What’s the Difference Between Glow Plugs vs Spark Plugs
Author: Toby Date Posted:4 July 2025
Spark plugs and glow plugs perform the same essential function—igniting combustion—but in completely different ways, depending on your engine type. Spark plugs generate a high-voltage spark to ignite the air-fuel mixture in petrol engines, while glow plugs preheat diesel engine cylinders to enable ignition under compression.
Understanding the distinction is crucial for diagnosing starting issues, scheduling maintenance, and choosing the right replacement parts—especially during cold starts, where ignition performance is most affected.
Comparison: Glow Plugs vs Spark Plugs |
||
|---|---|---|
| Feature | Spark Plugs | Glow Plugs |
| Used In | Petrol engines | Diesel engines |
| Primary Function | Create a spark to ignite fuel-air mix | Heat combustion chamber for cold starts |
| Activation Method | High-voltage electrical spark | Electrically heated element |
| Typical Symptoms When Failing | Misfires, poor fuel economy, hard starts | Hard starts, white smoke, rough cold idle |
| Number Per Engine (Common) | One per cylinder | One per cylinder |
| Replacement Interval | ~30,000 to 100,000 km (varies) | ~100,000 km or as specified |
Spark Plugs – For Petrol-Powered Combustion
If you’re driving a petrol-powered car—like a Toyota Corolla, Mazda 3, or Hyundai i30—your engine relies on spark plugs to ignite the air–fuel mixture inside each cylinder. These plugs create a high-voltage spark at just the right time during each engine cycle to keep things firing smoothly.
| How a Spark Plug Works | |
|
|
Inside the combustion chamber, the spark plug sits at the top of each cylinder. When the piston compresses the air-fuel mixture, the ignition coil sends a surge of voltage—often over 20,000 volts—through the plug. This creates an electric arc between the plug’s centre and ground electrodes, igniting the fuel at precisely the right moment. That mini-explosion drives the piston downward, creating the power that turns your wheels.
Signs your spark plugs need replacing:
|
They’re small, but they’re crucial. Worn spark plugs can throw off your timing and waste fuel, so replacing them at the manufacturer’s recommended intervals is a no-brainer.
Glow Plugs – For Diesel Engines in Cold Conditions
Glow plugs, on the other hand, are found in diesel engines—like those in the Toyota HiLux, Ford Ranger, or Isuzu D-MAX—and they work a bit differently. Instead of producing a spark, they heat up to assist the combustion process, particularly in cold weather. Since diesel engines rely on compression to ignite fuel, glow plugs provide the initial warmth needed to get things firing when temperatures drop.
| How a Glow Plug Works | |
|
Each glow plug sits inside the pre-combustion chamber or directly within the combustion chamber of a diesel engine. When you turn the key or press the start button, the glow plugs are activated and begin heating up rapidly—reaching temperatures over 1,000°C. This heat warms Signs your glow plugs might be failing:
|
|
You’ll never have both. Knowing which type your engine uses is key when diagnosing startup issues or planning a maintenance schedule.
Get the Right Plugs with Superspares
Whether you’re chasing glow plug replacements for a diesel workhorse or swapping out old spark plugs in your daily driver, don’t guess—get it right the first time. At Superspares, we’re your go-to auto parts online store for ignition components that suit Australian conditions and vehicles.
Explore our full range online and keep your engine firing the way it should—rain, shine, or frosty morning.
