A Guide to Glow Plugs for Diesel Engines
January 17, 2023

A Guide to Glow Plugs for Diesel Engines

Written By: ProSource Diesel

Glow plugs are devices that look similar to spark plugs, but they are used in diesel engines to aid in starting in cooler temperatures. There is a heating element on the tip of a glow plug that glows when warm, which is where it gets its name. This element ignites fuel when the engine is not hot enough for normal operation.

ProSource Diesel takes a closer look at what glow plugs do, how they work, and how to change them when they fail.

What Are Glow Plugs?

Glow plugs are diesel truck parts that assist in starting a diesel engine in cooler weather. The plugs have a heated element at the tip that glows when electrified. The purpose of glow plugs is to ignite fuel in a cold engine, which enables it to start quicker.

What Does a Glow Plug Look Like?

Glow plugs actually look very similar to spark plugs. They are both short, vaguely pencil-shaped metal parts with a connector at the top end for wires. Glow plugs have a slimmer shape, however, and the heating element at the bottom end is more like a toaster element vs the small hook-shaped electrode on a spark plug.

A Guide to Glow Plugs for Diesel Engines

What Do Glow Plugs Do?

The reason for glow plugs lies in how diesel engines work in comparison to gas engines. Diesel engines don’t need or use spark plugs for combustion. Instead, diesel engines start through high compression, which creates superheated air that then spontaneously combusts diesel fuel when it enters the air.

However, the pressure required to heat the air to a high enough temperature for spontaneous combustion is immense. When the engine is cold, some of the compressed hot air gets lost in the cold cylinder walls. Glow plugs in the cylinders or the manifold solve this problem.

How Do Glow Plugs Work?

In general, a diesel engine doesn’t need anything like a glow plug to start. Some direct-injected diesel engines don’t have glow plugs at all. In-cylinder glow plugs begin pre-heating for two to five seconds before the operator can start the engine. The wait-to-start relay goes off after the engine is running.

For heavy-duty diesel engines found in tractors and other diesel machinery, in-manifold glow plugs, also called Thermostart plugs are utilized. These are usually activated by a separate button rather than the normal ignition switch.

In the case of a warm engine, the in-cylinder plugs may not activate at all, and the engine can be started normally. With Thermostart plugs, it’s completely up to the driver whether to activate the plug.

A Guide to Glow Plugs for Diesel Engines

How Many Glow Plugs Are In a Diesel Engine?

There are two different types of glow plugs. One is the type that goes in the cylinder, the other is a type that goes in the manifold. For in-cylinder types of glow plugs, there is a plug in each cylinder.

For the in-manifold type, there’s only one glow plug that serves all the cylinders. So the number of glow plugs in a diesel engine will vary by the number of cylinders, or it will just be one Thermostart (in-manifold) glow plug. For example, an eight-cylinder engine using in-cylinder plugs would have eight plugs.

How Long Do Glow Plugs Last?

Glow plugs usually last for up to 100,000 miles. They will gradually wear out due to constant use. Some plugs may fail sooner if they’re poor quality or are used more frequently in hard or cold starting situations. Some signs that your glow plugs need to be replaced include:

  • Misfiring
  • Rough idle
  • Decreased fuel economy
  • White or black smoke when starting
  • Hard starting

Hard starting is the most common symptom of failing glow plugs. It may be accompanied by clouds of smoke as well.

A Guide to Glow Plugs for Diesel Engines

How To Test Glow Plugs

You’ll need a multimeter set to read in Ohms in order to test your glow plugs. You may need to set your multimeter to a higher sensitivity to get a read on your glow plugs. Disconnect the glow plug wires first, then touch the negative leads of the multimeter to the negative terminal on your battery.

Take the positive lead of the multimeter and touch it to the glow plug terminal. For most glow plugs, the range for resistance should be between 0.1 and 6 Ohms. If your glow plugs exceed or fall below that, then they may need to be replaced.

How To Change a Glow Plug

Glow plugs are relatively straightforward to change. The first step is to disconnect the battery cable, then you’ll need to remove the valve cover to locate the glow plugs. The glow plug wires will need to be disconnected first, then you can remove the old glow plugs. Insert the new glow plugs in each cylinder, then reconnect the cables.

How Hot Do Glow Plugs Get?

Glow plugs get extremely hot in less than two seconds. Modern ceramic glow plugs reach operating temperatures of about 1,832 degrees Fahrenheit and can maintain this temperature regardless of external conditions.

A Guide to Glow Plugs for Diesel Engines

How Much Are Glow Plugs?

There are many different types of glow plugs, and they can cost as little as $10 to as much as $320. The price is largely affected by the application and the make, model, and year of your vehicle. Labor costs to install glow plugs range between $80 and $160. For example, a set of eight glow plugs for a 2008-2010 6.4L Powerstroke cost around $160.

Rely on ProSource Diesel for a wide variety of diesel parts for trucks, including different types of glow plugs. We stock many replacement parts, upgrades, kits, and more for Powerstroke, Cummins, and Duramax diesel engines.

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