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How to tell if a spark plug is bad by looking at it

How To Tell If A Spark Plug Is Bad

Did you know you can determine a spark plug’s condition just by looking at it? Not all the time you have to face symptoms from bad spark plugs’, as their outlook will tell a lot about their condition.

How To Tell If A Spark Plug Is Bad

As a part of the inspection process, how will we know if the spark plugs have gone bad or not? That’s what we are going to share today in this article. To learn more, have a glance at it!

The 6 outlook of a bad spark plug

Outlook Of A Bad Spark Plug

1. Carbon deposits

If you notice foul carbon deposits caked on the spark plug, know that there was improper handling of the air-fuel mixture, dirty air carburetors, or frequent idling of the engine.

These issues arise only when the spark plugs are malfunctioning, and fuel is getting stuck on the tips.

2. Oily insulator tips

When the oil cylinder of the spark plugs leaks, the oil gets clogged up onto the insulator tips and on electrodes. Oil deposits and carbon deposits look almost the same, but the texture is different. If the oil freezes in those places, the spark plug may not work correctly.

3. Burnt

Have you seen white blisters on the spark plugs’ insulator tips? Then those arose as the spark plugs got too hot and burned. Specific factors get overheated; one includes loose spark plug or improper handling of air/ fuel ignition.

4. Wet

While you notice the spark plugs are wet, then know it happened due to engine flooding. Engine flooding occurs when you keep restarting the engine without letting it fire up. If the dampness is not dried, then it will corrode the components.

5. Broken

If you notice the broken spark plug’s electrodes, then it is your fault for choosing the wrong type for your vehicle. Too short spark plugs cannot cover the required mileage.

Hence the electrodes break, handling that much pressure. In this case, the spark plugs need replacement.

6. Worn electrodes

When the spark plug seems worn out, it is time to replace it as it has reached its last stage of life. A worn-out spark plug will not provide optimum power to the engine.

How To Check Spark Plugs Without Removing Them

First, ensure the tester’s clip is securely attached to the vehicle’s spark plug wire. You may need a spark plug socket and extension to attach it properly. Then, gently attach the tester’s main connector to the spark plug.

Once everything is connected, simply turn the vehicle’s ignition on—feel for the presence of a spark. If you feel it, the spark plug sends electricity and works properly. However, if you don’t feel a spark, the spark plug will likely be damaged and must be replaced.

What Do Bad Spark Plugs Look Like

Generally, spark plugs that need replacement to appear worn, with parts of the ceramic coating chipped away. If the spark plug is particularly worn, it may be covered in fuel deposits or have areas where the insulation on the electrodes has burned away.

What Happens When Spark Plugs Go Bad

The most common sign that your spark plugs have gone bad is that your engine starts running roughly or hesitates when you step on the accelerator.

The electrodes of the plug wear out over time, carbon can build up, and deposits or oil can collect on the plugs. This can cause the spark to be misdirected, which causes ignition problems and poor performance.

Other signs that your spark plugs may be going bad include poor cold starting, fuel economy, and excessive engine noise.

Bad Spark Plug Vs Good

The difference between a good spark plug and a bad one can be seen in the way it performs. A bad spark plug can cause misfires, poor fuel economy, and prevent the engine from running smoothly. In contrast, a good spark plug provides a stronger spark which causes the fuel to ignite completely and more easily, thereby improving engine performance.

Common Spark Plug Problems

Several common spark plug problems may be encountered. These issues range from minor problems, such as fouled spark plugs, to more severe problems, such as an engine misfire. Some other common sparks plug issues include:

  • Electrical Discharge Malfunction- this is the result of a broken wire or bad spark plug
  • Rough Idling- this is caused by a firing spark plug that does not completely ignite the air/fuel mixture
  • Misfiring- this can be caused by cracked or broken spark plug wires or worn spark plugs
  • Starting difficulty- this is often caused by spark plugs that are fouled from running on a lean air/fuel mixture

The 3 Reasons why spark plugs look bad

Reasons Why Spark Plugs Look Bad

#1. Improper placement:

One of the reasons why spark plugs start looking bad is the lack of installation knowledge. If the placement of spark plugs is too far away, they start overheating.

Of this, the components get burned soon. Also, not knowing to choose the right size spark damages their performance and outlook.

Too long ones can cause engine damage, and too short ones can hamper gas mileage, making your spark plugs break soon out of pressure.

#2. Lack of care:

Examining the spark plugs as routine maintenance is crucial. Even if you do nothing, just looking at the spark plugs can save them, whether they are okay or not.

If you see some damage is peaking, then you can immediately take action. Or else, after several months, those damages intensify, and the plugs die.

#3. Driving the car too slow frequently:

When you drive your car at a low speed more often, the air-fuel mixture cannot burn properly. Hence, day after day, the plugs get carbon-fouled.


Cleaning method of a bad-looking spark plug

Cleaning Method Of A Bad Looking Spark Plug

To clean the debris, oil, or carbon on your spark plugs, you just need a wire brush and brake cleaner. Spray the cleaner, sit for a few seconds and brush the debris off. If you know the right installation method, then you can dismantle the plugs for a deep clean.


How does a regular spark plug look like?

A perfectly fine spark plug will have a greyish-tan color on the electrodes. And the entire body will look like a clean metal.

 Can you clean a spark plug with gasoline?

Yes, you can clean them with gasoline. However, you should not keep the gasoline for more than five minutes per spark plug.

 Should spark plugs be wet or dry?

Dry. If your spark plugs are wet, then that is caused by engine flooding. You can dry the spark plugs yourself or wait till they dry themselves up.

What Do Bad Spark Plugs Sound Like?

It can vary from vehicle to vehicle, but generally speaking, it can be a knocking, thumping, tapping, pinging, or rattling noise from your engine. 

Do Spark Plugs Affect Acceleration?

The answer to this question is yes, spark plugs are a crucial part of an engine’s ignition system. Without spark plugs, an engine could not generate the power needed to accelerate.

How Can You Test A Spark Plug?

To begin testing, you’ll want to disconnect the spark plug’s wire from the spark plug itself. Once the wire is disconnected, use a spark plug removal tool to remove the spark plug from the engine. 

Before testing a spark plug, inspecting the spark plug for any signs of damage is always a good idea. If there is damage, like a crack in the porcelain coating or corrosion, the engine must be serviced since a damaged plug may cause damage to the engine.

How Long Do Spark Plugs Last?

Most spark plugs are designed to last between 30,000 to 50,000 miles, although it depends on the kind of car you are driving and the conditions you are in.

Should I Replace All Spark Plugs At Once?

Yes, replacing all your spark plugs simultaneously is often a good idea. This practice can help you to keep your car running efficiently and maximize its performance. However, there are a few considerations you should keep in mind before deciding to replace all your spark plugs.

How Long Do Ngk Spark Plugs Last?

To answer this question, it is important to know that NGK spark plugs are designed to last up to 40,000-50,000 miles, depending on the type and condition of the spark plug.

White Spark Plug Tip

White spark plugs are made of a special high-grade platinum alloy material. This material makes them special, as it has a higher melting point and much better thermal insulation than the standard copper core plugs. This means that white spark plugs retain more heat, which allows the spark plugs to work efficiently in performance even at high temperatures. 

Will New Spark Plugs Improve Speed?

The short answer is yes, new spark plugs can potentially speed up a vehicle. The spark plug works by igniting the fuel-air mixture in the combustion chamber of an engine. Replacing worn-out spark plugs allows for better fuel combustion and improved engine efficiency, increasing power and speed.


Is it too hard to visually realize your spark plugs have gone bad? We guess not! It is just a matter of examining them to treat them on time. Lastly, always rely on experts to choose the right type of spark plugs for your vehicle to install.

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