Electrics: glow plugs

Glow plugs are pencil-shaped parts most commonly made of metal but they also exist in ceramic, with a heating element at the end called the heating coil.

A diesel engine starts by the combustion between compressed air and diesel fuel. The diesel needs to be heated to between 700 ° C and 900 ° C this is what glow plugs do. This is even more important when the outside temperature is low! A diesel vehicle that has been started cold has an unstable engine speed, pollutes and consumes more fuel. Respecting the preheating time is important, so don’t skip it.

Each diesel engine has a glow plug for every cylinder, which is why different makes and models need different numbers of glow plugs. It’s difficult to determine the average lifespan of a glow plug: it all depends on the make, how often you start your car, the weather conditions, etc.

 The main signs of weak glow plugs are:

• Difficulty starting, with an extension of the preheating time.
• In modern vehicles, there’s a visual warning: the preheating warning light may start to flash a few seconds after starting.

Unlike spark plugs for petrol engines, glow plugs aren’t needed to run the engine, their job is to help start the vehicle and then “post-heat” for about 5 minutes in order to stabilize the combustion and reduce pollution.

This process also regenerates the particulate filter (DPF).

The principle is simple: Via an electrical voltage, the current flows through the glow plug from one end to the other. It enters through the connection end and circulates to the opposite end where the heating filament is located, which then rises quickly in temperature until it becomes incandescent (same principle as an electric convector).

It’s difficult to determine the lifespan of a glow plug: it all depends on the brand, the frequency of starting, the weather conditions, etc.

The main signs of weakness are either a more difficult start with an extended warm-up time or a visual warning on modern vehicles.

It’s important to monitor the condition of your preheating system so that you don’t miss the best time to change your glow plugs.

If your fuel system seems to be working fine, then your glow plugs may be the problem. You can check this by using an ohmmeter which will allow you to identify a problem with the continuity of the glow plug's electrical circuit. To do this you need to disconnect the electric wires that connect them to the relay.

If the link is interrupted, it means that the glow plug is faulty.

If the connection is good, then check that the resistance measured is the same on the other glow plugs. If not, then one of them is defective.

Note that contrary to what one might think, the ohmic value must be less than 2Ω to be effective, which is rather low.

Also, be careful that carbon does not form on your plugs! If you notice an excessive presence of carbon it is time to replace your glow plugs!

Obviously, like many parts, the glow plugs should be changed after a while otherwise they can cause:

- difficult or even impossible starting depending on the state of wear

- overconsumption of fuel and engine failures (for "post-heating" type glow plugs in particular)

- an increase in polluting emissions = a possible M.O.T. fail

Typically, their lifespan is around 120,000 km. However, this may depend on several factors such as the brand, the frequency of starting the vehicle or even the weather conditions.

It is important to change all of your glow plugs, even if only one is faulty, in order to avoid an imbalance between the cylinders when the vehicle is started.

Some examples of our glow plugs :

We offer genuine, OEM or aftermarket glow plugs.

For example for a Toyota HDJ 80 12s :

Aftermarket glow plug

Genuine glow plugs

For a  Nissan Patrol Y60 2.8 TD :

Genuine glow plugs

Glow plugs for the first three cylinders. Blue (to replace the factory green glow plugs), the first cylinder being at the cambelt end)

Genuine glow plugs

Glow plugs for last three cylinders (to replace original yellow factory glow plugs)

Genuine glow plug

Glow plug for first three cylinders (green)

Euro4x4parts offers a whole range of glow plugs for 12 and 24-volt vehicles.

Choose between:

• Genuine: genuine part delivered in the original car manufacturer packaging
• Aftermarket: "adaptable" quality part offering the same functions and characteristics as an original part, but not from the original supplier.

For certain vehicles, we also offer OEM parts ( Original Equipment Manufacturer): replacement parts manufactured by an equipment manufacturer who supplies one or more original manufacturers.

We offer a range of glow plugs for nearly 40 makes of 4x4 vehicles.

Glow plugs 

Glow timer & relay

Note: Euro4x4parts publishes this information as a service to its customers but can in no way be held responsible for the advice given here and its consequences in use.