Why spark plugs are black

The black sooting of the spark plugs can tell the car owner about the problems with his car. The causes of this can be low-quality fuel, problems with ignition, incompatibility of the air-fuel mixture or incorrectly set carburetor, and so on.

The black spark plugs

All these problems can be diagnosed quite easily just by looking at the black spark plugs.

Possible causes of sooting

Before answering the question of why the spark plugs are black, it is necessary to determine the way they have become black. After all, it depends on the search direction. In particular, spark plugs can blacken all together or only by one or two of the set. Also, the spark plug can turn black only on one side or to the entire diameter. There can also be the so-called “wet” and “dry” sooting.

It should be noted that the rate of appearance and the nature of the sooting depends on the existing faults (if any):

  • Sooting on new spark plugs begins to form at least after 200-300 operational kilometers. Moreover, it is desirable to drive along the highway at approximately the same speed and engine load. So the spark plugs will work optimally, and it will be possible to assess the condition of the car’s units more objectively.
  • The amount and type of sooting depends on the quality of the used fuel. Therefore, try to tank up at reliable fuel stations, and not to use gasoline or similar mixtures. Otherwise, it is difficult to determine the real cause of the sooting (if any).
  • In the carburetor engine idle speed should be set correctly.

Now let’s consider the question of the causes of sooting on the spark plugs. There are 11 key reasons:

  1. If you have noticed blackening on one side only, most likely, it is caused by the valve burnout. That is, sooting appears on the spark plug from the bottom to the side electrode (not to the center electrode).
  2. The cause of black spark plugs can be the valve burnout. The situation is similar to the previous one. Sooting can penetrate the bottom electrode.
  3. Incorrectly selected heat rating of a spark plug causes not only its damage in further use, but also uneven blackening. If the mentioned rating is low, then the shape of the sooting cone changes. If it is high, only the tip of the cone turns black and the body is white.
    Heat rating is the value characterizing the time during which a spark plug reaches the surface ignition. With a high heat rating, it heats up less. Install spark plugs with the heat rating specified by the manufacturer.
  4. Even black sooting on spark plugs indicates late ignition.
  5. Black spark plugs on the injector or carburetor can be due to the fact that the air-fuel mixture produced by them is too rich. As for the injector, there is a high probability of incorrect operation of the mass air flow sensor (MAFS), which provides information to the ECU about the mixture composition. Also depressurization of the fuel injectors is possible. Because of this, petrol gets into the cylinders even when the injector is closed. As for the carburetor, the causes can be as follows: misadjusted fuel level in the carburetor, depressurization of the needle gate valve, fuel pump overpressures (drive lifter outstands greatly), depressurization of the float or its contact with the chamber walls.
    «Dry» sooting on the spark plug

    «Dry» sooting on the spark plug

  6. Considerable wear or depressurization of the ball valve of the economizer high-power modes at carburetor engines. That is, more fuel enters the engine not only in high-power modes, but also in normal modes.
  7. The cause of a black spark plug can be a clogged air filter. Be sure to check its state and replace if necessary. Also check the air choke actuator.
  8. There can be problems with the ignition system, i.e. incorrectly set angle of ignition, defect of the high-voltage wire insulation, violation of the integrity of the cover or the distributor slider, malfunctions of the ignition coil, problems with the spark plugs. The listed factors can lead to interruptions in sparking or the occurrence of a weak spark. Due to this, not all the fuel burns and black sooting appears on the spark plugs.
  9. There can be problems with the engine valve mechanism. In particular, it can be the burnout of valves or their unadjusted expansion gaps. As a result, there are incomplete combustion of the air-fuel mixture and sooting on the spark plugs.
  10. When the fuel regulator has broken in injection cars, there is an overpressure in the fuel rail.
  11. Low level of compression in the cylinder corresponding to the black spark plug. You can read how to check the compression in another article.

As a rule, the following consequences occur in case of the set late ignition and operation on a rich air-fuel mixture:

  • ignition failure (There is the error P0300 in case of injector engines);
  • problems with engine starting;
  • uneven engine running, especially at idle, and as a result, an increased level of vibration.

Further we will tell how to eliminate the listed failures and clean spark plugs.

What to do in case of sooting

First of all, you should bear in mind that oil dirt and overheating, resulting in sooting on the spark plugs, are very harmful to the ignition system. Overheating is especially bad, because it causes the probability of failure of the electrodes on the spark plugs without the capability of their recovery.

If only one blackened spark plug has appeared in your car, you can detect the failure by changing the position of the spark plugs. If the new spark plug also turns black after that, and the old one becomes clean, it means that the problem is not in the spark plugs, but in the cylinder. If nothing has changed, the question then is the performance of the spark plug.


Oil on a spark plug

Oil on a spark plug

In some cases, spark plugs can be wet and black. The most common cause of this fact is the oil pumping. Additional signs of this fault are as follows:

Oil on a spark plug

  • difficult engine starting;
  • engine misfire;
  • engine moves sharply;
  • there is blue smoke from exhaust pipe.

Oil can get into the combustion chamber in two ways, i.e. from the bottom or from the top. In the first case, it gets through the piston rings. And this is a very bad sign, because it often results in the engine overhaul. In rare cases, you can carry out decarbonation of the engine. If the oil gets into the combustion chamber through the top, it goes from the cylinder head along the valve guides. The reason for this is the wear of the valve oil seals. To eliminate this problem, it is necessary to replace the seals.

Sooting on the insulator

Red sooting on a spark plug

Red sooting on a spark plug

In some cases, sooting that naturally appears in the combustion chamber can come off the piston and stick to the spark plug insulator at high engine speeds. The consequence of this is engine misfire. This is the most harmless situation, when spark plugs blacken. You can fix it by cleaning their surface or replacing them with new ones.

If black and red spark plugs have appeared in your engine, it means that you fuel up the car with an excess of additives with metals. Such fuel cannot be used for a long time because metallic deposits form a conductive soot on the surface of the plug insulator over time. Sparking will be worse and the spark plugs will soon fail.

Cleaning of spark plugs from sooting

Cleaning of spark plugs

Spark plugs should be cleaned and inspected regularly. It is recommended to do this in approximately 8-10 thousand kilometers. It is very convenient to do this at the time of replacing the engine oil. However, if the above described signs have appeared, it can be done earlier.

It should be noted that it is not recommended to clean the electrodes by the old method with sandpaper. The fact is that there is a risk to damage the protective layer on them. This is especially relevant for iridic spark plugs. Their central electrode is thin and covered with iridium, which is a semi-precious and rare metal.

To clean the spark plugs, you need:

  • detergent to remove sooting and rust;
  • disposable plastic cups (after the end of the cleaning procedure they should be disposed, they cannot be used for food products);
  • a thin brush with a hard bristles or a toothbrush;
  • rugs.

The cleaning procedure is performed according to the following algorithm:

To clean the spark plugs

The cleaning procedure

  1. The detergent is poured in a glass prepared in advance to a level to immerse the spark plug electrodes (without the insulator) completely.
  2. Immerse the spark plugs in the glass and leave for 30-40 minutes (a chemical cleaning reaction occurs in the process, which can be observed by the naked eye).
  3. After the specified time, the spark plugs are removed from the glass, and sooting is removed from the surface of the spark plug, especially the electrodes, with the help of a brush or a toothbrush.
  4. Wash the spark plugs in warm running water, removing the chemical composition and dirt from their surface.
  5. After washing, wipe the spark plugs dry with rugs prepared in advance.
  6. At the final stage dry the spark plugs on the battery, in the oven (at a low temperature of +60...+ 70 °C) or with a hair dryer or a fan heater (the main thing is that the remaining water in them should evaporate completely).

It is necessary to perform the procedure carefully, cleaning and removing all dirt and sooting on the surface. Remember that washed and cleaned spark plugs operate 10-15% more efficiently than dirty ones.


The appearance of a black spark plug on the carburetor or injector can be caused by various reasons. As a rule, there are several of them at once. For example, incorrectly selected spark plugs, extended engine operation at high speed, incorrectly set ignition, devective valve oil seals and so on. Therefore, we recommend to check the state of the spark plugs in your car periodically and when the above mentioned signs appear.

Inspect and clean the spark plugs with each oil change (8 — 10 thousand km). It is important that the correct clearance is set, and the spark plug insulator is clean. The spark plugs are recommended to be replaced in every 40-50 thousand kilometers (platinum and iridium plugs should be replaced in 80-90 thousand kilometers).

As the result, you not only increase the lifespan of the engine, but also maintain power and comfort when driving. For further information on how to diagnose a car's engine by the color of the spark plugs, you can read this article.

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