When it comes to starting, maintaining, and running a diesel engine, there are things you must consider that you don't need to do with a gasoline-powered engine. One such consideration is the glow plug.
Glow plugs are like mini heaters in your car's bonnet. They warm up the air and fuel in your vehicle's internal combustion system for efficient combustion. Glow plugs are an essential tool for making your diesel vehicle work. But to many people, they are a mystery, forgotten about unless it fails, and often mistaken for spark plugs.
So, let's appreciate the hidden and take a look under the bonnet to understand glow plugs. In this article, we’ll show you how they work, why they work and what makes them so different from spark plugs.
What are Glow Plugs?
To get started, we need to start with the basics - what is a glow plug?
Diesel engines use the energy from compressed gas to start an internal combustion engine. They need special procedures for this because they're so difficult to start when cold. As such, many diesel-powered vehicles use glow plugs to address this issue. The glow plug produces the heat necessary for a diesel engine to start, run and function optimally.
Several factors, including cold weather, the cylinder block and the cylinder head, reduce the heat in your engine. As such, glow plugs are installed in the combustion chamber to restore the needed heat into the engine. So, while spark plugs ignite the engine's air-fuel mixture whenever the vehicle runs, glow plugs act as a starting aid to get the engine going.
​​How Glow Plugs Work?
Each glow plug comprises of a terminal, a heated coil, sealed insulating ceramic powder and has a tip that protrudes into the engine's combustion chamber. Applying voltage to the terminal end of the plug allows electrical current to pass through a heating element in the plug's tip. The plug's body is threaded into the cylinder head and acts as an electrical ground for the heating element. When the glow plug is electrically energised its heated tip can reach up to 1000C in seconds.
A glow plug is a heating element that heats incoming fuel and air necessary to start and run a diesel engine. Factors like cold weather, the cylinder block, and the cylinder head in your engine reduce the heat that is necessary to start your car. To combat this, a glow plug is installed in your diesel engine to restore the needed heat. An electrical current is passed through its heating element by applying a voltage to the plug. This heated element provides the ignition to start your engine.
Essentially, the fuel combustion starts your engine so that you can drive. It's important to note that this is different to a spark plug which ignites the fuel/air mixture in the combustion chamber of a gasoline vehicle.
When a glow plug fails to work correctly, the combustion chamber doesn't have the heat it requires to ignite, and you will struggle or fail to start your vehicle. It is sometimes still possible to start the engine after several attempts if it's warm enough. However, in winter conditions, this is very unlikely. Some signs that your glow plug is faulty are:
- Black smoke: If black smoke comes out of your vehicle when you try to start the engine, this is indicative that the air/fuel combustion was unsuccessful. An imbalance causes black smoke in the air-fuel ratio (too much fuel and not enough air).
- Hard starting: In cold temperatures, the combustion chamber won't reach the necessary temperature to ignite without correctly functioning glow plugs. So, if you cannot start your engine and there are no problems with the fuel or battery, you should check your glow plugs.
- Engine misfiring: Running a diesel engine requires a high compression rate, with fuel injected at the right time and amount. If the glow plugs are faulty, the engine chamber won't reach the necessary temperatures. Fuel quality, compression and fuel injection are the usual culprits for misfires.
- White smoke: When there is not enough heat to burn fuel, the unburnt fuel particles. If it is cold and the engine has not warmed up, this can often cause white smoke.
- Rough idling: If one or more glow plugs fail, the engine will become increasingly hard to start as the temperatures drop and produce rough idling.
Essentially, both spark plugs and glow plugs are used to start a vehicle's engine. However, the spark plug is only for gasoline engines and the glow plug is for diesel.
A spark plug is essential to your ignition and vital to starting your gasoline car. Each spark plug has a metal body, ceramic insulator, internal resistor and a pair of electrodes. The electrical part of the spark plug is connected to your ignition coil. A high-voltage charge from the car's ignition system runs through the spark plug to the terminal, to the internal resistor and through to its electrodes at the tip. The voltage jumps the gap between the electrodes where it produces a spark causing combustion.
On the other hand, a glow plug ignites your diesel engine. While spark plugs ignite the engine's air/fuel mixture, glow plugs are a starting aid to get a diesel engine going. Loaded into each cylinder of the diesel engine, the glow plugs act as heating elements preheating the incoming fuel and air at the right time and amount, making it easier to start your vehicle.
With a faulty glow plug you may be able to get around town as long as you take it easy. However, we don't recommend driving at high speeds. Furthermore, delaying changing your glow plugs will strain on your vehicle, reducing fuel efficiency and increasing the vehicle's carbon footprint.
Of course, this is if the car gets started at all. The combustion chamber won't reach the necessary temperature to ignite in cold temperatures without correctly functioning glow plugs. So, if you cannot start your engine, it's likely due to your glow plugs. We recommend that if you believe your glow plugs are faulty, take your car to your local JAX mechanics. We will be able to make a diagnosis of the problems and recommend an appropriate course of action.
If your diesel vehicle has glow plugs, it will also come with a Glow Plug Light. The light will either come on and stay on, flash, or blink when it detects a fault within the engine management unit (ECU). We recommend that you take your car to your local JAX mechanic. There, we will be able to retrieve the information using a code reader suited to your particular make and model. Then, with that information, diagnose the problem and recommend a course of action.
Unlike spark plugs, glow plugs don't have a necessary service schedule, only requiring replacement when they fail or wear out. That said, most glow plugs should last for as long as 160,000kms; as they gradually deteriorate due to wear and tear. The good news is that glow plugs are both long-lasting and cost-effective to replace: depending on the quality and brand, they will roughly cost between $25-$50 to change.
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