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​Spark Plugs vs. Glow Plugs: Everything You Need to Know

Jan 30th 2024

four glow plugs


The fuel powering your engine needs air and heat to combust, which releases mechanical energy that spins the wheels forward. But where does the heat come from? In diesel engines, glow plugs heat the incoming air and fuel until it’s hot enough to burn, while gas-powered engines use spark plugs to produce a spark that ignites the air and fuel every time the system sends fuel to the engine.

Both systems have pros and cons and are subject to wear and tear; the one you use depends on your engine type. Learn about the similarities and differences between spark plugs vs. glow plugs to better understand the firing process.

What Are Spark Plugs?

Spark plugs are connected to the ignition coil in gas vehicles. The coil sends a high-voltage charge to the plugs, which mixes with the air and fuel to spark combustion. The connector at the head of each plug leads to a ceramic insulator followed by a gasket that seals the electrode. The plugs are mounted inside the combustion chamber and repeat this process every time the engine fires.

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What Are Glow Plugs?

Glow plugs are heating instruments inside diesel engines. They warm the incoming air and fuel to ensure the mixture burns thoroughly. The temperature of the fuel and air will fluctuate, especially during the winter months. The plugs allow the engine to fire by keeping temperatures consistent, which increases fuel efficiency and maximizes power.

Instead of sparking every time the engine fires, usually thousands of times per second, glow plugs only heat up during ignition. The element produces a red glow, which explains the name.

If you’re not sure what does an EGR valve do, you should first know that diesel engines use exhaust gas recirculation (EGR) coolers to reduce toxic emissions and prevent overheating. The system recycles the leftover exhaust gas through the engine instead of releasing it from the tailpipe. The EGR cooler chills the gas beforehand to ensure it reenters the combustion chamber at the right temperature.

If the exhaust gas gets too cold, the EGR valve can clog, leading to expensive repairs. That’s why glow plugs produce an intermediate glow that heats the exhaust gas while the engine is running. The excess heat decarbonizes the valve to keep it clean so the engine doesn’t overheat.

Spark Plugs vs. Glow Plugs: Performance

Both parts use electricity to start the engine, heating the air and fuel in less than a second, but glow plugs are considered more powerful because diesel engines run hotter than gas-powered vehicles. They can surpass 1,000 degrees Fahrenheit in the time it takes your engine to start.

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Spark Plugs vs. Glow Plugs: Maintenance

Glow plugs are built to last for years. Diesel experts at Hall Automotive say you need to replace them every 100,000 miles or less if you drive in cold weather, haul heavy loads or use an older model. Spark plugs tend to last 30,000 to 100,000 miles. They break down faster than glow plugs because they are used more consistently through the firing process. Leaks, excess dirt and poor ventilation can cause the part to fail, preventing the electricity from reaching the combustion chamber.

Maintaining your glow plugs will help keep your EGR cooler free of soot and carbon. If the valve clogs, it can be fatal for the engine. You will lose efficiency, and coolant can leak into the fuel supply, sending blue smoke from the exhaust. Replace the EGR cooler to avoid releasing NOx emissions into the atmosphere. Faulty glow plugs can damage the fuel system, including your injection pump, fuel filter and injectors. If the air inside the combustion chamber fails to reach the proper temperature, the fuel won’t burn, coating the parts that interact with it in debris.

car glow plugs

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Signs your glow plugs need to be replaced include:

  • Hard starts
  • Rough idling
  • Poor fuel efficiency
  • Engine misfires
  • Smoke coming from the tailpipe

Replace the injector pump, filter and injectors if the problem persists.

Spark Plugs vs. Glow Plugs: Which Is Better?

Spark plugs and glow plugs don’t last forever, but the latter is built to withstand hotter temperatures and more wear and tear. They also help unclog the EGR cooler valve by reducing the spread of harmful debris. Maintain these heating elements to keep your engine firing like new.