.