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IGNITION SYSTEMS
8D - 5
tives i n c e r t a i n fuels. These additives are designed to
change t h e chemical n a t u r e of deposits and decrease
spark p l u g misfire tendencies. Notice t h a t accumula-
t i o n o n t h e ground electrode a n d shell area m a y be
heavy b u t t h e deposits are easily removed. Spark
plugs w i t h scavenger deposits can be considered nor-
m a l i n condition and be cleaned u s i n g standard pro-
cedures.
GROUND
ELECTRODE
COVERED
WITH WHITE
OR YELLOW
DEPOSITS
CENTER
ELECTRODE
t i m i n g is over advanced or i f other operating condi-
tions are causing engine
overheating. (The heat
range r a t i n g refers to t h e operating temperature o f a
p a r t i c u l a r type spark p l u g . Spark plugs are designed
to operate w i t h i n specific temperature ranges de-
pending upon t h e thickness and length of the center
electrodes porcelain insulator.)
GROUND
ELECTRODE
STARTING
TO DISSOLVE
CENTER
ELECTRODE
DISSOLVED
J908D-12
Fig. 10 Scavenger
Deposits
Chipped Electrode Insulator
A chipped electrode insulator u s u a l l y results from
bending t h e center electrode w h i l e adjusting t h e
spark p l u g electrode gap. U n d e r certain conditions,
severe detonation can also separate t h e insulator
f r o m the center electrode (Fig. 11). Spark plugs w i t h
t h i s condition m u s t be replaced.
GROUND
ELECTRODE
CENTER
ELECTRODE
CHIPPED
INSULATOR
J908D-14
Fig. 12 Preignition Damage
Spark Plug Overheating
Overheating is indicated by a w h i t e or gray center
electrode i n s u l a t o r t h a t also appears blistered ( F i g .
13). T h e increase i n electrode gap w i l l be consider-
ably i n excess o f 0.001 i n c h per 1000 miles o f opera-
t i o n . This suggests t h a t a p l u g w i t h a cooler heat
range r a t i n g should be used. Over advanced i g n i t i o n
t i m i n g , detonation a n d cooling system malfunctions
can also cause spark p l u g overheating.
BLISTERED
WHITE OR
GRAY
COLORED
INSULATOR
J908D-13
Fig. 11 Chipped Electrode Insulator
Preignition Damage
P r e i g n i t i o n damage is caused by excessive combus-
t i o n chamber temperature. The center electrode dis-
solves
first
a n d t h e ground electrode
dissolves
somewhat l a t e r (Fig. 12). Insulators appear r e l a t i v e l y
deposit free. Determine i f the spark p l u g has t h e cor-
rect heat range r a t i n g for t h e engine, i f i g n i t i o n
J908D-16
Fig. 13 Spark Plug Overheating
SPARK PLUG
SERVICE
Use care w h e n disconnecting the spark p l u g a n d
coil cable boots and cables. T w i s t t h e boot 1/2 t u r n
and p u l l on t h e boot only to disconnect t h e cable.
W h e n replacing t h e spark plug and coil cables,
route t h e cables correctly and secure t h e m i n t h e ap-
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