Subaru LEGACY 2006 Manual page 740

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C: INSPECTION
Check the electrodes and inner and outer ceramic
insulator of plugs, noting the type of deposits and
the degree of electrode erosion.
(A)
(B)
(E)
(A) Spark plug 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
the city, weak ignition, too rich fuel mixture, dirty air
cleaner, etc.
Spark Plug
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.
(C)
(D)
IG-00010
4) Overheating:
White or light gray insulator with black or brown
spots and bluish burnt electrodes indicate engine
overheating. This is caused by inappropriate igni-
tion timing, loose spark plugs, and inappropriate fu-
el.
IG-00011
IG-00012
IG(H4SO)-5
IGNITION
IG-00013
IG-00014