GMC 4000 Series Service Manual page 420

usually indicates the engine is operating at speeds
or loads that are consistently greater than normal
or that a plug which is too "hot" is being used. In
addition, electrode wear may be the result of plug
overheating, caused by combustion gases leaking
past the threads and gaskets, due to insufficient
com pression of the spark plug gaskets, dirt under
gasket, or use of old gaskets. Too lean a fuel mix­
ture will also result in excessive electrode wear.
Spark plug life will also be affected by incor­
rect timing of the engine which results in ex cess­
ively high operating temperature.
Broken insulators are usually the result of
improper installation or carelessn ess, when re-
gapping the plug. Broken upper insulators usually
result from a poor fitting wrench or an outside
blow. The
cr a ck ed in su la to r m ay not m ake
evident immediately, but will as soon as oil or
moisture penetrates the fracture. The fracture is
usually just below the crimped part of the shell
and may not be visible.
Broken lower insulators usually result from
carelessn ess when regapping and generally are
visible. This type of a break may result from the
plug operating too "Hot" such as encountered in
sustained periods of high speed operation or under
extrem ely heavy loads, especially if not installed
correctly. Spark plugs with broken insulators
should always be replaced.
Spark plugs, to give good performance in a
particular engine, must operate within a certain
temperature range (neither too hot nor too cool).
If the spark plug remains too "cool," oil, soot,
carbon, and lead components will deposit on the
insulator, causing FOULING and MISSING. If the
plug runs too "Hot," the deposits accumulated on
the insulator surface during continuous slow or
stop-and-go driving may become blistered, elec­
trodes will wear rapidly, and under extreme con­
ditions, premature ignition (preignition) of the fuel
mixture result. EITHER CONDITION WILL SER­
IOUSLY AFFECT ENGINE PERFORMANCE.
Refer to "Specifications" at end of this section,
as the use of spark plugs in the proper Heat Range
is of vital importance to good engine performance.
Frequently, the wrong type of spark plug, one with
an improper Heat Range for the engine, may have
been installed when replacing spark plugs origin­
ally fitted by the engine manufacturer and such
misapplication may lead to poor performance.
ABNORMAL OPERATION
Where abnormal operating conditions cause
chronic carbon or oil fouling of the plugs, the use
of a type with one or two numbers higher (a "hot­
ter" type) than recommended in "Specifications,"
will generally remedy the trouble; and by the same
formula, where chronic preignition or rapid elec­
trode wear is experienced, a type with one or two
G M C S E R V I C E M A N U A L
numbers lower (a "cooler" type) will generally be
found satisfactory.
SPARK PLUG REMOVAL
blow all dirt out of plug sockets in cylinder head
with compressed air.
caution to avoid damaging wire terminals. Remove
w ires by firmly grasping large end of boot.
S-9704B (manufactured by Snap-On Tool Co.) (fig.
10), and unscrew plugs from cylinder head. Ordin­
ary wrenches may damage porcelain. If gaskets do
not remain on plugs, remove from cylinder head.
INSPECTION AND CLEANING
itself
ed points, and check point gap. Also check for loose
term inals. Replace plugs which have excessively
burned electrodes or cracked porcelain. Plugs
should be cleaned with an abrasive type cleaner.
If porcelain is badly glazed or blistered, the spark
plugs should be replaced. All spark plugs must be
of the same make and number of heat range. Use
a wire feeler gauge when checking spark plug gap.
POINT GAP ADJUSTMENT
and should be treated as such. Refer to "Specifi­
cations" at end of this section for proper gap di­
mensions. All plugs must be set to the same di­
mension, using a standard round feeler gauge.
IGNITION SYSTEM
IMPORTANT: Before removing any sparkplug,
1. Pull w ires off spark plug terminals, using
2. Use special spark plug wrench socket No.
Inspect plugs for cracked porcelain and burn­
Setting spark plug gap is a precision operation
CAUTION: Before adjusting gap, file
center electrode flat. In adjusting the
spark plug gap, never bend the center
electrode which extends through the por­
celain center as this may break the lower
insulator. Always make adjustment by
bending the ground or side electrode.
Figure 10— W rench Socket Installed on Plug
Sec. 6Y
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