Everything you need to know about the glow plug

The glow plugs in diesel engines are very different from the spark plugs used in gasoline-powered units. Their function is slightly different: they are not responsible for creating the spark necessary for ignition, but help increase the temperature in the combustion chamber. Glow plugs operate under harsh conditions: they heat up to very high temperatures in just a few seconds and are constantly exposed to high pressure, vibration and the corrosive effects of chemicals. Find out if a faulty glow plug can damage your engine.


What is a glow plug?

Glow plug in diesel engine are heaters, and their heating element is located in the intake manifold or (in the case of indirect injection engines) the pre-chamber or vortex chamber. The coil of the glow plug heats up thanks to the energy from the battery and later passes it along with the air to facilitate starting.

The earliest generation of these heating systems consisted of glow plugs in diesel engine connected in parallel by a special rail. In the event of a failure of one of the plugs, the entire system stops working, which is a major drawback of this solution. The second-generation preheating system consists of glow plugs in the diesel engine connected independently, which allows control of the operation of individual components and the recording of any errors. Advanced electronic systems control the operation of these components.


Where are glow plugs located on a diesel engine?

You may be wondering where glow plugs are located in diesel engines. Like spark plugs, they are also located in the engine head. Their most important component is a heater, which - thanks to the energy drawn from the battery - can heat up to 1,000 degrees C in a matter of seconds or so! Glow plugs start working as soon as the engine is started. The driver is informed of this through the indicator light, which should only be on for a moment.

In modern diesel units, glow plugs also perform other functions. If special sensors are installed in the combustion chamber, they can properly correct the engine's low-speed operation, which leads to a reduction in exhaust emissions. Glow plugs, which used to be a simple component, today can have a much more complex design. Also contributing to this is the fact that they are much smaller than they were years ago.


Construction of diesel engine glow plugs

Glow plugs in diesel engines are manufactured using two types of technology. There are glow plugs with a metal or ceramic heating element. Glow plugs with a metal heating rod are made of special metal alloys, which ensure that the operating temperature is reached quickly, and that heat is emitted into the combustion chamber. The performance of candles with a metal heating element varies depending on their diameter and length. This affects the glowing speed of the engine glow plug.

Glow plugs with a ceramic heating element are designed for the most demanding conditions. They reach operating temperatures faster, which, moreover, are higher than those reached by their metal counterparts. Ceramic glow plugs are also characterized by greater resistance to sudden changes in temperature, which contributes to a longer service life. Although glow plugs with a metal or ceramic heating element can have similar performance, they should not be used interchangeably.

Glow plugs differ in their thermal properties, heating time, operating voltage, shape, and size. The replacement of glow plugs should be preceded by a thorough analysis of the engine specifications of any salvage car sold on Copart or IAAI. This will help you buy glow plugs that will fit the mounting hole and provide optimal operating conditions for the drive unit. You can find information on the size and parameters of the glow plugs that your car's engine is equipped with in the car manual.


Basic parameters of a glow plug in a diesel engine

The basic parameters of a glow plug include:

- voltage - given in volts [V],

- current - given in amperes [A],

- size of the wrench needed to install or remove the device - given in millimeters [mm],

- thread size - given in millimeters [mm].

What all the glow plugs have in common is high-quality workmanship and performance. The use of the latest technology in the manufacture of glow plugs by reputable manufacturers means that the time to reach operating temperature has been reduced from a dozen to just 2-3 seconds. This results in a significant reduction of engine start-up time, even during severe frosts.

Another important factor is the durability of the glow plug design. The operating conditions are demanding, as each time the engine is started, there are significant temperature changes. The glow plugs must reach the right temperature each time so that diesel ignition is possible.

Once the combustion chamber is heated, the components are affected by the high temperature generated by the power unit. When glow plugs wear out, their design still needs to be strong enough to be removed from the engine head.


Bad diesel engine glow causes

Failure of engine glow plugs can result from a wide variety of causes – but among the most common are:

– Glow plug stem failure – as a result of overheating, the heating coil becomes brittle and prone to breakage. The failure occurs when fuel injection starts too early, or when too much tightening torque is used when installing the plug. The lack of a gap causes the heat insulation to be damaged because too much heat is transferred to the glow plug housing. Overheating of the heating coil then occurs (the control coil cannot perform its function properly).

– Cracking or melting of the glow plug stem – often occurs due to its overheating, which can result from an early start of fuel injection, contamination of the injector nozzles, dripping of fuel from the injector nozzle, or its leakage. The source of the problem can also be a broken heating coil, too high a supply voltage, or a hung relay.


Failure of glow plugs – symptoms

What are the symptoms of glow plug failure? The check engine light on the dashboard is often the first sign of a problem. If you notice it, remember that there is nothing to wait for - the car should be in the workshop as soon as possible! However, the light being on is not the only sign of failure. A vehicle in which the glow plugs are not functioning properly will usually behave in a rather characteristic way. What should draw your attention?

– Difficult starting – problems with starting the engine are a classic symptom of defective glow plugs. It is particularly pronounced in cold weather. Then, if the plugs are not working properly, the combustion chamber cannot warm up to the necessary temperature for ignition initiation. With the failure of glow plugs, ignition dropout can also occur. You can read more about this in our article: Ignition loss - symptoms, causes and repair.

– Uneven engine operation – the failure of the engine's glow plugs is sometimes manifested by uneven idling. When the glow plugs are damaged or contaminated with carbon deposits, starting the car gradually becomes more difficult - and when it succeeds, the car runs unevenly.

– Increased fuel consumption – fuel consumption is affected by many factors. Glow plugs do not play the biggest role. However, a malfunction can noticeably contribute to an excessive appetite for diesel fuel. Faulty plugs make it difficult to reach the required temperature in the combustion chamber - and then the engine's performance becomes limited. The mechanism has to work harder to compensate for the malfunction.

– Escaping smoke – when there is not enough heat in the combustion chamber, only part of the fuel is burned. The remaining particles escape from the exhaust pipe, and you can then observe white smoke coming out of it. Usually, this effect is accompanied by an intense diesel smell. Remember, however, that when it's cold outside, as long as the engine has not warmed up, white smoke should be nothing to worry about - it's perfectly normal in such conditions. The warning signal is sometimes not only white, but also black smoke, which appears when there is an imbalance in the fuel-air mixture (there is too little air and too much fuel in it). Damage to the engine's glow plugs disrupts the combustion process. It can contribute to your car emitting black smoke.


Can faulty glow plug damage engine?

When the ambient temperature is above 10 degrees C, most efficient diesel engines should have little trouble firing up. Modern diesel units inject fuel at such high pressure that starting should also be possible at lower temperatures. However, it should be remembered that glow plugs are also responsible for maintaining the correct temperature in the combustion chamber. Ignoring glow plug failures can lead to costly engine failures.

Photo by Unsplash