Spark Plugs: What They Do and When to Change Them

Spark Plugs: What They Do and When to Change Them

Last Updated March 3, 2022 | Meghan Drummond

Internal combustion engines (ICE) are powered by a series of small explosions that occur inside your engine. These explosions require fuel, air, and, finally, a spark, which is what the spark plugs are responsible for.

Despite their simplicity, spark plugs are critical to the continued functioning of your engine and understanding how they work, when and how to replace them, and the different types of spark plugs that are available will help you maintain your engine.

How Do Spark Plugs Work?

An open engine, preparing to change spark plugs

Inside your engine, there are a number of cylinders. You could have an inline-four, which would have four cylinders, or a big V8, which would have eight.

Each cylinder has its own spark plug and a piston. The one exception to this is a Hemi which has two spark plugs per a cylinder because Dodge has to be extra like that. For every other engine type, you’ll know how many spark plugs you will need because it will match the number of cylinders that you have.

When the piston moves down, it draws in fuel and air. When it goes up, the piston compresses that air-fuel mixture. Then, the spark plug creates a spark, and that spark ignites the air/fuel combo and pushes the piston back down, giving your vehicle power. The crankshaft turns, and the piston pushes up, forcing the exhaust out, and then the piston goes back down, pulling in more fuel and air. This process is ongoing, and when it’s working together, smooth and efficient.

Your engine has to do this, perfectly, thousands of times per minute. And every cycle requires the spark plug to ignite. When it doesn’t, it will cause an engine misfire. If the fuel isn’t burnt, it can get dumped into the exhaust, which will damage the catalytic converter. Even one bad spark plug can cause a great deal of damage.

Despite their relatively small size, spark plugs are a critical component of your engine and need to be in working order for your car to run correctly.

Types of Spark Plugs

Cold vs Hot Spark Plugs

The spark plug’s other major duty, besides creating the initial spark, is then disposing of heat. Different types of spark plugs are better at this than others.

You’ll find cold plugs in high horsepower engines with high compression rates. These plugs aren’t as insulated, so heat is easily transferred away. Hot plugs have more insulation, which keeps the spark plug hot enough to burn away carbon deposits.

The difference between cold vs hot plugs is kind of like the difference between regular and premium gasoline. If you aren’t sure, err on the side of a plug that’s too cold. You might need to change it more often, but it’s less likely to cause engine-knock.

Different Metals for Spark Plugs

Two spark plugs side by side, one labelled nickel and the other platinum

Different metals have different melting points, and these different melting points allow different precious metals to require less energy to initiate a spark. This allows manufacturers to use a small electrode, which is able to dissipate heat more easily.

In short, the science adds up on this premium price difference. Platinum spark plugs are better than copper spark plugs, iridium spark plugs are better than platinum spark plugs, and double iridium is even better than that.

Though precious metal spark plugs are more expensive, they do last longer, and won’t need to be replaced as frequently. Even just making the upgrade from copper to platinum can change your expected longevity from around 30,000 miles to 100,000.

There are different pitches, thread diameters, and lengths as well, but those differences aren’t about performance as much as they’re about what will fit with the other components of your engine. Follow your manufacturer’s instructions about this to the letter. Spark plugs that are the wrong length, in particular, will wreck your day in a hurry.

Replacing Spark Plugs

Getting peak fuel efficiency and performance out of your engine requires functioning spark plugs. Having spark plugs that misfire can cause a rough idle, lack of throttle responsiveness, and many other problems you don’t want to deal with.

Though most people have noticed that manufacturer’s recommendations for oil changes tend to be on the low side, the opposite is true for how long they expect your spark plugs to last. Manufacturers tend to have an optimistic opinion of spark plug’s longevity that just doesn’t line up with most people’s actual experiences.


  • Compressed Air
  • Torque Wrench
  • Gap Gauge (and your vehicle’s gap setting)
  • Anti-Seize Lubricant

Drive your car for a few miles, then stop and wait for the engine to reach about room temperature. This will help with a smooth removal. Disconnect your negative battery terminal, and then clean away any grossness that’s accumulated around your plugs. You don’t need to clean until shiny, you just don’t want anything to fall in while you have your spark plug out.

Disconnect electrical connections, remove the bolt, and then gently pull up on the coil pack. You can wiggle lightly if it gets stuck. Then use a spark plug socket to loosen the plugs. Then, use the spark plug boot to reach in and pull the spark plug out gently. Be warned that sometimes spark plugs will break off and then you’ll need to get a plug extractor. It’s just easier to not break it.

Now, it’s time to take a look at your spark plug.

Four spark plugs, brown and covered in grime

What Your Spark Plug Can Tell You

A “good” spark plug will still have brown deposits along the coils. If this is the only thing you see going on with your spark plug, it’s fine. Go ahead and put it back in.

If your spark plug is black, check to see if it’s oily or dry. Dry and black means you probably need a new air filter or that you should look into getting hotter spark plugs. Oily and black means you’ve got an oil leak somewhere.

If it’s wet, your engine flooded, go ahead and dry them out and then reinstall them. Pretty much anything else you can find from blisters to broken and worn electrodes means it’s time to drop in some new spark plugs.

Installing the New Spark Plugs

Let’s be honest. First of all, cars have their spark plugs in different places based on make and model. Some of them would require an elite gymnast to access. Mechanics have the correct tools to reach these spark plugs and they’re cheaper than a visit to a doctor because you got your arm stuck reaching somewhere you shouldn’t have been.

But, if you can see all of your spark plugs and you’re confident that you can reach them without bending inappropriately, then you can change your own spark plugs.

Gap gauges can be picked up inexpensively at almost any hardware store. Using the gap gauge, you can check the distance between the electrode and the arm. It should be a precise fit, so the wire on the gauge should touch ever so slightly, but not so much that you can’t easily pull it out.

Applying anti-seize lubricant to the base of a new sparkplug

Then, apply a thin layer of anti-seize lubricant on the threads of your spark plugs, screw them in, and torque them to manufacturer specifications. Be gentle during this process. Though most people note that installing spark plugs is almost impossible to mess up, the one way you can is by overtightening and breaking a spark plug, especially if it goes down in a cylinder.

To get the boot back on correctly, many recommend using a small amount of dielectric grease. It certainly helps it to go on easier and it may help prevent misfires.

Then, reinstall the coil packs, re-bolt, and reattach electrical wires. Repeat process for each spark plug.

When you’re finished, don’t forget to reattach your battery and then go ahead and start your car up. With new spark plugs, you should notice an immediate improvement in the way your vehicle functions. Most people agree that changing your own spark plugs is an easy part of car maintenance that requires few tools, but if you’re uncomfortable there’s nothing wrong with asking a professional to do it for you. But hopefully, now that you know more about how spark plugs function and what to look for, you’ll know when to change them and what to look for in your next set of spark plugs.

Meghan Author Photo

About the Author

Meghan is a Classic Mustang geek with a soft spot for four-eyed Foxes. She has over 300 in-depth articles to her credit that have been cited by some of the top news sites in the US. Read full bio →

Sources: Diagnosing Spark Plugs, Know Your Parts | What You Should Know About Spark Plugs, Their Adjustment, and Their Replacement, New Gate School | How to Replace Spark Plugs, Family Handyman Image Credit: Mobil Oil

Spark Plugs: What They Do and When to Change Them

Spark plugs provide the ignition that keeps an internal combustion engine functioning. Knowing when and how to change your spark plugs is an important step in keeping your engine running well.