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Glow plugs are a unique heating element found in diesel engines. They look similar to spark plugs, and they help diesel engines start up in colder temperatures. Their namesake comes from the heating element on the tip of a glow plug, which glows when it gets warm. The heating element then ignites the fuel if the engine is not hot enough to operate normally.
ProSource Diesel takes a closer look at what glow plugs do, how they work, and how to change them when they fail.
As mentioned before, glow plugs are unique diesel truck parts that help a diesel engine get started in cold weather. Engines can be difficult to start in cold weather, so a glow plug ignites fuel to help. They ignite fuel using the heating element on the tip, which glows when electrified, hence the name “glow plugs”.
Although they are a different part, glow plugs look very similar to spark plugs. Both are short, vaguely pencil-shaped metal pieces. Both have a connector at the top end for wires. There are differences, though: glow plugs have a slimmer shape, and the heating element on their bottom end is more like a toaster element, instead of the small hook-shaped electrode spark plugs have.
Glow plugs are needed because diesel engines work differently from gas engines. Diesel engines don’t need spark plugs for combustion; instead, diesel engines start through high compression: superheated air that 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.
Getting the air hot enough for spontaneous combustion requires immense pressure. When the engine is cold, some of the compressed hot air escapes into the cold cylinder walls. Putting glow plugs in the cylinders or the manifold solves this problem.
Generally, diesel engines don’t actually need a glow plug to start the engine. 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.
The heavy-duty diesel engines found in tractors and other machinery use in-manifold glow plugs, also called Thermostart plugs. These plugs are usually activated using a separate button rather than by the normal ignition switch.
If the engine is already warm, the in-cylinder plugs might not activate, and the engine can be started normally. Thermostart plugs can be activated if the driver chooses to activate them.
There are two different types of glow plugs. One goes in the cylinder, and the other goes in the manifold. In-cylinder glow plug models have a plug in each cylinder. In-manifold types only use one glow plug that serves all the cylinders.
The number of glow plugs in a diesel engine varies by the number of cylinders. If it’s a Thermostart (in-manifold) glow plug, then there will only be one. For example, an eight-cylinder engine using in-cylinder plugs would have eight plugs.
Glow plugs usually last for up to 100,000 miles. They gradually wear out due to constant use. If the glow plugs are of poor quality, or you live in a cold area where they’re frequently activated, then they might fail sooner. Some signs that your glow plugs need to be replaced include:
Hard starting is the most common symptom of failing glow plugs. It may be accompanied by clouds of smoke as well.
You’ll need a multimeter set to read in Ohms 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.
Changing glow plugs is relatively straightforward. First, you must 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 before you can remove the old glow plugs. Then you can insert the new glow plugs in each cylinder, and then reconnect the cables.
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.
There are several different types of glow plugs on the market. Price varies too; they can cost as little as $10 or as much as $320. Price is largely affected by the application, make, model, and year of your vehicle. For example, a set of eight glow plugs for a 2008-2010 6.4L Powerstroke costs around $219. Labor costs for installing glow plugs can add $100 to $200 per plug.
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.
