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.

Fig 1: Identifying Spark Plug With Gap

Courtesy of SUBARU OF AMERICA, INC.

1. Normal:

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

   Fig 2: Identifying Normal Spark Plug

   Courtesy of SUBARU OF AMERICA, INC.
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.

   Fig 3: Identifying Carbon Fouled Spark Plug

   Courtesy of SUBARU OF AMERICA, INC.
3. Oil fouled:

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

   Fig 4: Identifying Oil Fouled Spark Plug

   Courtesy of SUBARU OF AMERICA, INC.
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.

   Fig 5: Identifying Overheating Spark Plug

   Courtesy of SUBARU OF AMERICA, INC.