Why are the spark plugs black?

Spark plugs can become black because or carbon deposits. They can get soot or deposits on their entire diameter, or only on one side, they all can become black or just one or two spark plugs. In any form soot is deposited, it is good to know that this is a problem and a check is needed.

The main causes of black spark plugs vary. For example, a low-quality fuel will be responsible if the soot is wet or dry or how much deposits are there on the spark plugs.

Carbon deposits or soot on the spark plug begin to form shortly after they are used at about 150 miles, and deposits can be accelerated by driving constantly in crowded places and using low-quality fuel. Thus it is better to fuel the car with a quality fuel to avoid fast carbon deposits on the spark plug.

Causes and easy fixes of a black spark plug

If a spark plug has black deposits only on one side, this could be due to a burnout valve that is responsible for low compression in the cylinder, appearing from the bottom to the side electrode.

  • A high heat rating of a spark plug can cause it to have black deposits on one side. If the spark plugs have a high heat rating then you should consider changing the with the correct spark plugs for your car. You can check the car manual.
  • Too high fuel-air ratio. If a spark plug is black this could mean that in the combustion chamber the temperature is not optimal.
  • Late ignition. This can cause the spark plugs have soot or carbon deposits on their entire surface.
  • Clogged air filter. Although a less common cause, a clogged air filter can be responsible for deposits on a spark plug
  • A bad fuel pressure regulator. A failing pressure regulator can cause pressure above the limits in the fuel rail which can cause spark plugs to go black
  • Ignition system problems. These are caused by a damaged wire insulation, ignition coil faults, damaged or worn spark plugs such as a weak spark or spark interruptions which cause the fuel not to be fully burned thus leading to black deposits on the spark plug.
  • Incomplete combustion of the air-fuel mixture cause by problems with the engine valve mechanism due to expansion gaps.

Late ignition and rich fuel-air mixture can cause the engine to have problems when starting, especially because of ignition failures, the electronic control unit (ECU) will signal the P0300 error. Other signs will be that the engine will not as smooth and vibrations especially at idle.

Sometimes you can find oil deposits on the spark plugs. This can happen because of engine misfires, difficult engine start, black smoke from the exhaust, and engine vibrations.

Oil can enter the combustion chamber through piston rings which is not good because it can indicates problems with the engine. Also, oil can enter in the combustion chamber through worn out engine valve oil seals and this problem can be resolved by replacing the seals.

Carbon deposits form naturally in the combustion chamber and, at high engine speeds, they can come off the piston and stick to the spark plug insulator. Being due to engine misfire, a natural cause, the easy fix is to simply change the spark plugs.

How to check the black spark plug problem yourself?

Checking the spark plugs with carbon deposits or black soot on it can save you from having bigger issues with the ignition system over time. If you see a black spark plug you can try to change its place with a good one. If the good spark plug also turns black, and the black one that you changed becomes cleaner than the problem lies in the cylinder head.

But if nothing happens, than spark plugs can also be problematic. Try buying a new set of plugs and repeating the above steps to clearly see if the cylinder head is responsible.

How to clean spark plugs from black deposits?

From time to time is best to do this operation, especially it is best cleaning the spark plugs if the above problems occur. Checking the plugs every 5000 to 8000 miles is a best practice to avoid maybe bigger problems. Clean the sparks carrefully, removing all deposits on them.

  1. First of all, try to avoid cleaning the electrode with sandpaper, this can do more harm than good.
  2. You would need a rust cleaner, plastic glass and a small brush. A toothbrush works fine.
  3. Pour the cleaner in the plastic glass to a level that only the spark plugs electrodes can enter, without the inslator.
  4. Put the spark plugs in the glass, respecting the above step and leave them there for about 35 minutes.
  5. After that, take the spark plugs out and start brushing without pushing hard, pay attention to the electrodes.
  6. After brushing, put the spark plugs under warm running water.
  7. Wipe the spark plugs with a dry cloth
  8. Be sure that water evaporates completely, you can let the spark plugs dry for a longer time or use a hair dryier or a heated spot to put the spark plugs on.


As you can see, there are a few causes of black spark plugs. The often problem I faced when dealing with this problem are choosing the wrong spark plugs for the engine, so, before choosing, be certain about what spark plugs you need for your car.

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