SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or
yellow (Fig. 10). They may appear to be harmful, but
are a normal condition caused by chemical additives in
certain fuels. These additives are designed to change
the chemical nature of deposits and decrease spark
plug misfire tendencies. Notice that accumulation on
the ground electrode and shell area may be heavy but
the deposits are easily removed. Spark plugs with
scavenger deposits can be considered normal in condi-
tion and cleaned using standard procedures.
CHIPPED ELECTRODE INSULATOR
A chipped electrode insulator usually results from
bending the center electrode while adjusting the spark
plug electrode gap. Under certain conditions, severe
detonation also can separate the insulator from the
center electrode (Fig. 11). Replace spark plugs with
chipped electrode insulators.
PREIGNITION DAMAGE
Excessive combustion chamber temperature can
cause preignition damage. The center electrode dis-
solves first and the ground electrode dissolves some-
what later (Fig. 12). Insulators appear relatively de-
posit free. Determine if the spark plug has the correct
heat range rating for the engine, if ignition timing is
over advanced or if other operating conditions are
causing engine overheating. (The heat range rating
refers to the operating temperature of a particular type
spark plug. Spark plugs are designed to operate within
specific temperature ranges depending upon the thick-
ness and length of the center electrode and porcelain
insulator.)
SPARK PLUG OVERHEATING
Overheating is indicated by a white or gray center
electrode insulator that also appears blistered (Fig.
13). The increase in electrode gap will be considerably
in excess of 0.001 in per 1000 miles of operation. This
suggests that a plug with a cooler heat range rating
should
be
used.
Over
advanced
ignition
tim-
Fig. 10 Scavenger Deposits
Fig. 12 Preignition Damage
Fig. 9 Electrode Gap Bridging
Fig. 11 Chipped Electrode Insulator
.
IGNITION SYSTEMS
8D - 5