Page "Spark plug" Paragraph 50

A light brownish discoloration of the tip of the block indicates proper operation ; other conditions may indicate malfunction.

For example, a sandblasted look to the tip of the spark plug means persistent, light detonation is occurring, often unheard.

The damage that is occurring to the tip of the spark plug is also occurring on the inside of the cylinder.

Heavy detonation can cause outright breakage of the spark plug insulator and internal engine parts before appearing as sandblasted erosion but is easily heard.

As another example, if the plug is too cold, there will be deposits on the nose of the plug.

Conversely if the plug is too hot, the porcelain will be porous looking, almost like sugar.

The material which seals the central electrode to the insulator will boil out.

Sometimes the end of the plug will appear glazed, as the deposits have melted.