Subaru Impreza 2003 Service Manual page 1172

IGNITION
1) Normal:
Brown to grayish-tan deposits and slight electrode
wear indicates 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.
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.
SPARK PLUG
4) Overheating:
White or light gray insulator with black or 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.
IG-00011
D: ADJUSTMENT
Clean the spark plugs with a wire brush. Clean and
remove the carbon or oxide deposits, but do not
wear away porcelain.
If deposits are too stubborn, replace the plugs.
After cleaning the spark plugs, correct the spark
plug gap using a gap gauge.
NOTE:
Do not use spark plug cleaners, because the spark
plugs are applied with platinum tips.
Spark plug gap: L
0.7 — 0.8 mm (0.028 — 0.031 in)
IG-00012
NOTE:
Replace with a new spark plug if this area (A) is
worn to "ball" shape.
IG-00013
IG(H4DOTC)-6
IG-00014
L
(A)
IG-00015