Spark Plug: Testing and Inspection

SPARK PLUG VISUAL DIAGNOSIS

NOTE: These spark plugs have a ceramic insulator that is 3.175 mm (1/8 in) longer than earlier model spark plugs. Use a spark plug socket that is deep enough for these longer plugs (J 39358). Failure to do so could cause cracking of the insulator and arcing inside the plug resulting in an engine misfire.






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.

Spark Plug Visual Diagnosis Chart:

^ 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.
^ The heat resistant, insulating boot covers the spark plug terminal.
- The boot extends over a portion of the spark plug insulator.
- These boots prevent engine misfiring.
^ 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.