Spark Plug: Inspection

Check the spark plug for abnormalities. If defective, replace the spark plug.

Fig 1: Checking Spark Plug For Abnormalities
Courtesy of SUBARU OF AMERICA, INC.
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: Checking Spark Plug Condition - Normal
  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: Checking Spark Plug Condition - Carbon Fouled
  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: Checking Spark Plug Condition - Oil Fouled
  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: Checking Spark Plug Condition - Overheating
  Courtesy of SUBARU OF AMERICA, INC.
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.
NOTE:
- Never use a plug cleaner.
- Do not use a metal brush as it may damage the electrode area.
Spark plug gap L:

Standard

0.50 - 0.55 mm (0.020 - 0.022 in)

Fig 6: Checking Spark Plug Gap
Courtesy of SUBARU OF AMERICA, INC.