Getting to know glow plugs

With the summer sun shining, this week’s “Getting to Know” guide sheds some light on glow plugs which can be found in diesel-powered cars.


What is a glow plug?


A glow plug is a pencil-shaped piece of metal with a heating element at the tip. It essentially consists of a housing with screw-in threads and a pencil element pressed into the housing. The single-pole connecting pin is glued to the housing by means of a non-detachable round aluminium nut.


What is the role of glow plugs?


While petrol engines use spark plugs during ignition, diesel engines use glow plugs to start. The principal role of glow plugs is to heat up the air in a combustion chamber of a diesel engine to allow it to reach the necessary temperature. There may be as many as 10 glow plugs, one for each cylinder of the engine.


How do glow plugs work?


When electrified, the heating element heats due to its electrical resistance and begins to emit an orange-red light, hence the term glow plug, which is similar to what happens with a heating element in a toaster. The air-fuel spray generated by the injector ignites close to the red hot tip of the glow plug, which initiates combustion.


How do you know if the glow plugs are working?


When you turning on the ignition, the glow plug symbol (an orange squiggle) illuminates on the dashboard, and will then go out after a few seconds. When a car’s ignition is started with a button rather than a key, the car’s computer knows to delay starting the engine until the glow plugs have been activated.


How do you know if the glow plugs are not functioning correctly?


The principal symptoms are:


  • The engine may become more difficult to start, especially in cold weather.


  • There is black smoke when starting the engine, indicating the presence of unburned fuel due to an incomplete combustion process.


Where can I find a garage to get my glow plugs looked at or replaced?


To find your local Motor Ombudsman-accredited franchised car dealership or independent garage to get your glow plugs looked at or replaced, visit The Motor Ombudsman’s online Garage Finder.


To read a Motor Ombudsman case study on glow plugs, click here.


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