Updated at: 03-08-2023 - By: Lucas

Carbon deposits may cause spark plugs to turn black. They can all turn black, or just one or two plugs, depending on whether they accumulate soot or deposits across their full diameter or only on one side. It’s comforting to know that soot buildup, in whatever form it takes, is a problem that necessitates inspection.

Black spark plugs can be caused by a variety of factors. Whether the soot is moist or dry, for example, or how many deposits are on the spark plugs, a poor-quality fuel will be to blame.

If you drive in a congested area and use low-quality fuel, you may notice that your spark plugs begin to accumulate carbon deposits or soot within 150 miles. Thus, it is best to use a high-quality fuel in order to avoid quickly building up carbon deposits on the spark plug.

Causes and easy fixes of a black spark plug

Causes And Easy Fixes Of A Black Spark Plugs-1

There may be a burnout valve in the cylinder responsible for low cylinder pressure, which can be seen on one side of a spark plug with black deposits.

  • 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.

An electronic control unit (ECU) will display the P0300 error if it detects a rich fuel-air combination or late ignition while the engine is trying to start. Other symptoms include a sluggish engine and vibrations, particularly at idle.

Spark plugs can occasionally be found with oil residue. Engine misfires, difficulty starting the engine, black exhaust smoke, and engine vibrations can all cause this.

As a result of piston rings leaking oil into the combustion chamber, the engine may be in trouble. In addition, worn-out engine valve oil seals might allow oil into the combustion chamber, which can be remedied by replacing the seals.

Carbon deposits naturally accumulate in the combustion chamber and might adhere to the spark plug insulator at high engine speeds. The simple solution is to replace the spark plugs, which are the natural source of engine misfire.

How to check the black spark plug problem yourself?

Spark plugs that have carbon deposits or black soot on them should be checked regularly to avoid more serious ignition system problems. Spark plug replacement can be attempted in the event that a black one is found. It is possible that the cylinder head is to blame if both the good and the bad spark plugs turn black.

Spark plugs, on the other hand, can be a hazard even if nothing happens. New plugs can be purchased, and the previous processes can be repeated to determine the cause of the problem.

How to clean spark plugs from black deposits?

Causes And Easy Fixes Of A Black Spark Plugs-3

Even if none of the concerns listed above are present, it is recommended that you do this maintenance procedure on a regular basis. Checking the spark plugs on a regular basis will help you avoid more serious issues down the road. Remove all traces of pollution from the sparks by scrubbing them well.

  • First of all, try to avoid cleaning the electrode with sandpaper, this can do more harm than good.
  • You would need a rust cleaner, plastic glass and a small brush. A toothbrush works fine.
  • Pour the cleaner in the plastic glass to a level that only the spark plugs electrodes can enter, without the inslator.
  • Put the spark plugs in the glass, respecting the above step and leave them there for about 35 minutes.
  • After that, take the spark plugs out and start brushing without pushing hard, pay attention to the electrodes.
  • After brushing, put the spark plugs under warm running water.
  • Wipe the spark plugs with a dry cloth
  • 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, black spark plugs can be caused by a variety of things. Choosing the wrong spark plugs for the engine is a common error I encountered when addressing this issue, so be sure you know exactly what spark plugs your vehicle requires before making a purchase.