Spark Plug: Inspection

1. Check the spark plug for abnormalities. If defective, replace the spark plug.
   Fig 1: Checking Spark Plug For Abnomalities

   Courtesy of SUBARU OF AMERICA, INC.
2. Check the spark plug electrode and condition of the insulator. If abnormal, check and repair the cause and replace the spark plug.
   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 the insulator and electrode are mostly caused by slow-speed driving in town, weak ignition, too rich fuel mixture, etc.

      Fig 3: Identifying Carbon Fouled Spark Plug

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

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

      Fig 4: Identifying Oil Fouled Spark Plug

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

      A white or light gray insulator with black or brown spots and bluish burnt electrodes indicate engine overheating, wrong selection of fuel, or loose spark plugs.

      Fig 5: Identifying Overheated Spark Plug

      Courtesy of SUBARU OF AMERICA, INC.
3. Using a nylon brush, etc., clean and remove the carbon or oxide deposits from the spark plug. If deposits are too stubborn, replace the spark plugs. After cleaning the spark plugs, check the spark plug gap "L" using a gap gauge. If it is not within the standard, replace the spark plug.

Spark plug gap L: 

Standard 

1.0 - 1.1 mm (0.039 - 0.043 in) 

Fig 6: Measuring Spark Plug Gap

Courtesy of SUBARU OF AMERICA, INC.