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: Inspecting Spark Plug

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: Indicating Correct Spark Plug Heat Range

   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 Spark Plug

   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. If the same condition remains after repair, use a hotter plug.

   Fig 4: Identifying Oil Fouled Spark Plug

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

   Fig 5: Identifying Overheated Spark Plug

   Courtesy of SUBARU OF AMERICA, INC.