How To Read Spark Plugs
Read your spark plugs by inspecting them for damage, corrosion, or leaks. Problems with spark plugs can cause a charred, broken, or oily appearance.
Written by John Davis
Reviewed by Melanie Reiff
When you remove your spark plugs, look for signs of damage, corrosion, or oil leaks. These various problems will cause visible markings and discolorations to your spark plugs, ranging from charred and black-looking plugs to those that appear broken or coated with oil buildups.
Spark plugs are a vital component in internal combustion engines. As you might have guessed, they supply the electric current or “spark” necessary for fuel combustion inside your motor. It’s important to replace the spark plugs every 80,000-100,000 miles to ensure your engine performs efficiently—and when you do replace them, don’t forget to visually inspect them for signs of damage, corrosion, or oil leaks.
If you’re wondering how to read spark plugs, you came to the right place.
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How to read spark plugs
Knowing what to look for when removing spark plugs—i.e., being able to read spark plugs—is a valuable skill that can help you quickly diagnose and troubleshoot problems with your engine.
When you remove spark plugs from your engine’s cylinder head, don’t just throw them away. Take a few minutes to inspect the plug for damage. But before you can read them, you’ll need to understand the different parts of spark plugs.
The top of the spark plug has a terminal nut followed by a corrugated insulator. This is where you connect the spark plug to the plug wires or coil pack. Next is the hexagon that allows you to tighten the spark plug with a socket.
Below the hexagon is the metal shell. This is the threaded part of the spark plug that screws into the cylinder head. The curved metal piece at the bottom is the ground electrode (also known as the ground strap), which should be just above the center electrode.
There are more parts to a spark plug than the ones listed here, but learning these essentials should be enough to help you get started reading spark plugs.
The main area you should concern yourself with is the firing end—the metal shell that screws into the cylinder head—where the electrodes are.
Normal spark plugs should be a light tan or gray color, while spark plugs with damage can be black, charred, wet and oily, or broken.
Key Takeaway Learning how to read spark plugs can help you diagnose various engine issues. Normal spark plugs should appear light tan or gray on the firing end.
What are spark plugs?
Spark plugs are small devices that supply the electrical current or spark necessary to ignite the fuel mixture in your engine. In addition to igniting the fuel in your engine, spark plugs also transfer heat away from the combustion chamber.
These surprisingly precise pieces of machinery can greatly affect your engine performance. The spark plug heat range must be between 500 and 850 degrees celsius (932-1562 ℉). If a spark plug’s temperature falls below 500 ℃, it will not be hot enough to burn off carbon deposits from the combustion chamber; temperatures hotter than 850 ℃ may melt the electrodes and can lead to overheating and pre-ignition/detonation.
What to look for on your spark plugs
So, you bought some new spark plugs and are planning to give your car a tune-up. That’s great because it’s an important part of any automotive maintenance routine and will help your engine continue to perform well.
But when you remove the old spark plugs, don’t forget to inspect them. These old parts can tell you a lot about the condition of your engine. And learning how to read them can greatly improve your ability to diagnose problems with your car.
Before you begin, consider purchasing an inexpensive magnifying glass. It can help you read your spark plugs and inspect them for damage.
MORE: How to do summer car maintenance
Normal spark plugs
Normal spark plugs should be light to dark tan or gray. They may look dirty, especially if it’s your first time doing this job. But a little discoloration and darkening of the metal are normal.
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Deposits may appear in the form of oil or an ashy buildup. If your spark plugs have deposits, you may experience an engine misfire.
Oil deposits on the spark plug’s firing end or the insulator tip may indicate oil fouling and leaks.
Spark plugs with oil deposits may appear grimy; the oil may be fresh and wet or more like a muddy gel. This condition is usually the result of an oil leak around the engine. Common causes include a bad valve cover gasket and leaking or malfunctioning valve guides and seals.
Ashy or dry soot deposits occur during combustion and are typically caused by poor-quality fuel.
Wet and dry fouling
Wet and dry fouling cause symptoms similar to deposits. And both conditions can cause your car to have trouble starting or to misfire.
Dry fouling, also known as carbon fouling, looks like a black sooty buildup on the firing end. It is typically the result of an overly rich situation—when your car gets too much fuel and not enough air—and there may be a problem with the air filter or carburetor.
Wet fouling usually causes the firing end to appear wet, shiny, and, sometimes, oily. It is commonly the result of a bad head gasket or poor oil control by the piston rings.
Lead fouling
You won’t experience lead fouling unless you drive a racecar that uses leaded gasoline. This condition results in yellow deposits on the insulator nose and can cause misfires at high RPMs.
It is caused by using fuel that contains too much lead. However, since spark plugs in racing engines are frequently replaced, this issue is of little to no concern.
Breakage
Breakage usually results in a broken spark plug insulator nose surrounding the center electrode. Occasionally, the ground electrode will also be bent.
This condition is typically caused by a highly lean air/fuel ratio, incorrect gasoline octane rating, or advanced timing. These issues result in detonation and rapid temperature change, causing the spark plug to break.
Melted spark plugs
Melted spark plugs will have center and ground electrodes that appear scorched or melted. This condition can cause your engine to lose power.
NGK, the company that manufactures many of the spark plugs in use today, states that melting occurs when plugs are tightened incorrectly—specifically when they are under-tightened.
All vehicle manufacturers provide torque specs for the components on your car. Spark plugs are no different. When you change your spark plugs, you must follow your car maker’s guidelines to prevent melting.
Lead erosion
If your spark plugs have lead erosion, the ground electrode will appear chipped and may be thinner than normal.
This is another condition that only affects racecar drivers that use leaded gasoline. Too much lead can cause chemical reactions with the electrodes at high temperatures, weakening the metal and causing it to appear brittle.
Corrosion and oxidation
Corrosion and oxidation can occur when the plugs are exposed to leaded gasoline over extended periods.
This condition generally causes the plug gap to expand, and the spark plugs to appear green with pitted or rough-looking electrodes.
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What to do if your spark plugs are bad
If your spark plugs are bad, replacing them should be your first step. This will allow you to visually inspect the old ones for damage—which can help you figure out what caused your plugs to fail. And often, replacing your old spark plugs with new ones can resolve your issue.
Luckily, changing spark plugs is a relatively simple job that you can easily do in your driveway. You’ll need these tools:
- A ratchet wrench and socket set
- An extension
- A spark plug socket
- A torque wrench
- Dielectric grease
If you’re ready to change your spark plugs, follow the steps below.
Step 1: Safely park your car and allow the engine to cool completely.
Step 2: Unplug the electrical connector and remove the spark plug wires or coil boots. You may have to use a pair of spark plug pliers to remove the wires or coil boots. Be very gentle and carefully twist the wire or boot back and forth until it pops off the plug.
Note: Depending on your make and model, you may have to remove the valve cover to access your plugs. If this is the case, you’ll need to replace the valve cover gasket when you replace the spark plugs.
Step 3: Unscrew and remove the old spark plugs.
Step 4: Install the new spark plugs. Be very careful not to cross-thread your plugs when performing this step. A good trick is to spin the plugs backward until you feel the threads line up; then, finger-tighten them until they are snug.
Step 5: Use your torque wrench to finish tightening the plugs to the manufacturer’s specifications.
Step 6: Add a dollop of dielectric grease to the end of each coil boot or plug wire and attach them to the spark plugs. Then, plug in the electrical connectors and finish putting the rest of the parts back together.
After you complete these steps, turn on your car and listen for any misfires. If everything sounds good, take it for a test drive to ensure the engine runs well.
What can cause a lean spark plug?
A lean spark plug occurs when your engine is burning too little fuel or too much air. While a variety of things can cause this issue, it is most likely a problem with the fuel system. Some common causes of an engine running too lean are a
clogged fuel filter
or a bad fuel pump
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FAQs
What does a flat spark plug mean?
If the electrode on your spark plug is flattened or broken, it probably means they are the wrong size plugs.
How do you read spark plug sizes?
Spark plug sizes are coded using various letters and numbers. These letters and numbers are often manufacturer-specific and refer to the hexagon and thread sizes and the types of electrodes.
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