By Mike McGlothlin
It’s no secret that diesel engines are notorious hard starters in cold weather. And this is exactly why hidden components known as glow plugs are so important. Concealed in the cylinder head(s), these electrically energized heating elements make it possible for your engine to both start and produce fewer emissions in frigid temperatures. For an in-depth look at this often overlooked yet vitally important part, we’re examining what glow plugs are, how they’re made, how they work, their different types, how long they last, and how you can test and replace them when they fail.
What Is A Glow Plug?
A glow plug is a starting aid for a diesel engine. Its appearance is reminiscent of a spark plug found in a gasoline engine, but it’s actually comprised of a heating coil inserted into a steel tube—with that tube also being made to hold up to the electric current it sees. On the opposite end of the heating coil, there is a connection point for your vehicle’s glow plug wiring harness. Once electrically energized, the tip of the heating coil glows, hence the name glow plug. Without this simple component, many diesel engines would struggle to fire up on those cold winter mornings.
Why They’re Needed
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Diesel engines rely on high compression (highly compressed intake air) to ignite fuel in-cylinder without the aid of a spark plug, which makes them hard to start in colder climates. So why are they so hard to start? Because a portion of the compressed (and super-heated) in-cylinder air escapes into the engine’s cylinder walls and block when the engine is cold. By rapidly increasing the air within the cylinders, a glow plug provides for ample heat to remain in those chilled cylinder walls until the engine can be started. From there, the glow plugs remain active until the engine’s internal combustion process sufficiently produces enough heat for the glow plugs to no longer be required.
Where They’re Located
An engine’s glow plugs are located within its cylinder head(s), and each cylinder is equipped with its own glow plug (i.e. a six-cylinder engine will have six glow plugs). More specifically, in an indirect injection (IDI) diesel engine the glow plugs are found in the pre-chamber. In a direct injection (DI) diesel engine, they’re positioned with the tip slightly protruding into the combustion chamber. Depending on the engine, its glow plugs can be found either internally (beneath the valve covers) or externally. For example, the popular 7.3L Power Stroke conceals its glow plugs under the valve covers (shown). In contrast, the glow plugs on the well-known 6.6L
Duramax LBZ are externally accessible.
How They Work
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In addition to heating the compressed air in the cylinders, glow plugs serve to warm the diesel fuel being injected. There are three basic stages to a glow plug’s functionality. 1) First is the pre-heat stage. Here, the glow plugs, triggered by keying on the ignition, heat up rapidly to warm the cylinder. 2) Next comes the temperature sustainment stage (or stand-by stage), where the glow plugs maintain their temperature during and directly after engine startup, providing for efficient and optimized combustion. 3) In the post-heat stage, the glow plugs continue to be operational for a period, which serves to limit tailpipe emissions. Note that not all glow plug systems utilize a post-heat stage (think older diesel engines, where pre-heat and temperature sustainment may be the only stages employed).
Types Of Glow Plugs: Steel, Ceramic, and Pressure Sensor
There are three specific types of glow plugs, with two being the most common. Steel glow plugs are the traditional versions that’ve been in use for decades. These glow plugs don’t heat up as quickly as their ceramic counterparts or match their temperature output, but they are highly reliably and call for less voltage to operate. Ceramic glow plugs feature a ceramic insulator around the heating coil, which expedites the heating process. They can achieve higher operating temperatures (more than 1,800 degrees F) quicker than standard steel versions can, and they can hold that higher temperature for longer periods of time. Pressure sensor glow plugs utilize cutting-edge sensor technology (usually a piezo resistive type) to determine the required heating strategy of the glow plug in any given condition. They’re designed to reduce exhaust particle emissions.
3 Kinds Of Glow Plugs
1. Steel: traditional type, found in most older diesel engines
2. Ceramic: quicker heating and hotter temps than steel versions
3. Pressure Sensor: designed for use in helping to limit tailpipe emissions
How Long Do They Last?
Time has proven that well-built, high-quality glow plugs such as OEM or OEM equivalent units last the longest. Thanks to the quality of the building materials used in producing them, OEM glow plugs allow for very minimum deformation over the course of the heating element’s life. Additionally, they resist warping and corrosion much better than aftermarket or cheap knockoffs do. In particular, OE glow plugs that employ welded filaments as opposed to wire filaments offer more consistent heating performance over the course of their service life. On the other hand, wire filament types tend to be more reliable in engines with excessive NVH.
How To Diagnose A Bad Glow Plug
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Unfortunately, unless a check engine light calls attention to it, glow plug failure can go unnoticed until cold weather sets in. And because the lifespan of a glow plug can be anywhere from one
year to 10 or more, annual inspections of glow plug health are advisable. To check your glow plugs, you’ll need a multimeter that reads and is set to ohms, the higher the sensitivity setting the better. Disconnect the glow plug wiring harness from the glow plug, attach the negative lead of the multimeter to the negative battery terminal, and touch the positive multimeter lead to the glow plug’s terminal. For most glow plugs, the measured resistance should range between 0.1 and 6 ohms. Higher or lower than that, and the glow plug needs to be replaced.
Removal, Replacement, And Cost
Because glow plugs are only operational during startup, they can last a very long time. Unfortunately, that also means they can be a bear to remove when it’s finally time to replace them. With their threads prone to seizing in cylinder heads, it pays to use a penetrating lubricant prior to breaking them free. As previously stated, always replace a failed glow plug with an OEM or equivalent replacement. Once in place, the manufacturer-recommended torque spec must be adhered to for a proper combustion seal (never over-tighten them). For most engines, it’s more cost-effective to replace glow plugs as a complete set rather than one at a time. The price of replacement glow plugs varies from engine to engine (and brand to brand) but expect to pay between $55 and $400 for a set.
Supporting Parts
As you might’ve guessed, glow plugs don’t operate on their own. In fact, there is an entire network of components involved in facilitating what they do. For starters, the engine’s computer (called an ECM, ECU, or PCM in most instances) commands the glow plug module, usually according to engine oil temperature, intake air temperature, barometric pressure, or a combination of all three. The glow plug module itself regulates the electric current supplied to the glow plugs, and some modules control the interval and amount of heat required. Then you have the glow plug relay (pictured), which determines whether voltage from the battery will be sent to the glow plugs or not. Of course, the vehicle’s battery(s), its ignition switch, and its fuse box play a supporting role, too.
Improving Emissions
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Again, when glow plugs heat up the compressed air and pressurized fuel present in the cylinders of a diesel engine, tailpipe emissions are reduced. Ensuring complete combustion takes place from the first moment the engine is running—where a diesel arguably pollutes most—is key in meeting modern emission standards. Beyond initial startup, and namely during the temperature sustainment stage, glow plugs safeguard against emissions as the engine begins its climb to normal operating temp. Even further yet, glow plugs with post-heating capability can decrease particulate emissions by as much as 40-percent during the engine warmup period, not to mention they can also aid the regeneration process in late-model diesel vehicles equipped with a diesel particulate filter system.
Glow Plug Replacement Tips
*Use penetrating lubricant (threads are prone to corrosion and seizure in cylinder head(s))
*Replace them as a set, even when only one has failed (cost-effective)
*For best quality and longest service life, only use OEM or OE-equivalent glow plugs
*Always tighten glow plugs to the manufacturer’s recommended torque spec and never over-tighten them (they can break off and fall in-cylinder)
*Steel glow plugs and ceramic glow plugs are NOT interchangeable
*The price of new glow plugs will vary from engine to engine (and brand to brand), expect to pay between $55 and $400 for a set
