Spark Plug: Inspection

Check the electrodes and inner and outer ceramic insulator of plugs, noting the type of deposits and the degree of electrode erosion.

Fig 1: Checking Sparkplug: Electrode, Inner & Outer Ceramic Insulator

Courtesy of SUBARU OF AMERICA, INC.

1. Normal:

   Brown to grayish-tan deposits and slight electrode wear indicates correct spark plug heat range.

   Fig 2: Inspecting Sparkplugs For Normal Wear: Brown To Grayish-Tan Deposits & Slight Electrode Wear

   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 city, weak ignition, too rich fuel mixture, dirty air cleaner, etc.

   It is advisable to replace with plugs having hotter heat range.

   Fig 3: Inspecting Sparkplugs For Carbon Fouling: Fluffy Carbon Deposits On Insulator Electrode

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

   Wet black deposits show excessive oil entrance into combustion chamber through worn rings and pistons or excessive clearance between valve guides and stems. If the same condition remains after repair, use a hotter plug.

   Fig 4: Inspecting Sparkplugs For Oil Fouling: Oil Fouled

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

   White or light gray insulator with black or gray brown spots and bluish burnt electrodes indicates engine overheating. Moreover, the appearance results from incorrect ignition timing, loose spark plugs, wrong selection of fuel, hotter range plug, etc. It is advisable to replace with plugs having colder heat range.

   Fig 5: Inspecting Sparkplugs For Overheating

   Courtesy of SUBARU OF AMERICA, INC.