Spark Plug: Inspection

Check electrodes and inner and outer porcelain 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.

   Fig 2: Locating Brown To Grayish-Tan Deposits & Slight Electrode

   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, dirty air cleaner, etc.

   It is advisable to replace with plugs having hotter heat range.

   Fig 3: Identifying Carbon Fouled

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

   Wet black deposits show excessive 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 gray brown spots and bluish burnt electrodes indicate engine overheating. Moreover, the appearance results from incorrect ignition timing, loose spark plugs, wrong selection of fuel, hotter range plug, etc. It is advisable to replace with plugs having colder heat range.

   Fig 5: Identifying Overheating

   Courtesy of SUBARU OF AMERICA, INC.