Subaru 2002 IMPREZA Service Manual page 1158

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C: INSPECTION

Check the 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

(E) Damaged gasket

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

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.

4) Overheating:

White or light gray insulator with black or gray

brown spots and bluish burnt electrodes indicates

engine overheating. Moreover, the appearance re-

sults 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.

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2001 impreza