Spark Plugs Versus Glow Plugs: What Is the Difference?

Do you know the primary function of your engine’s plugs? If you said it’s to spark the ignition of the fuel, you’re right! Whether they’re spark plugs or glow plugs, they’re the reason your vehicle’s internal combustion engine (ICE) can burn the fuel that makes it go. That’s why checking your plugs is an important part of routine maintenance on your vehicle. So, what’s the difference between spark plugs and glow plugs?

How plugs work in a motor vehicle

A closeup diagram of a A KLG spark plug type KSS1 from 1935
A KLG spark plug | SSPL/Getty Images

Champion knows a thing or two about plugs. The site explains that plugs are one of the three components that all ICE vehicles need. Those three essential components are fuel, air, and an ignition source. Without all three of them, your vehicle isn’t going anywhere!

The fuel won’t burn without just the right amount of added air and plugs that start the air/fuel mixture burning. In a gasoline-powered car, the plugs produce thousands of sparks per minute to keep the constantly arriving fuel mixture lit.

Glow plugs don’t have to work nearly as hard to keep the diesel mixture burning. Once they’ve ignited the fuel mixture, their job is over until the next time you start the engine. The lack of spark plugs is one reason diesel engines are easier to maintain.

Spark plugs vs. glow plugs 

So, what’s the difference between spark plugs and glow plugs? It’s mainly the type of engine in which they’re found. Gasoline engines require sparks to ignite the fuel, while glow plugs are sufficient to ignite diesel fuel, which starts burning at a relatively low temperature. In fact, diesel will ignite even without the help of the glow plug if the engine or the day is warm enough.

Even though the diesel may ignite without glow plugs under ideal conditions, it won’t burn cleanly and will produce more emissions. Glow plugs will ensure more thorough combustion, fewer emissions, and an engine that will start regardless of the weather.

Unlike diesel, gasoline won’t ignite without an actual spark. That’s because its ignition temperature is higher than diesel — around 120° higher. That kind of temperature difference will require more than a really hot day to get a gasoline engine started!

How to know if your plugs are going bad

Knowing what a new glow plug looks like will help you spot problems when they appear. You might notice subtle signs like a probe tilted slightly so that it’s coming in contact with the body of the glow plug. This was caused by poor installation, usually resulting in a tightening torque that was too high. The same thing probably happens if you notice damage to the threads. 

Not-so-subtle indications that it’s time for a new glow plug are a swollen, damaged, or misshapen tip. Also, of course, a probe tip that’s missing entirely! 

With spark plugs, you’ll probably notice visible changes to the plug. There might be discoloration caused by fuel additives that have damaged the plug or vertical burn streaks from a current discharge. A wet, oily appearance indicates too much oil fouling that’s preventing the plug from “sparking” properly. 

Other indications that it’s time to replace your spark plugs because they’re going bad are unusual deposits like ash or soot, a burned appearance, and black or gray speckles. A spark plug that’s been running correctly but is worn out will have an excessively wide gap and electrodes that are rounded from wear. 

That wider-than-usual gap will make the entire ignition system work harder, leading to poor fuel economy, misfires, and possible damage to other parts of the ignition system. Why not give your engine a new set of hardworking plugs? Your engine will start easier, your vehicle will run better, and you’ll save money on gas. That sounds like a spark plug trifecta!

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