Auto glow plug system parts

Glow plug system: key insights into component performance, mounting, diagnostics, and durability

Ignition and preheating systems are directly involved in starting the engine and also perform different functions. The former is used in petrol engines and is designed to produce a spark that ignites the air-fuel mixture. The latter is used in diesel engines. Its main function is to heat up the air in the combustion chambers (swirl chamber or prechamber) to make starting a cold engine easier. This is especially important at low ambient temperatures.

Design of ignition and preheating systems

The operating principle of an ignition system consists in converting the low voltage of the onboard system into high voltage and producing a spark for ignition of the air-fuel mixture. The voltage is converted by an ignition coil. The current it creates is transferred to the distributor and then, through high-voltage cables, to the spark plug of the corresponding cylinder. A high voltage discharge is produced between the spark plug electrodes. The system also includes a relay and sometimes a contact breaker, which is also involved in controlling the system. Modern cars are often equipped with a distributorless ignition system (DIS) which has one ignition coil per spark plug and is connected to it directly, without any high voltage cables.

A preheating system has a different working principle. The glow plugs used in it are electric heating devices. Their operation is controlled either by a relay or by a separate control unit. When the glow plugs are on, the corresponding indicator lights up on the dashboard.

Maintenance tips

• Make sure all cables and connectors are properly attached. For example, incorrectly connected high-voltage cables may cause the ignition coil, contact breaker, or other components to burn out.
• Don’t leave the ignition on for a long time with the engine shut down. This may result in overheating of the coil, a burnt-out resistor, or a loosened contact breaker spring.
• Don’t leave the engine idling for a long time. This results in increased build-up of soot on the spark plugs.
• Observe the tightening torque specified by the manufacturer when installing new spark or glow plugs. Don’t apply too much force if the component gets stuck. This is especially true for glow plugs, which are easy to break. To remove them, the first thing to do is to grease the thread area with synthetic oil and leave it overnight. Then start the engine or supply power to the working components to let them warm up. After that, you may try to unscrew them, but remember to follow the recommended tightening torque. If you still can’t unscrew the components, heat them up again. Use a special reamer to clean glow plug holes.
• Only use high-quality fuel. Otherwise, your spark or glow plugs will soon be covered with soot.

Signs of malfunction

Detecting ignition and preheating systems malfunctions is not that easy, as most of them have similar symptoms. For example, difficulties in starting a petrol engine or the inability to start it at all may indicate damaged high-voltage cables, a faulty ignition coil or spark plugs, a defective distributor cap or sensors, or a faulty timing advance mechanism or electronic control unit. All these issues may come with reduced engine power and increased fuel consumption.

No matter which component of the preheating system is faulty, all diesel vehicles have problems with starting a cold engine. The engine may idle rougher than usual.

Causes of malfunction

Low-quality fuel, wear and tear of piston rings and valve stem seals, or an incorrect ignition advance angle may cause soot deposits on the spark plugs. If power leads are damaged, the ignition coil may overheat. Malfunction of preheating system components might be caused by clogged fuel injectors. Furthermore, parts of ignition and preheating systems often fail due to voltage fluctuations in the vehicle’s electrical system.

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