Subaru 2009 IMPREZA Service Manual page 392

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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) Spark plug gap

(B) Carbon accumulation or wear

(E) Damaged gasket

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 and dirty

air cleaner.

3) Oil fouled:

Wet black deposits show oil entrance into the com-

bustion chamber through worn rings or pistons, or

excessive clearance between valve guides and

4) Overheating:

White or light gray insulator with black or brown

spots and bluish burnt electrodes indicate engine

overheating, incorrect ignition timing, wrong selec-

tion of fuel, and loose spark plugs.

D: ADJUSTMENT

Clean spark plugs with a nylon brush, etc. Clean

and remove the carbon or oxide deposits. If depos-

its are too stubborn, replace the spark plugs. After

cleaning the spark plugs, measure the spark plug

gap using a gap gauge.

• Never use plug cleaner.

• Do not use metal brush or it will wear insulator.

Spark plug gap L:

1.0 — 1.1 mm (0.039 — 0.043 in)