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8D - 4
IGNITION SYSTEMS
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cleaning, file the center electrode flat w i t h a s m a l l
point f i l e or jewelers file. Adjust the gap between t h e
electrodes t o t h e specified dimension (Fig. 7). T h e
gap for 2.5L and 3.9L engines i s .88mm (.035 i n ) .
J908D-10
Fig. 7 Setting Spark Plug Gap - Typical
A l w a y s t i g h t e n spark plugs to the specified torque.
Over t i g h t e n i n g can cause distortion r e s u l t i n g i n a
change
i n t h e spark p l u g gap. Tighten a l l spark
plugs t o 4 1 N»m (30 ft-lbs) torque.
SPARK PLUG
CONDITION
Normal Operating Conditions
The few deposits present w i l l probably be l i g h t t a n
or s l i g h t l y gray i n color w i t h most grades of commer-
cial gasoline (Fig. 8). There w i l l not be evidence o f
electrode b u r n i n g . Gap growth w i l l not average more
t h a n approximately 0.025 m m (.001 in) per 1600 k m
(1000 miles) of operation. Spark plugs t h a t have nor-
m a l wear can usually be cleaned, have its electrodes
filed a n d regapped, and t h e n reinstalled.
Some fuel refiners i n several areas of the U n i t e d
States have introduced a manganese additive ( M M T )
for
unleaded fuel. D u r i n g combustion, fuel
w i t h
M M T causes the entire t i p of the spark p l u g to be
coated w i t h a rust colored deposit. This rust color can
be misdiagnosed as being caused b y coolant i n t h e
combustion chamber. Spark p l u g performance is n o t
affected b y M M T deposits.
Cold Fouling (Carbon Fouling)
Cold f o u l i n g is sometimes referred to as carbon
fouling because the deposits t h a t cause cold f o u l i n g
are basically carbon (Fig. 8). A dry, black deposit o n
one or t w o plugs i n a set may be caused by s t i c k i n g
J908D-15
Fig. 8 Normal Operation and Cold (Carbon) Fouling
valves o r defective spark p l u g cables. Cold (carbon)
f o u l i n g o f t h e entire set may be caused b y a clogged
air cleaner.
Cold f o u l i n g is n o r m a l after short operating p e r i -
ods. The spark plugs do not reach a h i g h enough op-
e r a t i n g temperature d u r i n g short operating periods
to b u r n t h e carbon deposits off.
Electrode Gap Bridging
Electrode gap b r i d g i n g may be traced to loose de-
posits i n t h e combustion chamber. These deposits ac-
cumulate on the spark plugs d u r i n g continuous stop-
and-go
d r i v i n g .
W h e n
t h e engine
is
suddenly
subjected t o a h i g h torque load, the deposits p a r t i a l l y
liquefy a n d bridge t h e gap between t h e electrodes
(Fig. 9). T h i s short circuits t h e electrodes.
Spark
plugs w i t h electrode gap b r i d g i n g can be cleaned us-
i n g s t a n d a r d procedures.
GROUND
ELECTRODE
J908D-11
Fig. 9 Electrode Gap Bridging
Scavenger Deposits
F u e l scavenger deposits may be either w h i t e or yel-
low (Fig. 10). They may appear t o be h a r m f u l , b u t
this is a n o r m a l condition caused b y chemical addi-
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