Glow plugs in diesel engines - work, replacement, prices. Guide

Glow plugs are essential for the correct start of a diesel engine. Many motorists remember this fact only in winter.

A characteristic feature of a diesel engine is the combustion process, which is different from the combustion process of a gasoline engine. While in the latter the mixture is ignited by an electric spark from a spark plug, in a diesel engine the air is first compressed to a very high pressure (hence the name of these units - diesel). Compressed air reaches a high temperature and then fuel is injected - ignition occurs.

However, with a cold diesel, it is necessary to preheat the combustion chamber in order to initiate the ignition of the air-fuel mixture. That's what glow plugs are for.

It must be remembered that the temperature of the air sucked into the combustion chamber must reach at least 350 degrees Celsius. Therefore, starting a diesel in such conditions without a glow plug would be a miracle.

Glow plugs heat the air in the combustion chamber to the optimum temperature in a matter of seconds. They work when an orange light (usually with a spiral symbol) lights up on the dashboard. It lights up when we turn the key in the ignition. You need to wait until the engine starts until it goes out. Glow plugs do not work while driving. If the glow plug indicator lights up while driving, you should contact the service center.

Heater in diesel engine

The first glow plugs were a simple heater screwed into the engine casing. They didn't even have shielded heating elements, their durability was very poor.

They were replaced by glow plugs with a heating element placed inside a hermetically sealed tube. At present, the so-called second-generation pencil glow plugs with a metal heating tip, which at an external temperature of 0 degrees Celsius reach 4 degrees in just 850 seconds and even 10 degrees C after 1050 seconds.

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Ceramic glow plugs are more modern and more and more popular. They are made of heat-resistant ceramic material that heats up to 1000 degrees C in just one second, reaching a maximum temperature of 1300 degrees C.

Temperature difference

Glow plugs work in extreme conditions. This is especially true in the cold season. A spark plug in a cold engine must warm up to 1000 degrees C in a few seconds, after which its heating element is exposed to the high temperature resulting from the combustion process. When the user turns off the engine, the spark plug cools down again.

All these factors do not contribute to the durability of glow plugs, although they are still made of very durable materials (especially ceramic candles).

Exhaust scoring and prolonged engine start time regardless of weather conditions are typical external symptoms of worn glow plugs.

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Access to them is not easy, replacement or repair requires the use of specialized tools. To access the glow plugs, most often you have to remove the engine cover. A specially shaped torque wrench is used to tighten the spark plugs.

The glow plug tells you the truth about the health of your diesel engine

The technical condition of a gasoline engine can be determined by the appearance of the spark plug electrodes. The same applies to glow plugs - the condition of the diesel engine and the injection system can be determined by the appearance of their heating element.

A blackened candle with visible traces of soot indicates an incorrect combustion process. On the other hand, if you notice a white coating on the spark plug, then the fuel is sulphated.

Oil and carbon deposits indicate excessive oil consumption or damage to the injection pump. Part of the heating element falling off can be caused by too early injection of fuel with insufficient atomization. On the other hand, overheating of the plug may indicate insufficient cooling of the socket or a burnt head gasket. And pitting on the heating element is caused by voltage being too high at startup.

Experts point out that the service life of glow plugs also depends on the quality of the fuel. The more water in the fuel, the faster the spark plugs corrode and the shorter their service life.

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Glow plugs cost from PLN 20 to PLN 200, depending on the brand and technical and operational features. Of course, the so-called fakes, but they can cause a lot of trouble to the engine. Improper spark plugs can break down and even cause a short circuit in the electrical system. Replacing candles costs PLN 10-20 apiece.

According to the expert

Adam Kowalski, Auto Moto Service from Slupsk:

– Unlike spark plugs, car manufacturers do not plan to replace their glow plugs periodically. They should be checked if there are any signs of wear and replaced if they do not work properly. Under normal operating conditions, a set of glow plugs is enough for about 15 start-up cycles and about 100 thousand kilometers of the car. Provided that only glow plugs recommended for a specific power unit are used. The service life of the spark plugs is affected by the technical condition of the engine, the quality of the fuel and oil used, as well as the way the car is operated. If the car is driven only in the city, the spark plugs may wear out faster. This is affected by a large number of engine starts, and then the candles are most loaded. For example, taxi drivers know this well. If one glow plug is damaged, it is best to replace the entire set. The point is that they should all have the same useful life. Of course, candles must be of the same type. 

Wojciech Frölichowski