Glow plugs are miniature heaters with a heating element on one side and contacts on the other. Each combustion chamber has one glow plug. Before starting the engine, voltage is applied to the glow plugs, and the plugs quickly warm up, increasing the temperature in the combustion chamber. The working part of the glow plug is located at the base of the fuel injector. As a result, the injected fuel passes near the glow plug and heats up, which improves the combustion process. In addition, small particles of fuel, passing near the glow plug, ignite, which improves the ignition process of the fuel.
The length of time that voltage is supplied to the glow plugs is regulated by the control unit. On early models, the control unit monitors the air temperature in the engine compartment. On later models, the control unit monitors the coolant temperature based on information from a sensor mounted in the cylinder head. Based on information received from the temperature sensors, the control unit regulates the time that voltage is supplied to the spark plugs.
When the indicator light comes on in the instrument cluster, it indicates that the spark plugs are working. Within a few seconds, the plugs heat up to a high temperature and the indicator light goes out, which is the signal to start the engine. If no attempt is made to start the engine after the indicator light goes out, a timer will cut off the voltage to the glow plugs to prevent the battery from being discharged and the glow plugs from overheating. If on later models the indicator light comes on after the engine has started, there is a fault in the pre-heating system.
It should be noted that on later models, the glow plugs are not switched off immediately after the engine is started, but after some time, allowing the engine to reach normal operating mode. The duration of the glow plugs' operation after the engine is started depends on the coolant temperature.
The failure of even one glow plug leads to difficult engine starting with interruptions in operation and strong vibration.
Burnt and broken glow plugs are often the result of faulty fuel injectors. If there is a burnt plug, then a simple replacement is not enough. It is necessary to check the injectors, including determining the shape of the spray torch, the operating pressure and tightness. It is important that the injectors have a clear fuel cut-off.