Glow plugs are essential components for starting a Diesel car. It is possible to test the operation of each plug and the preheating relay to determine whether replacement is necessary.
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How to identify a glow plug?
Do you own a Renault Scenic, an Opel Zafira, or any other car with a diesel engine under the bonnet? Follow our recommendations on how glow plugs work!
The glow plug is a pencil-shaped part, most commonly made of metal (it also exists in ceramic), with a heating element at the tip (heating coil).
It is an aid for starting Diesel engines. The purpose of this mechanical part is to increase the temperature in the combustion chamber, allowing the car to start smoothly.
Paradoxically, it falls into the category of engine parts used for cooling.
In Diesel engines, the air must be hot enough to ignite the fuel, especially in cold weather. This is where the glow plug comes in, raising the temperature (above 800°C) to enable starting.
However, its role does not stop there because if the temperature is too low, the diesel fuel burns poorly. The engine then runs poorly and emits a lot of pollution. This can damage the catalytic converter (and consequently the lambda sensor).
Note that there are as many glow plugs as there are cylinders in an engine. Generally, French cars have four cylinders and therefore four glow plugs.
Functioning
Unlike spark plugs for petrol engines, glow plugs are not used during engine operation but only when starting the engine. They are used for about 5 minutes after starting to stabilise combustion and reduce pollution: this is referred to as "post-heating".
The same process is used to enable the regeneration of the particulate filter (DPF).
The principle is simple: via an electrical voltage, the current flows through the glow plug from one end to the other. It enters through the connection terminal and travels to the opposite end, where the heating filament is located, which then heats up very quickly to become incandescent (similar to an electric heater).
The air in the combustion chamber then heats up, causing the diesel fuel to ignite and allowing the air-fuel mixture to reach an optimal temperature.
Preheating relay
To function, glow plugs require electricity, which is the role of the preheating relay.
This electrical box controls the delivery of current to the plugs and also manages the engine temperature and the preheating time.
It is therefore responsible for the display of the preheating indicator light, a starting indicator shown on the dashboard.
Replacing glow plugs
Of course, like many automotive parts, glow plugs need to be replaced after a certain period to avoid causing damage:
- Difficult or impossible starting depending on the wear condition
- Increased fuel consumption and engine failures (especially for "post-heating" type plugs)
- Emission of polluting particles = reason for a failed MOT retest
Generally, the lifespan of a glow plug is around 120,000 km.
However, this lifespan can depend on several factors such as the brand, the vehicle's starting frequency, or even climatic conditions.
It is important to replace all glow plugs, even if only one is faulty, to avoid an imbalance between the cylinders when starting the vehicle.
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Testing glow plugs
It is therefore essential to monitor the condition of your preheating system to avoid delaying the replacement of your glow plugs if necessary.
Diagnosing the preheating system
To avoid going to a garage, we recommend performing an engine diagnostic on your car. This will allow you to identify a fault in the preheating circuit by reading the corresponding code (P0380 and P0670).
Additionally, note that with such tools, you can easily discover the source of a fault without necessarily consulting a specialist!
Preheating indicator light
The preheating indicator light comes on when the driver turns on the ignition and then disappears, informing the driver that they can start the engine.
If the indicator light does not come on, it is likely due to a problem with the preheating relay. Conversely, if it stays on, this also indicates a malfunction.
In both cases, you can check the current supply to the plugs with a voltmeter (while another person turns on the ignition).
You can also refer to your technical guide to check the power supply to the preheating relay.
If the power supply system works, then the relay itself is the problem, not necessarily the plugs.
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Checking the condition of glow plugs
If your power supply circuit seems to be working correctly, your glow plugs may be the issue.
You can check their condition using an ohmmeter, which will help you identify a problem with the electrical circuit continuity of the plug. However, you will need to disconnect the electrical wires connecting them to the relay.
- If you notice that the connection is interrupted, it means the plug is faulty.
- If the connection is good, then check that the resistance measured is the same on the other glow plugs. If not, one of them is defective.
Note that contrary to what one might think, the ohmic value must be below 2Ω to be effective, which is relatively low.
Also, be careful to ensure that soot does not accumulate on your plugs! If you notice excessive soot, the plugs need to be replaced!
Glow plugs are important components of your engine block, optimising the temperature rise of your combustion engine (useful in cold weather!).