Spark Plug: Inspection

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

   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 Spark Plug (Normal)

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   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, etc.

      Fig 3: Identifying Spark Plug (Carbon Fouled)

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   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 Spark Plug (Oil Fouled)

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   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 Spark Plug (Overheating)

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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: Identifying Spark Plug Gap

Courtesy of SUBARU OF AMERICA, INC.