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: Spark Plug Electrode And Condition Of Insulator - Normal
  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 the city, weak ignition, too rich fuel mixture, etc.
  
  Fig 3: Spark Plug Electrode And Condition Of Insulator - 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: Spark Plug Electrode And Condition Of Insulator - 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: Spark Plug Electrode And Condition Of Insulator - 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

0.7 - 0.8 mm (0.028 - 0.031 in)

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