What Is the Difference Between Spark Plugs and Glow Plugs?

What Is the Difference Between Spark Plugs and Glow Plugs? | Westside Car Care

You may have heard of spark plugs and glow plugs, but knowing the difference between them can be confusing. These small components play a big role in starting your car, but they work in entirely different ways. Whether you drive a gasoline or diesel engine vehicle, understanding the purpose of each type of plug will help you keep your engine running smoothly and know when something might need attention. So, let’s look into the spark plugs and glow plugs, how they function, and why your vehicle depends on them.

What Are Spark Plugs

Let’s start with spark plugs found in gasoline-powered engines. These little devices ignite the air-fuel mixture inside the engine’s cylinders, generating the combustion needed to power your vehicle. Without a functioning spark plug, your car simply won’t start.

A spark plug is connected to the ignition system, and as the name suggests, it generates a spark that ignites the air and fuel inside the combustion chamber. This is the key process in starting a gasoline engine and keeping it running. When the air-fuel mixture is ignited, it causes a small explosion, creating the force needed to move the pistons in the engine. This repeated combustion process is what makes your car move.

Why Are Spark Plugs Important

Spark plugs need to be in good condition for your vehicle to run efficiently. Worn or faulty spark plugs can lead to misfires, rough idling, or difficulty starting your car. Over time, spark plugs wear out, and regular maintenance is needed to replace them. Keeping your spark plugs in check ensures that your engine runs smoothly and that you maintain optimal fuel efficiency.

What Are Glow Plugs

Now, let’s shift gears to glow plugs. Unlike spark plugs, glow plugs are used in diesel engines. While diesel engines don’t rely on a spark to ignite the fuel, they do need a little help when starting in colder temperatures. This is where glow plugs come in.

Glow plugs don’t create a spark but instead heat up the air inside the engine’s cylinders. Diesel engines work through compression rather than ignition, meaning that the fuel-air mixture is compressed to a point where it self-ignites. However, in colder weather, the air isn’t hot enough to achieve combustion on its own, so glow plugs preheat the air to get the process going. Once the engine is warm, the glow plugs are no longer needed, and the engine can run independently.

Why Are Glow Plugs Important

Glow plugs are essential for starting a diesel engine, especially in cold weather. Without them, your diesel engine may struggle to start, and you could experience long cranking times or failure to start altogether. Diesel engines are known for their durability and longevity, but glow plugs play a crucial role in making sure they start efficiently, no matter the weather.

Key Differences Between Spark Plugs and Glow Plugs

Now that you know what each type of plug does, let’s break down the main differences between spark plugs and glow plugs:

  • Type of Engine: The most obvious difference is that spark plugs are used in gasoline engines, while glow plugs are used in diesel engines.
  • Function: Spark plugs generate a spark to ignite the air-fuel mixture in a gasoline engine. In contrast, glow plugs heat up the air in a diesel engine to help achieve combustion, particularly in cold weather.
  • Starting Process: Spark plugs are necessary every time a gasoline engine fires, while glow plugs are only used to help a diesel engine start, especially when the engine is cold.
  • Maintenance Needs: Spark plugs need to be replaced more frequently, generally between 20,000 to 30,000 miles, depending on the vehicle. Glow plugs, on the other hand, tend to last longer but still require periodic checks to ensure they’re functioning properly.

When to Replace Spark Plugs and Glow Plugs

Keeping an eye on the condition of your spark plugs or glow plugs is important for maintaining your vehicle’s performance. Here are some signs that it may be time for a replacement:

For Spark Plugs
If you notice rough idling, frequent misfires, poor acceleration, or trouble starting your gasoline engine, it could be a sign that your spark plugs need replacing.

For Glow Plugs
If your diesel engine is taking longer to start or you notice increased smoke from the exhaust when starting, it might be time to check your glow plugs, especially if you live in a colder climate.

Routine checks and timely replacement of these plugs can prevent bigger engine problems down the line. Always consult your vehicle’s manual for the recommended maintenance schedule.


Don’t wait until your engine gives you trouble. Schedule a maintenance check at Westside Car Care. Our trained professionals will inspect your spark plugs or glow plugs and ensure your vehicle is ready to perform at its peak.