Editing Spark plug (section)

==Reading spark plugs==<!-- This section is linked from [[Carburetor]] -->
The spark plug's firing end will be affected by the internal environment of the combustion chamber. As the spark plug can be removed for inspection, the effects of combustion on the plug can be examined. An examination, or "reading" of the characteristic markings on the firing end of the spark plug can indicate conditions within the running engine. Engine and spark plug manufacturers will publish information about the characteristic markings in spark plug reading charts.

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 [[engine knocking|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.

An idling engine will have a different impact on the spark plugs than one running at full [[throttle]]. Spark plug readings are only valid for the most recent engine operating conditions and running the engine under different conditions may erase or obscure characteristic marks previously left on the spark plugs. The most valuable information is gathered by running the engine at high speed and full load, immediately cutting the ignition off and stopping without idling or low speed operation and removing the plugs for reading.{{Citation needed|reason=Who decided this method works the best?|date=August 2014}}

Spark plug reading viewers, which are simply combined flashlight/magnifiers, are available to improve the reading of the spark plugs.

[[File:2spark plug viewers.jpg|thumb|right|Two spark plug viewers]]