What are glow plugs and where are they located?

Glow plugs, sometimes spelled as glowplugs, are used to help diesel engines start in cold weather. Unlike gasoline engines, which use spark plugs to ignite fuel in the combustion chambers, diesel engines, fuel spontaneously combusts under high pressures and temperatures. Diesel engines use long stroke cylinders to achieve very high air pressures. The high pressure leads to high temperatures, which cause the fuel to combust as it is sprayed into the chamber.

In cold conditions, though, it can be difficult to get the temperature high enough in the chamber to achieve spontaneous combustion. To solve this problem, many diesel engines use glow plugs to heat the air in the chamber when the engine is cold so that it can start. The glow plug is a long, pencil-like piece of metal with a heating element at the tip, usually located at the top of the cylinder at the engine head. It receives an electrical charge and heats up. If you were to look at a heated glow plug (which you just might do, if you’re testing one—as you’ll read later), you would see it start to emit visible light, hence the name “glow” plug.  The heat that is then generated by the glow plug goes through the cylinders, warming the engine block surrounding the cylinders. Once the engine is warm, the vehicle is able to be started.

How do I know if my glow plugs need to be replaced?

Glow plugs can fail for a number of reasons. The high heat they experience can eventually cause damage to the part. This can be exacerbated if the glow plugs stay on too long or are on too frequently; for example, if the glow plugs come on when the engine has recently been run and is still warm. Some trucks will have a manual glow plug control. Leaving this on too long can damage the glow plugs. Some diesel engine glow plugs are run by an electronic controller. Problems with the controller may cause the plug to stay hot for too long, or to experience too great of an electric charge. Power surges can also damage the glow plugs. Glow plugs can also become corroded or simply wear out over time. 

Failing glow plugs will make a diesel engine run poorly, particularly in cold weather, and shortly after its been started (assuming you can get it started). Difficulty in starting is, in fact, one of the major symptoms of failing glow plugs. Even once the engine has started it may not run smoothly. Since the fuel won’t combust completely, the engine may run roughly or misfire. Uncombusted fuel can also find its way to the exhaust where heat may cause it to combust. This will lead to thick white smoke coming out of the exhaust, as though your truck has selected a new Pope. Eventually, the diesel engine’s normal processes will cause it to heat up and complete combustion will be achieved. This will cause the roughness and smoke to go away—until the next time you start the engine cold. These symptoms, especially when encountered for a brief period upon starting, are the sure sign of glow plug trouble. 

If you are still uncertain of whether or not your glow plugs are working correctly, there are a couple ways to test them. You could use a test light. Connect the test light to your battery’s positive terminal and touch the probe to each glow plug. Any plugs that do not cause the test light to light need to be replaced. There is also a way to test the glow plugs with a battery charger. Connect the negative clamp to the glow plug body and the positive clamp to the plug terminal. The plug terminal should eventually start to glow red hot. If it does not, then it should be replaced. Do not apply charge to the glow plug any longer than is necessary to determine if it is working. Leaving it on too long may damage the glow plug. 

Can I replace glow plugs myself?

Replacing glow plugs on a diesel engine should prove no more difficult than replacing the spark plugs on a gasoline engine. Depending on your model, you may need to remove the valve cover, or other parts to access the glow plugs. With that done you’ll have to disconnect the glow plug wires, use a socket to remove the old glow plugs, install the new ones, reconnect the wires, and replace any other parts you removed. When installing the new glow plugs, be sure not to over-tighten them, as doing so could damage them. 

Need replacement glow plugs for your diesel engine?

If you are in need of replacement glow plugs for your vehicle’s diesel engine, then look no further than 1A Auto. We offer a large selection of new and aftermarket glow plugs for many makes and models, and at great prices so that your vehicle will never go cold.

At 1A Auto, we also make shopping for replacement diesel engine glow plugs for your vehicle easy - we're here to help you select the right parts! Call our customer service toll free at 888-844-3393 if you have any questions about our new and aftermarket glow plug replacement parts, warranty, compatibility or to purchase, or you can buy online.