Diesel Glow Plugs

Because the air in the cylinders of a diesel engine must get very hot for the fuel to ignite, a diesel can be difficult to start when ambient temperatures are extremely cold. Not only is the intake air cold, but the cold metal of the engine can absorb the heat of compression, preventing ignition. To aid cold starting, many diesel engines have glow plugs threaded into the combustion chamber or pre-chambers. As the name suggests, glow plugs contain a heating element that gets so hot it glows when current is applied. Operation varies according to vehicle, but in general the glow plugs should operate for a period of time when the engine is cold, to heat the air and ignite the incoming fuel.

Gas engines are called spark ignition engines, because they rely on a spark provided by the ignition system to ignite the air/fuel mixture in the combustion chamber. Combustion in a diesel engine, which is called a compression ignition engine, is initiated when atomized fuel is injected into highly compressed hot air in the combustion chamber or a pre-chamber. To raise the temperature of the air in the cylinder to the diesel fuel’s self-ignition temperature, diesel engines have much higher compression ratios than gas engines, typically 17-25:1. Reaching self-ignition temperature is easy for initial starting at warmer ambient temperatures or when a warm engine is restarted, but when it’s very cold outside starting aid is needed to get a diesel engine running.

There are a variety of ways to add some warmth to a diesel engine vehicle and make it less difficult to start on a frosty morning. A block heater is typically installed in place of a freeze plug. It has a heating element, which is connected by a cord to house current, that projects into the coolant. Block heaters warm the coolant and also raise the temperature of the engine oil. Engine oil can also be heated with a dipstick heater, which directly heats the oil in a similar manner. Cold oil is more viscous, which makes cranking difficult. Cold weather adversely affects car batteries, slowing internal chemical processes and reducing their cranking capability. Keeping the battery warm with a battery warmer will help the battery function normally and deliver the current needed to turn over the engine and supply power to the glow plugs. A battery warmer wraps around the battery and is also 120-volt powered.

Some diesel engines have a grid heating element in the air intake to heat air entering the engine and aid cold starting, but glow plugs are a more common way of introducing heat into the cylinders of a cold diesel engine. Glow plugs are pencil-shaped electrical devices that are powered by the vehicle’s battery. They are threaded into each cylinders’ pre-chambers on indirect injected diesel engines or into the combustion chambers on direct injected engines. When the ignition key is turned to the on position, a light will illuminate on the dash indicating the glow plugs are heating up. On modern diesel engines, glow plugs generally require a period of seconds to reach their normal operating temperature, then the light will go out and the engine can be started. The glow plugs will raise the temperature in the combustion chambers sufficient for the diesel fuel to ignite when injected. On some vehicles the glow plugs will continue to operate after the engine has started to improve combustion efficiency and reduce emissions.

Glow plug operation is governed by a glow plug control module using input from the engine control computer. When the ignition key is turned on, the module establishes the amount of current and glow plug on-time duration, then activates the glow plugs via a glow plug relay. The control module also determines the length of time the glow plugs should continue to operate after the engine starts in order to achieve complete combustion with minimal emissions.

Since glow plugs are essential for cold starting on many diesel engines, an engine that is difficult to start, starts but smokes and runs rough, or fails to start, may indicate defective glow plugs. A defective control module can result in current being applied to the glow plugs for an excessive time period, raising their temperature to a point where they overheat and fail. To test a glow plug, connect a 12-volt test light between the glow plug terminal and the positive terminal of the battery. If the test light doesn’t illuminate, the glow plug is defective. Glow plug integrity can also be determined by measuring resistance and current draw.

No matter whether you’re working on a diesel engine car or truck, or an import or domestic vehicle, if you’re experiencing starting problems, we have the replacement glow plugs and related components to restore easy starting in cold weather, and optimal cold weather engine performance with minimal emissions. We offer components that are manufactured to original equipment specifications, so when repairs are completed you can count on the performance your vehicle was designed to deliver.