Spark Plug: Inspection

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

Fig 1: Identifying Spark Plug Gap

Courtesy of SUBARU OF AMERICA, INC.

1. Normal:

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

   Fig 2: Identifying Brown To Grayish-Tan Deposits

   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 city, weak ignition, too rich fuel mixture and dirty air cleaner.

   Fig 3: Identifying Carbon Fouled

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

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

   Fig 4: Identifying Oil Fouled

   Courtesy of SUBARU OF AMERICA, INC.
4. Overheating:

   White or light gray insulator with black or brown spots and bluish burnt electrodes indicates engine overheating. Moreover, the appearance results from incorrect ignition timing, loose spark plugs, wrong selection of fuel, hotter range plug, etc.

   Fig 5: Identifying Overheating

   Courtesy of SUBARU OF AMERICA, INC.