Glow plugs explained: How they work, types & replacement tips

Glow plugs explained: How they work, types & replacement tips
Have you ever found yourself turning the key on a chilly morning and hearing the starter motor spinning, but nothing happens? 
Well, that's a common scenario for diesel car owners. And it often points to one possible culprit – glow plugs. 
Glow plugs help diesel vehicles start and run smoothly by warming the air in the combustion chamber. This guide explores their types, differences from spark plugs, signs of wear, and replacement tips to keep your diesel engine in good shape. 

What are glow plugs?  

Glow plugs are heating elements that preheat the combustion chambers for incoming fuel and air to encourage efficient fuel combustion in a diesel engine. As a result, the vehicle can start smoothly. 
Unlike gasoline engines that use spark plugs to ignite fuel, diesel engines rely on air compression to heat up and ignite air-fuel mixture. However, in cold conditions, the air may not be sufficiently warmed by compression alone, making it harder for the engine to start. 
When the temperature drops, glow plugs come into play. 

A close-up image of a car's side mirror covered with a thin layer of frost, indicating cold winter conditions

What does a glow plug look like?  

Glow plugs come in various designs but generally feature a similar structure. They often have a slim threaded middle section with a hexagon head, allowing for easy installation and removal with a wrench. This thread ensures a secure fit into the cylinder head. 
The heating element, with a heating coil inside, is located at one end of the glow plug. It's the part that heats up to help start the engine by warming the air-fuel mixture. 
On the other end of the glow plug is the electrical connector. This connector provides the power needed for the heating element to function. The connector ensures a steady electrical supply for the glow plug's operation. 

A close-up photo of several used glow plugs laid out on a white surface

How do glow plugs work?  

Glow plugs work on a simple yet effective scientific principle. Electrical energy is converted into heat. This heat then increases the temperature inside a diesel engine's combustion chambers. 
A glow plug's operation begins when the vehicle's ignition is turned on. Electrical current from the vehicle's battery is supplied to the glow plug, focusing on the heating element at the plug's tip. This element quickly heats up due to the electrical resistance it provides. 
The heating element warms up and raises the temperature in the combustion chamber. This helps to ignite diesel fuel more effectively for ideal combustion. Once the engine starts running smoothly, the glow plugs turn off since they have completed their task. 

hot glowing car glow plug

Types of glow plugs  

A few types of glow plugs are available in the market, catering to specific engine requirements and conditions. 
Pressure Sensor Glow Plugs 
Pressure Sensor Glow Plugs (PSG) are "smart" plugs with a sensor in the tip that measures combustion chamber pressure. This information is sent to the Engine Control Unit (ECU) to adjust the heating pattern accordingly. They provide precise control for optimizing engine performance. 
Ceramic Glow Plugs 
Designed for extreme temperatures, Ceramic Glow Plugs (CGP) can reach up to 1500°C in under three seconds. This rapid heating helps start engines quickly. It's beneficial in low-compression diesel engines facing Environmental Protection Agency (EPA) restrictions. 
High-Tech Steel Glow Plugs 
They come in two kinds: GE and GN. GN plugs are modern three-phase glow plugs that heat up quickly, remain hot during ignition, and cool down after three minutes. The plugs are long, pencil-like. 
Older diesel engines used GE-type plugs, which could take up to 30 seconds to heat, but modern high-tech steel allows them to heat in just seven seconds. GE plugs are typically shorter and more similar in size to spark plugs. 

 A 3D rendering of three glow plugs on a neutral background.

Glow plugs vs. spark plugs  

Despite their similar names, glow plugs and spark plugs operate differently to meet the specific needs of diesel and gasoline engines. 
As mentioned earlier, glow plugs are used in diesel engines to heat the air in the combustion chamber before starting the engine. Unlike spark plugs, glow plugs don't produce a spark. Their primary function is to keep the inside of the ignition chamber properly heated for efficient fuel ignition. 
In comparison, spark plugs play an essential role in gasoline engines. They generate an electrical spark that ignites the fuel-air mixture for every combustion stroke. This spark crosses a small gap at the plug's tip, resulting in combustion. 

Spark plug and glow plug positioned against a textured black background with a red border, suggestive of a mechanic's work mat.

Signs of bad glow plugs  

When glow plugs start to fail, several signs can alert you. Each of them affects the operation of the engine. 
The check engine light or glow plug light 
Seeing the check engine or glow plug light on your dashboard can mean an issue. Still, it's not always about the glow plugs. The easiest way to pinpoint the exact cause is by using an OBD2 scanner, which can read the fault codes generated by your vehicle's engine control unit. 
Before starting the engine, it's normal for the glow plug light to illuminate briefly. This indicates that they are active and warming up the engine. 
However, if this light stays on longer than a few seconds or starts flashing, it might be a problem with the glow plugs or their system. 

A close-up view of a car dashboard displaying an illuminated glow plug warning light, symbolized by a yellow icon resembling a coiled wire
Vehicle starting issues 
If you struggle to start your car in cold weather, one of the causes could be failing glow plugs, unable to heat the combustion chamber to the optimal temperature. 
Engine misfiring 
Faulty glow plugs can cause the engine to misfire. You might notice the car jerking or hesitating as you try to start it. 
Rough idling 
Another sign of bad glow plugs is rough idling, where the engine runs unevenly or shakes when the car stops or moves slowly. This issue happens when fuel isn't igniting evenly in all the cylinders. 
Further Reading: Car shakes when idle 
Decreased fuel efficiency 
If you find your vehicle is using more fuel than usual, it might be due to inefficient combustion caused by faulty glow plugs. This inefficiency requires more fuel to maintain the same level of performance. 
White smoke 
When glow plugs are failing, you may notice white smoke from the exhaust, especially when starting the car. This is due to unburnt fuel coming through the engine and exhaust system from incomplete combustion. 
Black smoke 
Alternatively, black smoke may indicate that too much fuel is being burnt. This problem may occur when the engine overcompensates for one or more cylinders not firing correctly, often due to bad glow plugs. 
These signs may indicate that you need to replace glow plugs to keep the car running reliably. 

When to replace glow plugs? 

If you are experiencing any of the issues listed above, replace the glow plugs as soon as possible. Not only will this help to solve the listed issues, but it will also improve the efficiency of your engine. 
If you haven't noticed any problems, the 150,000 km (100,000 miles) mark is still a good point to replace the glow plug. Replacing them at this point helps to prevent future problems. It improves engine efficiency, ensuring smooth starts and operation under various conditions. 
A hand in a blue glove holds a metal glow plug socket wrench with a glow plug inserted, ready for installation

How to replace glow plugs? 

Replacing the glow plugs is relatively straightforward, though some diesel engines are more difficult to access than others.  
Note: Don't do this on a hot engine; give it at least an hour to cool down. 
  1. Locate the glow plugs in the cylinder head 
  2. Work on one plug at a time. Disconnect the connector from each one. 
  3. Glow plugs can be difficult to remove. Spraying a splash of lubricating fluid on the plugs after switching off the engine to loosen the threads is advisable. 
  4. Unscrew the glow plug with a deep socket wrench or specialized tool (if necessary) 
  5. Take extra care when removing the plug from the engine. Longer plugs are likely to break at the tip. 
  6. Install the new plug in the reverse order of its removal 
  7. Repeat the procedure with the next plug 
This simple maintenance task can significantly improve your engine's efficiency and longevity.