Forester S F4-2.5L DOHC (1998)
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
guides and stems. If same condition remains after repair, use a hotter plug.
4) Overheating
White or light gray insulator with black or gray brown spots and bluish burnt electrodes indicate 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.
D: CLEANING AND REGAPPING
Clean spark plugs in a sand blast type cleaner. Avoid excessive blasting. Clean and remove carbon or oxide deposits, but do not wear away
porcelain.
If deposits are too stubborn, discard plugs. After cleaning spark plugs, recondition tiring surface of electrodes with file. Then correct the spark
plug gap using a gap gauge.
Spark plug gap: L
1.0- 1.1 mm (0.039 - 0.043 in)
