Subaru XT 1988 Service Manual page 1208

ENGINE ELECTRICAL SYSTEM 6-1
3) Oil fouled
Wet black deposits show excessive oil entrance into combus-
tion chamber through worn rings and pistons or excessive
clearance between valve guides and stems. If same condition
remains after repair, use a hotter plug.
A9-166
Fig. 66
4) Overheating
White or light gray insulator with black or gray brown spots
and bluish burnt electrodes indicate engine overheating. More-
over, 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.
A9-167
Fig. 67
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 firing surface of
electrodes with file. Then correct the spark plug gap to 1.0 to
1.1 mm (0.039 to 0.043 in) using a gap gauge.
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