Chevrolet 70 1969 Series, Руководство по ремонту

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ENGINE ELECTRICAL 6Y-17
Dirty or leaded plugs may be evident by black
carbon deposits, or red, brown, yellow or blistered
oxide deposits on the plugs. The black deposits
are usually the result of slow speed driving and
short runs where sufficient engine operating temp­
erature is seldom reached. Worn piston rings,
faulty ignition, over-rich fuel mixture and spark
plugs which are too "cold” will also result in car­
bon deposits. Red, brown, or yellow oxide deposits,
a consequence of the combustion of leaded fuel,
usually result in spark plug failure under severe
operating conditions.
The oxides have no adverse affect on plug
operation as long as they remain in a powdery
state. But, under high speed or hard pull, the pow­
der oxide deposits melt and form a heavy glaze
coating on the insulator which, when hot, acts as
a good electrical conductor, allowing current to
follow the deposits and short out the plug.
Excessive gap wear on plugs of low mileage
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, electode wear may be the result of plug
overheating, caused by combustion gases leaking
past the threads and gaskets, due to insufficient
compression 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 excess­
ively high operating temperature.
Broken insulators are usually the result of
improper installation or carelessness. Broken
upper insulators usually result from a poor fitting
wrench or an outside blow. The cracked insulator
may not make itself 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
carelessness 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
extremely 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 F O U L IN G 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. E IT H E R C O N D IT IO N W I L L S E R ­
IO U S L Y A F F E C T T H E P E R F O R M A N C E O F T H E
EN G IN E .
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.
A B N O R M A L O P E R A T IO N
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
numbers lower (a "cooler" type) will generally be
found satisfactory.
SPARK PLU G R E M O V A L
IM P O R T A N T : Before removing any sparkplug,
blow all dirt out of plug sockets in cylinder head
with compressed air.
1. Pull wires off spark plug terminals, using
caution to avoid damaging wire terminals. Remove
wires by firmly grasping large end of boot.
2. Use special spark plug wrench socket No.
S-9704B (manufactured by Snap-On Tool Co.) (fig.
7) and unscrew plugs from cylinder head. Ordin­
ary wrenches may damage porcelain. If gaskets do
not remain on plugs, remove from cylinder head.
Figure 7— Wrench Socket Installed on Plug (Typical)
CHEVROLET SERIES 70-80 H EAVY DUTY TRUCK SH O P M A N U A L