When removing spark plugs, you should also follow all the above safety precautions. First of all, you should disconnect the battery. Spark plugs must be replaced within the timeframes specified in the service plan.
When removing the spark plugs, make sure that no foreign objects get into the holes into which the spark plugs are screwed. Failure to comply with this requirement will result in damage to the valves, valve seats or even the cylinder head after installing the spark plugs and starting the engine.
Based on the spark plug's appearance, certain conclusions can be drawn about its functioning, mixture formation, and the engine's condition (pistons, piston rings, etc.).
The spark plug is normal if there is a light powdery coating on the electrodes and insulator, ranging from grey-yellow to grey-brown in colour. At the same time, this indicates that the engine is also in order. The glow plug heat rating is also selected correctly and corresponds to the operating loads.
If the spark plug body, insulator and electrodes are covered with a thick layer of powdery soot (carbon deposit) of a black-gray color, this means that the mixture formation is adjusted incorrectly and the air-fuel mixture is oversaturated. The air supply is too low, the choke is engaged for too long, the gap between the electrodes is too large, the glow plug is too high and its temperature during engine operation is below the required operating temperature.
The presence of oily carbon deposits on the insulator, electrodes and spark plug housing indicates that engine oil is getting into the combustion chambers. This may be due to wear of the pistons or piston rings.
The presence of glassy, stuck-on deposits of a dark brown or even grey-black colour on the insulator, which sometimes have the shape of drops, with simultaneous damage to the electrodes, especially the central one, and the spark plug body, is evidence of a lean mixture, a loosely tightened spark plug, or poorly closing valves. The reason for this may be the spark plug itself, because its heat range does not correspond to the engine's needs.
If the car is fueled with leaded gasoline, then the insulator of a normally working spark plug will have a gray tint. It is recommended to remove deposits that accumulate between the insulator and the central electrode using a sand blast from a spark plug tester. Before installing the spark plugs, be sure to clean their threads.