What is a Glow Plug?

November 6th, 2022 by

Car technology has come a long way since the Model T hit the road back in 1908. While new advances are immediately noticeable and celebrated in areas such as performance, fuel economy, and safety, other components work quietly behind the scenes to keep your car running smoothly. The glow plug is one of them.

Glow plug technology has advanced significantly in recent years, dramatically improving the time it takes for your engine to start in cold weather. But what exactly is a glow plug? What do glow plugs do? How much does it cost to replace them? And what causes them to fail in the first place? In this informative guide, we’ll answer all of those questions and more. So stay tuned as we explore everything you need to know about glow plugs.

What Is a Glow Plug?

Glow plugs are essentially small heater elements used to preheat air in the combustion chamber of a diesel engine prior to starting. Thankfully, you don’t really need glow plugs to start a diesel engine when the weather is warm due to the warmer temperature of the intake air. However, starting a diesel engine when the weather is cold is generally much more difficult.

Consequently, glow plugs are used to make starting a diesel engine in cold weather conditions much easier by preheating the air in the combustion chamber, allowing it to reach its ignition temperature more quickly. Glow plugs are activated automatically when you turn on the ignition and typically only remain active for a few seconds. In most cases, modern glow plugs are made from ceramic or metal and are electrically powered.

How Do Glow Plugs Work?

When you turn on the ignition, electricity flows through the glow plugs and creates extremely high heat (1,300 degrees Celsius for ceramic glow plugs). This heat warms up the air in the combustion chamber, making it easier for the engine to start. The greater temperature also ensures that the fuel burns efficiently, reducing emissions.

Modern glow plugs heat up quickly, with some models reaching maximum temperature in as little as two seconds or less when used with an Instant Start System (ISS). Even when starting in temperatures as cold as minus 30 degrees Celsius, a diesel engine equipped with ISS and glow plugs can fire up in a matter of seconds.

What Causes Glow Plugs To Fail?

Glow plugs can last for years but will eventually need to be replaced. Here is a list of common reasons for premature glow plug failure:

  • Glow plugs that are the wrong type for the engine: There are several types of glow plugs, each designed for a specific engine type. Consult your owner’s manual or a certified technician to ensure you’re using the right glow plug for your engine.
  • Excess carbon buildup in the pre-chambers: Excess carbon buildup can lead to overheating or shorting of the glow plugs, causing them to fail.
  • Incorrect spray pattern of the fuel injectors: This can cause excess squirting of raw fuel onto the glow plug, forcing rapid cooldown and eventual failure.
  • Glow relay not turning off: The glow relay is responsible for supplying power to the glow plugs. If the glow relay does not turn off, it can cause the glow plugs to burn out.
  • Glow cycle is too long: If the glow plugs are on for longer than necessary, they can overheat and fail. This could be due to a sticking relay, the relay being faulty, or a feedback circuit not turning the relay off soon enough.

Signs That You Need To Replace Your Glow Plugs

Over time, glow plugs will inevitably need to be replaced. If you’re wondering how to tell when it might be time to replace your glow plugs, here are a few telltale signs to be on the lookout for:

  • Your car takes longer than usual to start in cold weather.
  • The fuel fails to ignite within the cylinder, causing the engine to run roughly or misfire.
  • You notice a decrease in fuel economy, which may be due to incomplete combustion. This means that unburned fuel is passing through the engine.
  • Black, sooty smoke comes from the tailpipe, possibly indicating that the air-fuel mixture is not burning completely.
  • Your car produces white smoke, meaning the combustion chamber might not be getting hot enough to burn fuel.

If you notice any of these symptoms, it’s essential that you immediately have your glow plugs checked by a qualified mechanic. Replacing them can help to improve engine performance and prevent further damage.

How Long Do Diesel Glow Plugs Last?

Diesel glow plugs typically last for approximately 100,000 miles. However, this depends on several factors, such as the make and model of your vehicle, your driving habits, and whether or not you regularly perform maintenance on your car.

If you take good care of your car and perform regular maintenance, your glow plugs can last even longer. Be sure to consult your owner’s manual to find out how often to replace your glow plugs.

How Much Does It Cost To Get Glow Plugs Replaced?

The cost of replacing glow plugs will vary depending on the make and model of your vehicle, but it is typically between $235 and $287. In some cases, you may also need to have the glow plug controller replaced. This can add between $283 and $305 to the cost of repairs. However, replacing your glow plugs is much cheaper than having to replace your entire engine, so it’s worth getting them checked regularly.

Glow plugs are essential to any diesel engine, and keeping them in good working condition is vital to the health of your car. It’s best to have them checked regularly and replace them as needed to prevent any issues. Sweeney Chevrolet offers a wide range of services to keep your vehicle running like new. We are also proud to offer a wide selection of new and used cars. Don’t hesitate to get in touch with us to schedule a test drive or service appointment.

Shiny new glow plug ready for action. by Tyrone Adams is licensed with CC BY-SA 2.0


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