What are glow plugs?

What are glow plugs used for?

Glow plugs pre-heat the engine cylinders, so the engine can start smoothly and quickly when cold. One glow plug is fitted to each cylinder. Therefore, an engine with four cylinders will have four glow plugs.

Old 'indirect' diesel engines used to require long periods of pre-heating, when the glow plugs would need to be operated, before the starter motor could be operated.

Newer 'direct' injection diesel engines, with their higher compressions and sophisticated high-pressure fuel injection, rely on glow-plugs less. Only on very cold mornings do modern engines require pre-heating.

However, glow plugs play other roles. They can be activated, when the engine is running, to speed warm-up times.

Working glow plugs are also essential for effective diesel particulate filter cleaning. Should one, or more, glow plugs fail, the first noticeable symptom is a diesel particulate filter problem.

How do glow plugs work and how do they fail?

Glow plugs seem to be very simple components. Yet, they are precision-made parts. They comprise one, or more, heating elements at one end, with an electrical connector at the other. The metal body is threaded, so the plug can be screwed into the engine cylinder-head.

Glow plugs have a tough life. They must withstand temperature increases of hundreds of degrees within seconds. They also must face the full force of diesel combustion. It is little wonder that they do not last forever. They can suffer from various ailments but the most common issue is their internal heating element failing. This results in the plug not heating up and a problem being flagged by the engine management system.

What types of glow plugs are there?

Despite looking similar, different types of glow plugs exist. Unsurprisingly, some types are more expensive than others. However, resist the temptation to downgrade replacement glow plug specifications. Fit like-for-like parts, at the very least.

In brief, the 'quick glow' plug employs two heating coils at the end of a metal probe, some of which are controlled by an external control unit that governs glow plug probe temperatures, by varying the electrical current supply.

As its name implies, self-regulating plugs possess a regulating heating coil internally, meaning that they should not be used on cars with external regulators.

Ceramic glow plugs possess a probe made from silicon nitride, rather than metal, the higher melting point of which permits higher temperatures to be reached faster than possible before. These types can also be controlled either internally, or externally.

The latest in glow plug technology uses twin heating coils that can reach their maximum working temperatures from cold in only several seconds. The voltage supply is controlled by a computer that allows the system to consider inputs from other sensors on the car.

Where are glow plugs located?

Glow plugs screw into the cylinder-head. The probe emerges either in the engine cylinder, or within a dedicated chamber inside the cylinder-head, dependent on the engine design.

Accessing the plugs to test, remove, or replace them depends on how the engine is installed within the car. Sometimes they can be between the engine and the bulkhead/firewall, making them very tricky to locate.

How do you test if glow plugs are working?

A good quality diagnostic tester, which plugs into the car's diagnostic socket, can provide data about glow-plug performance. However, you can test them directly. While dedicated glow plug testers are available, you can use a simple multimeter to check the integrity of each plug's heating coils. Should your result record infinite resistance (i.e. open circuit), you have a broken internal coil. Expect a healthy plug to give you a value of five ohms, or above - apart from ceramic plugs, many of which record less than one ohm.

What glow plug replacement tips can GEM share?

Glow plugs may be hardy and endure a tough life but they are also delicate. Never apply excessive force, when removing them.

A problem is that they can corrode into the cylinder head, so if you apply too much pressure to unscrew them, or apply force at an angle, there is a real risk of them snapping. Removing the remaining portion of the glow plug can be a nightmare and could dictate extensive dismantling to remove the cylinder-head and have the glow plug's remains machined out.

We recommend that you use a dedicated glow plug removal tool.

With the glow plug removed, clean out any carbon from within the cylinder-head recess, using a dedicated reaming tool.

Never replace a single failed glow plug - change them, as an engine set.

Never overtighten new glow plugs, nor lubricate the threads. Tighten them to the torque value stated on the packet, or the manufacturer's technical literature.

Faulty relays can overload the glow plugs, causing premature failure. Many suppliers recommend that the relay is tested at the very least. Should there be any doubt about its condition, it should be replaced at the same time as the glow plugs.

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