How to Read a Spark Plug | Autance

Gasoline engines rely on the spark plug to deliver the “spark” that will ignite the air-fuel mixture in the combustion…

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How to Read a Spark Plug | Autance © How to Read a Spark Plug | Autance

Gasoline engines rely on the spark plug to deliver the “spark” that will ignite the air-fuel mixture in the combustion chamber. What some of us don’t understand is that the spark plug can tell us a lot of things about our engine. Are we running lean or too rich? Do we have an issue with the ignition timing or the oil ring? These are some of the things that a spark plug can tell us. As such, it is important that we, too, learn how to read this critical component of the modern combustion engine.

Remove and Read the Spark Plugs One at a Time

Check the owner’s service manual to determine the exact location of the spark plugs. They often come with specific instructions on how to remove the spark plug as well as the tools that are necessary for their removal. Make sure you have these tools ready whenever you remove plugs from their sockets.

Don’t pull out all the spark plugs in one go. Confusion can set in when it is time to put these spark plugs back into their slots. You might slot a spark plug in the wrong socket and cause problems. Unless you are going to replace your old and worn-out spark plugs with the best spark plugs, it is better to remove them one by one.

Remove, inspect, and re-connect one spark plug before you go to the next. If you read that the spark plug needs replacement, then you need to replace it. But if you do not have a replacement yet, make sure to take note of the location of the problematic spark plug.

Inspect the Spark Plugs for Soot

It is wise to have a brand new spark plug in hand to serve as a reference for you. This works well for those who are new car owners who may not have sufficient knowledge about automotive things. You can use a brand new spark plug for comparison purposes. This will help make reading the status of spark plugs a lot easier.

The first thing you need to check is the presence of soot. This often comes with a dark gray or black color. You can see dry and black soot on the insulator tip and the electrode of the spark plug. This is an indication of a carbon-fouled spark plug. One of the most common causes of carbon fouling is having a very rich fuel-air mixture. It is also possible that you can have carbon deposits on the plug because of dry air filter. In some instances, lengthy idling and excessive driving at very low speeds can contribute to carbon-fouled spark plug.

If the issue is due to very rich fuel-air mixture, then you don’t need to replace the spark plug. A simple adjustment to the carburetor or fueling is often enough.

Look for Visible White or Oily Deposits

If you notice an ashy-looking deposit on the spark plug’s electrode or insulator, there’s a chance that you have an oil-fouled plug. This often happens if there is a leak somewhere in your engine. There may be issues in the valve guide seals or the cylinders themselves. It is also possible that you have a problem in the piston oil control rings. An oil-fouled spark plug often feels moist or wet to the touch. By comparison, a carbon-fouled plug is dry.

This is a very serious issue. Before you start replacing your spark plug, it is best to have a mechanic look at the problem. He needs to isolate the cause of the leaking oil. You can replace your spark plug with a new one. But if the leak is still present, then you will still end up with an oil-fouled spark plug.

Check for the Presence of Blisters or Signs of Burning

Look at the spark plug and check if you can see blisters or signs of burning. These are often an indication of overheating issues. Blistering is typical on the insulator tip of the plug. You may also notice other signs of damage due to excessive heat. This can include burned metal or melted plastic.

The most common cause of blistering in spark plugs is insufficient engine coolant. It is also possible that you have a leak somewhere in the engine cooling system. Since there is a leak, the coolant is not able to circulate well throughout the system. Before replacing your spark plug, it is best to address the issue of overheating first.

It is also possible that the main issue is not an overheating engine, but an incorrect spark plug. Check your owner’s manual on the correct type of spark plug for your vehicle’s model and make. Take note of the correct heat range of the spark plug. It is also possible that blistering can occur because of incorrect ignition timing, loose plug, or very lean fuel-air mixture.

Check for Worn or Broken Electrodes

Look at the hook-shaped ground electrode of the spark plug. It should have a smooth and even surface. If not, you are looking at a worn-out spark plug electrode. This means only one thing – you need to replace it with a new one.

If you cannot see the “hook” section of the ground electrode, this is a sign of a broken electrode. This occurs because of an incorrect type of spark plug. If it’s too long it can damage the engine. If it’s too short, your fuel economy can suffer.

Take Note of Black Spots

The presence of black, pepper-like spots on the spark plug nose is a possible sign of light detonation. If you also notice chipping or cracking in the spark plug’s insulator, then there’s a serious problem at hand.

Have a mechanic perform a more thorough assessment of your engine. Serious detonation can lead to irreversible damage the engine’s cylinders, pistons, intake valves, and rings. Make sure the replacement spark plug you use is of the correct heat range.

Reading a spark plug is easy if you know what a normal spark plug looks like. Coupling what you see in the spark plugs with other symptoms can help establish the engine’s current state.


  1. How Automobile Ignition Systems Work – howstuffworks
  2. How to Read a Spark Plug – wikiHow
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