What Causes A Spark Plug To Turn Black? 13 Possible Reasons


Suppose you have a job interview to attend in the morning. You woke up early and prepared yourself to bring your A-game. You sit in the car and put the pedal to the medal, but you’re surprised, and the car isn’t running.

So, you check the engine and notice signs of soot on your spark plug. You don’t know how to solve this issue. The clock keeps ticking. You are definitely in a fix.

The above situation is common and happens to the best of us. Most of us ignore the relatively minor parts of our car. But, these tiny parts can create a big problem like creating a gap if not taken care of regularly.

Let’s see how you can take care of a spark plug to turn black and hear our insights and hacks to fix them. But first, let’s know why they turn black in the first place.

What Causes A Spark Plug To Turn Black?

If your spark plug gets harmed or has turned black, chances are you will face trouble with the oiler and starting the engine. At the very beginning, you must read the spark plug to know what occurred.

Examine the spark plug, and if it proves to be black, it could be due to one of these two reasons:-

  • Carbon Fouled
  • Oil Deposits

Carbon Fouled

When the spark plug is black, dry, and dusty, it would indicate that the engine is burning overly rich. It substantially means your engine is not burning what it’s supposed to burn.

However, this doesn’t mean it’s oil-contaminated; instead, it’s too rich in the air and fuel mixture.

Let’s get into the details of why a spark plug may show that it’s carbon fouling, shall we?

Rich Mixture

When there is excessive carbon growth on the engines, the spark plug’s dark color is visible. The rich mixture causes this increased carbon.

Your car will be acquiring excessive fuel and insufficient air if you have a high mixture. As a result, it will cause inconveniences such as low gas mileage and slow acceleration.

Heat Range

You can have this problem if the heat range is too cold.

It will depend on the weather or the altitude that you currently live in. It can occur in both the two-cycle and four-cycle engines.

Gap Too Wide

If your spark plug gap is too broad, then it will not burn correctly. Hence, it will cause damage, and the spark plug will be covered by soot and dust.

Low Compression

Sometimes your car engine can have compression problems. As a result, the car fails to get any power for low or no compression problems and doesn’t start.

Dirty Air Filter

The dirty air filter works the same as choking your engine turning your spark plug dark.

Inaccurate Adjustment of Carburetor

Carburetor allows air-fuel mixture adjustments with the help of low-speed and high-speed screws. These screws control the fuel that launches into the engine. If you adjust the carburettor improperly, then the engine’s power will decrease drastically.

It happens especially for the two-cycle engines.

Oil Deposits

If you observe that your spark plug looks black and wet and when you hold it with your fingers, they are stained by grease, and then it is called oil fouling.

You have already seen the carbon fouling reasons. Now let’s dive to see what causes the oil fouling on a spark plug.

Rich Mixture of Oil

If you put an extreme amount of oil in the two-cycle engine, the excess oil will enter into the engine cylinders. As a result, it will cause the spark plugs’ tips to blow up, becoming wet and oily.

Because of this, the engine can’t burn the oil it should burn to work efficiently.

Incorrect Set Up of Carburetor

You can set up your air-fuel mixture at high speed or low speed at the carburetor for the two-cycle engine. If it’s not set up correctly, then the spark plug becomes dark and oily.

Blown Head Gasket

Your engine may work fine, but if the head gasket is blown, it leaks oil internally. Thus it causes the spark plugs to become covered with oil.

That stops the proper forming of sparks and leads to hazardous situations such as misfires and unsatisfactory performance.

Strained Piston Rings

It could be that your piston rings are so worn out that the oil is slipping past your piston. Later on, the slipped oil leaks into the cylinder, a place where oil should not go in. It causes the spark plug to become greasy and black.

Worn Out Valve Guides

In the ancient four-cycle engines, the valve guides are sometimes worn out. It arises when the engine is used for ages. Such cases are sporadic, though.

The oil slips past the valve guides and creates problems with the spark plug.

What Can You Do To Stop Your Spark Plugs from Turning Black?


Before anything else, you should check your engine to know why your spark plug is turning black. Either it will be dry and dusty or wet and oily.

In both cases, knowing the reason beforehand is a necessity.

So you found the reason, but you don’t know what to do!

Don’t worry because here, we’re going to provide comprehensive solutions for each problem you might be facing with your spark plug.

Cleaning Air Filter

The most familiar reason for this is you might have a clogged air filter. You should check the engine first.

To clean the air filter, you should start with a bucket full of water and a laundry detergent mixture.

Next, you may place the filter into the water and circulate it. Use your hands to clean the dirt and dust out of the filter.

Finally, you may take out the filter from the water before rinsing it.

Bumping Heat Range

If the heat range is too cold, you may bump your heat range up to one or two harder on your spark plug.

Narrowing the Spark Plug Gap

You need a spark plug gap tool to modify the gap.

You may run your tool through the gap and see for yourself if the gap is too broad or not. If the tool does not touch the electrodes, this means you must narrow the gap.

You have to tenderly curve the ground electrode with the tool to reduce the gap.

Fixing The Air-Fuel Mixture

You have to set up your mixture correctly to the right proportion. The ideal air-fuel mixture ratio should be 14.7:1 for a proper engine.

Adjusting the Carburetor

Your high-speed and low-speed screws should be correctly adjusted so that it’s burning accurately. Additionally, the engines’ power will significantly work better if you adjust the carburettor appropriately.

Clean Your Spark Plug

At times, the spark plug should be cleaned and examined. Of course, it should be done while changing the oil engine. But if you see sooting on your spark plug, you can clean it at that time as well.

However, it is not recommended to clean your spark plug without any safety precautions. We’ll go more on to the cautions next.

What Safety Precautions Must You Take?

If you want to solve your spark plug problem by yourself, then you must take some precautions. A spark plug may be small, but working with it can prove to be extremely risky.

Better safe than sorry!

  • Do not use a shot blaster or abrasives to clean a spark plug. Instead, it’s better to use a wire brush or spray-on plug cleaner.
  • Please do not use a glass bead while cleaning as it cements the gap between the center electrode and the massive electrode spark plugs.
  • Do not drop the spark plugs. Dropping the spark plug will harm the inside, and it won’t function perfectly.
  • Do not physically gap the plugs using the electrodes. Instead, you should observe the gaps gently. Then, with the help of appropriate tools, you may narrow or widen the gaps of the spark plugs.

Which Spark Plug Doesn’t Turn Black (Lasts The Longest) Easily?

Now, you may have an idea how a spark plug turns black. But what if you can buy a plug that won’t turn into other colors that easily?

If you use spark plugs that last long, this problem won’t occur frequently!

Let’s see some of the top-notch and highly-rated spark plug options, shall we?

NGK 6619 Iridium IX

NGK 6619 Iridium IX
Photo: Amazon


This spark plug features endurance and survival with one of the most robust materials. In addition, it can take 25% more than platinum and copper plugs.

Denso Platinum TT Plug

Denso Platinum TT Plug


This unit has the reputation of being a great brand name in the spark plug industry. It performs consistently and has the durability to last long.


Should Spark Plugs Be Black?

Spark plugs are standard when the tip of the plug is tan-browned or light brown. Any other color is an indication of more significant drawbacks.

Thus, spark plugs should not be black or even white.

How Can You Tell If A Spark Plug Is Bad?

You might have trouble running the engine or starting the car if a spark plug fails to function correctly. Additionally, your car might accelerate slowly compared to other cars.

Reading a spark plug can help you recognize if a spark plug is wrong or not.

Final Words

Before solving the issue, you first have to know the reason behind your black spark plug. So here we covered everything there is to know about the causes and solutions to each of the problems.

To rewind a few, you can clean your air filter or narrow the spark plug with the aim that your engine can run perfectly!

You may now check if your spark plug is functioning perfectly or not. If not, you know what to do!

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