What are Glow Plugs?

A Glow Plug is a heating device that starts a diesel engine.

A petrol engine requires a spark to start and a diesel engine relies on pressure to compress the air within the cylinder to get started. This compression increases the temperature of the air inside the cylinder to a level that will allow the spontaneous combustion of the diesel droplets that are injected into the cylinder. It is the role of glow plugs to heat the cylinders sufficiently to start the engine.

When cold starting a diesel engine, the low air temperature, as well as the cold piston and cylinder surfaces, make it difficult for high enough combustion temperatures to be reached.

When glow plugs deteriorate, a diesel vehicle will be slow to start and once it fires up, will run roughly, miss and produce lots of white smoke.

Older diesel vehicles need to manually preheat glow plugs prior to starting whereas most modern vehicles automatically preheat the glow plugs.

“Older engines with Thermostat manifold plugs used a glowing period of up to 20 seconds whereas more modern engines use around a 6 to 8 second heat period and provide afterglow at a reduced voltage.”


If one glow plug goes, it is recommended to change all at the same time according to Bosch. Once one fails, it won’t be long before the rest go.

It’s Time To Change Your Glow Plugs When:

  • Your Diesel Vehicle Fails To Start
  • Vehicle Only Starts After Excessive Cranking
  • Vehicle Idles Roughly After Initial Starting
  • Excessive Exhaust Emissions After Initial Starting

Although glow plugs can easily last more than 100 000 km they could stop working at any time and need to be replaced when the above symptoms are present.

If you have any concerns with your vehicle contact us or book online for any of the services we offer.

Alternatively, follow us on Facebook and contact us via Facebook Messenger at m.me/fixnfitauto

Header Image © Fix n Fit Auto Limited (For Decorative Purposes Only) 
Informational Blog Disclaimer
Verified by MonsterInsights