Spark Plug Reading: What It Is and Why It’s Done

As your spark plugs wear out, the color and appearance of their tips can change. While spark plugs can last for a long time, they typically need to be inspected every 30,000 miles. You can have a mechanic inspect your spark plugs or you can inspect them yourself. If you’re doing it yourself, then you need to learn how to do spark plug readings.

Why Do You Need to Read Spark Plugs?

Reading spark plugs means evaluating their condition. This is typically done by inspecting the spark plug tip’s coloration, and checking if the tip is damaged. An engine that’s functioning properly will have spark plugs with a brown or light gray color around the tip. However, if the spark plug tips have a different color, then it can mean that your engine has a problem with its ignition or fuel system.

What Does Spark Plug Colors Mean?

Different spark plug tip colors can correspond to specific problems with your engine.

Black Spark Plug Tip

A black spark plug tip could mean oil is getting inside the combustion chamber through faulty valves or seals, creating carbon deposits on the spark plug tips. It could also mean that the engine is running rich. Aside from an internal oil leak, a black fouled tip can also be caused by a clogged air filter, excessive low-speed driving, a rich fuel-air mixture, or idling your car for a long period of time.

If the tip is black, that usually means an engine is running too rich; carbon deposits on the plug indicate oil burning.

–Anthony Harlin, ASE Certified Master Automobile Technician

White Spark Plug Tip

If your spark plug has white deposits on the tip, then it could be a sign that the spark plug is overheating. This can be caused by engine overheating, a loose spark plug, faulty ignition timing, or a fuel-air mixture that is too lean. If a loose spark plug is the culprit, your vehicle will also exhibit signs of an exhaust leak.

If there are ash-like deposits on the insulator or tip, then the engine could also be consuming oil or fuel additives. Oil leaks are a serious issue, as they can be caused by faulty piston rings or valve guide seals, among other gaskets.

A white spark plug tip can also have blisters or melted electrodes on the tip. If your spark plug has these issues, it must be replaced.

Tips on How to Read Spark Plugs

While a spark plug tip’s color is a great indicator of problems, you should still be on the lookout for signs of corrosion, melting, or damaged electrodes. If the spark plug deposits are wet, then your engine might have a leaking head gasket. Your engine could also be running a very rich air-fuel ratio or have valvetrain problems.

Remember that if your spark plug tips have signs of fouling or overheating, the problem must be quickly diagnosed to prevent any problems. If ignored, these problems can interfere with your engine’s performance and can even cause engine damage, which is expensive to repair.

Want to learn more about how to read spark plugs? There are online resources that have videos and images that illustrate what a spark plug looks like when they’re up for replacement.

If you’re not confident in your ability to read spark plugs, then don’t be afraid to seek professional help from a trusted mechanic.