What Are Glow Plugs in a Diesel?

Although gasoline and diesel engines both operate on similar principles, they have a variety of mechanical differences that make them unique from each other. One of these is the absence of spark plugs and the inclusion of glow plugs.

Why Don’t Diesel Engines Need Spark Plugs?

The answer comes down to how the combustion process happens in a diesel engine. Unlike gasoline, which needs an external spark to ignite in the combustion chamber, diesel engines rely on high compression to initiate combustion.  

Spark Plugs vs. Glow Plugs

So if diesel doesn’t need an electrical spark for combustion, what are glow plugs needed for? Glow plugs are only used for initial startup, as they heat the inside of the combustion chamber to the minimum temperature to get the engine started. Once the cycle begins, the heat for combustion is naturally generated in the compression of the diesel fuel. 

Diesel glow plug
Donar Reiskoffer, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons

How Does a Glow Plug Work?

Glow plugs are similar in shape to spark plugs, with one extending into each cylinder of the engine. Using a filament made of materials such as platinum or iridium, the glow plugs quickly turn red hot and heat the air within the combustion chamber. 

In older diesel vehicles, the glow plugs could require up to 20 seconds of manual activation to reach the proper temperature. This is less of an issue on newer diesels, where more advanced glow plugs can reach operating temperature in just a few seconds. 

Not all diesel engines need glow plugs, however. They’re primarily used on smaller diesel engines that cannot maintain the necessary heat for ignition. Many direct-injection diesel motors forgo glow plugs, as well as larger engines that have less trouble maintaining the right internal air temperature. 

Symptoms of Faulty Glow Plugs

  • Illuminated glow plug light
  • Trouble starting vehicle (especially in the cold)
  • Misfiring engine
  • White/black smoke from exhaust
  • Rough idle

How Often Should I Replace My Glow Plugs?

In general, modern glow plugs should be replaced anywhere between 80,000-100,000 miles. Their lifespan can depend on your vehicle, driving style, and various other factors. 

Glow plugs are also one of the cheaper engine components to replace, meaning you won’t have to break the bank to help ensure your diesel vehicle is running at its best. 

Glow Plug Replacement in the Salt Lake Valley

If you’re looking for quality diesel service and glow plug replacement, the ASE Certified technicians at Matson Point S in Riverton, Utah have the experience and equipment to properly take care of your vehicle! Call or schedule an appointment online with us today!

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