Pontiac FIREBIRD 1972 Service Manual page 797

ENGINE ELECTRICAL 6E-35
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insulator just above the shell This ring is some-
times mistakenly regarded as evidence that
combustion gases have blown out between shell
and insulator.
All AC spark plugs have a type number on the insulator
which designates thread size as w l l as relative position of
the plug in the Heat Range. Type numbers starting with
4 are 14 mm. thread size.
The last digit of the type number indicates the Heat Range
position of the plug in the AC Heat Range System. Read
these numbers as you would a thermometer-the higher the
last digit, the hotter the plug will operate in engine; the
lower the last digit the cooler the plug.
INSPECT
Use of leaded fuels results in lead deposits on spark plugs
and can cause misfiring at mileages less than 12,000 miles.
Spark plugs must be replaced at 6,000 miles when operating
with leaded fuels. Where misfiring occurs prior to 6,000
miles, spark plugs in good condition can often be cleaned,
tested and reinstalled in an engine with acceptable results.
TAPERED
SEAT
Worn or dirty plugs may give satisfactory operation at
idling speed, but under operating conditions they fre-
quently fail. Faulty plugs are indicated in a number of
ways, such as: poor fuel economy, power loss, loss of
speed, hard starting and general poor engine performance.
Spark plug failure, in addition to normal wear, may be due
to carbon or lead fouled plugs, excessive gap or broken
insulator.
Fouled plugs may be indicated by checking for black car-
bon 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 suffi-
cient engine operating temperature is seldom reached.
Worn pistons, rings, faulty ignition, over-rich carburetion
and spark plugs which are too cold will also result in
carbon deposits. Red, brown, etc., oxide deposits, a conse-
quence of the use of leaded fuel, usually result in spark
plug failure under severe operating conditions. The oxides
have no adverse effect on plug operation as long as they
remain in a powdery state. But, under high speed or hard
pull, the powder 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 indi-
cates the engine is operating at high 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, cuased by combusiton
gases leaking past the threads and gasket, due to insufi-
cient compression of the spark plug gasket (V-8 only), dirt
under the gasket seat, or use of old gaskets. Excessively
lean carburetion will also result in excessive electrode
wear.
Broken insulators are usually the result of improper instal-
lation or carelessness when regapping the plug. 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 mois-
ture penetrates the fracture. The fracture is usually just
below the crimped part of shell and may not be visible.
Broken lower insulators usually result from carelessness
when regapping and generally are visible. In fairly rare
instances, this type of break may result from the plug
operating too "hot" such as encountered in sustained peri-
ods of high-speed operation or under extremely heavy
'loads. When regapping a spark plug, to avoid lower insula-
tor breakage, always make the gap adjustment by bending
the ground (side) electrode. Spark plugs with broken in-
sulators should always be replaced.
Fig. 6E-48 Spark Plug