Spark Plug: Testing and Inspection

Spark Plug

INSPECTION

Check the electrodes and inner and outer ceramic insulator of plugs, noting the type of deposits and the degree of electrode erosion.






1. Normal:
Brown to grayish-tan deposits and slight electrode wear indicate correct spark plug heat range.






2. Carbon fouled:
Dry fluffy carbon deposits on insulator and electrode are mostly caused by slow speed driving in the city, weak ignition, too rich fuel mixture and dirty air cleaner.






3. Oil fouled:
Wet black deposits show oil entrance into the combustion chamber through worn rings and excessive clearance between valve guides and stems.






4. Over-heating:
White or light gray insulator with black or brown spots and bluish burnt electrodes indicate engine overheating, incorrect ignition timing, wrong selection of fuel, and loose spark plugs.