Subaru Forester 2001 Service Manual page 7

Ignition
C: INSPECTION
Check electrodes and inner and outer porcelain of
plugs, noting the type of deposits and the degree
of electrode erosion.
(A) Electrode gap
(B) Carbon accumulation or wear
(C) Cracks
(D) Damage
(E) Damaged gasket
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 elec-
trode 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.
SPARK PLUG
S181003A10
S6M0420A
G6M0087
G6M0088
IG(DOHC TURBO)-6
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.
G6M0089
G6M0090