Spark Plug Visual Diagnosis

Spark Plug Visual Diagnosis Chart:

Resistor type spark plugs with tapered seats are used. No gasket is needed on these plugs. These spark plugs are a High Efficiency (HE) spark plug that has a fine wire electrode, nickel plated shell and a platinum pad across from the electrode.
^ Normal service is a mixture of idling, low speed, and high speed operation.
- Occasional high speed driving is essential to good spark plug performance.
- Increased combustion heat burns away excess carbon deposits, which accumulate from frequent idling or stop-and-go driving.
^ A broken upper insulator is usually the result of improper installation or carelessness when regapping the plug.
- Broken UPPER insulators usually result from a poor fitting wrench or an outside impact.
- The cracked insulator may not show up right away, but will as soon as oil or moisture penetrates the crack.
^ Broken LOWER insulators usually result from carelessness when regapping and generally are visible.
- This type of break may result from the plug operating too hot, which may happen in periods of high-speed operation or under heavy loads.
- When regapping a spark plug, always make the gap adjustment by bending the ground (side) electrode.
- Always replace spark plugs that have broken insulators.
^ Corona discharge is a steady blue light appearing (in darkness) around the insulator, just above the shell crimp.
- Do not mistake this for a shorted insulator.
- This is visible evidence of a high-tension field.
- This has no effect on ignition system performance.
- This discharge repels dust particles.
- This leaves a clear ring on the insulator just above the shell.
- This is not evidence that combustion gases are escaping.