Spark Plug: Inspection

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

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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: Spark Plug Electrode And Insulator Condition (Brown To Grayish-Tan Deposits) - Normal

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   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: Spark Plug Electrode And Insulator Condition (Dry Fluffy Carbon Deposits On Insulator And Electrode)

      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 Insulator Condition (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: Spark Plug Electrode And Insulator Condition (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 

0.50 - 0.55 mm (0.020 - 0.022 in) 

Fig 6: Spark Plug Gap L

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