What Is a Glow Plug?

Glow plugs allow for easier and more efficient cold-weather starts in diesel engines.

Andy Stonehouse | 

Aug 28, 2024 | 3 min read

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If there's a diesel engine under the hood of your truck or you are the owner of an increasingly uncommon diesel-powered car or SUV, you know that diesel motors employ glow plugs instead of the spark plugs found in gas engines.

The electric-powered glow from these ignition sources helps with cold starts. As with spark plugs, regular glow-plug maintenance is required to keep your diesel vehicle running efficiently.

Glow Plugs Are the Diesel Engine's Spark Plugs

While gas-powered internal-combustion engines use spark plugs as an electrical source to ignite a mixture of fuel and oxygen and drive pistons in their cylinders, diesel engines instead use high compression to cause that combustion, making them more fuel efficient than gasoline engines.

But to get a diesel engine started, especially in cold temperatures, glow plugs are used to bring it to ignition temperature. Diesels can start without glow plugs in warm temperatures, but cold starts require a glow plug to kick-start the combustion cycle.

Glow Plugs Only Work During the Initial Ignition Process

Unlike spark plugs, which fire an electrical current on every single combustion cycle while the gasoline engine is running — thousands and thousands of times — glow plugs are only required during the initial ignition process in a diesel engine.

A glow plug is a long, thin, pencil-shaped metal device featuring a heating element at the top that literally glows hot enough to emit light when the plug energizes.

This element warms the combustion chamber for easier starting. Intake air is compressed before the fuel injector sprays fuel on the glowing tip of the plug, producing a tiny explosion — even in cold temperatures.

In older diesel vehicles, it was necessary for a driver to manually turn on the glow-plug system and wait up to 20 seconds for the plugs to warm up. Modern diesels automatically activate the glow plugs and allow faster starts, even in cold weather.

Glow Plugs Are Relatively Simple Devices

Glow plugs are essentially heating coils surrounded by a metal tube filled with ceramic powder, which works as an electrical insulator. In just moments, the tip of the glow plug can exceed 1,800 degrees Fahrenheit, leading to an efficient initial combustion.

A variation on all-metal glow plugs is ceramic glow plugs. Somewhat like a gasoline engine's spark plugs, they are encased in a silicon-nitride ceramic coating to heat up even more quickly, adding to their fuel efficiency.

Glow Plug Maintenance Is Critical to Diesel Performance

Though glow plugs can last as much as 100,000 miles, they can wear out quickly, especially if you often use your diesel engine in a very cold climate. Symptoms of worn glow plugs include difficult start-ups with black or white exhaust smoke, misfires, a rough idle, or a drop in overall fuel economy. An engine warning light is also a sign that glow plugs should be inspected or replaced.

Andy Stonehouse

Andy Stonehouse literally fell into the world of auto writing while working as a ski-town journalist, and has not looked back since. A childhood spent dealing with the eccentricities of a 1976 MG Midget has made any subsequent auto experience a more safe and reliable drive. He has been blessed with nearby mountain trails and snowy roads in Colorado to do TV-adventure-styled test drives on a weekly basis.