Forester S F4-2.5L DOHC (1998)
Spark Plug: Testing and Inspection
INSPECTION
Check electrodes and inner and outer porcelain of plugs, noting the type of deposits and the degree of electrode erosion.
1. Normal
Brown to grayish-tan deposits and slight electrode wear indicate correct spark plug heat range.
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.
3. Oil fouled
Wet black deposits show excessive oil entrance into combustion chamber through worn rings and pistons or excessive clearance between valve
