Engine Ignition System (H4DOTC) (Except Sti): Diagnosis Subaru Impreza GH

INSPECTION

Check the electrodes and inner and outer ceramic insulator of plugs, noting the type of deposits and the degree of electrode erosion.

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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 electrode are mostly caused by slow speed driving in the city, weak ignition, too rich fuel mixture, dirty air cleaner, etc.
3. Oil fouled: Wet black deposits show oil entrance into combustion chamber through worn rings or increased clearance between valve guides and stems.
4. Overheating

White or light gray insulator with black or brown spots and bluish burnt electrodes indicate engine overheating, incorrect ignition timing, improper fuel, or loose spark plugs.

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For inspection procedure, refer to "Diagnostics for Engine Starting Failure". Ref. to IGNITION CONTROL SYSTEM , Diagnostics for Engine Starting Failure.