Pontiac Fiero 1988 Service Manual page 416

IGNITION SYSTEM 6D4-3
an adapter. DO NOT pull on the wire to remove. Pull
on the boot, or use a tool designed for this purpose.
Spark Plugs
Resistor type, tapered seat spark plugs are used
on all engines (except aluminum heads). No gasket is
used on these tapered seat plugs. See Figs. IB and 1C
for an explanation of coding on spark plugs.
PREFIX ANDSUFFIX
L E T T E R S IDENTIFY
A SPECIFIC TYPE
S P A R K P L U G . NUMBERS
R E L A T E TO THREAD
SIZE AND HEAT RANGE
AS FOLLOWS:
R = Resistor
4 = 1 4 m m Thread
6 = Heat R a n g e
T = Taper Seat
S = Extended Tip
TAPERED SEAT
NO GASKET
G20199-6D
Fig. 1B Spark Plug Example
4 - 14 mm 2 =
8 = 18 mm
5 =
10 = 10 mm 6 =
12 = 12 mm 7 =
1st number denotes THREAD SIZE
1 /2-inch taper
1 /2-inch
%-inch
%-inch
2nd number denotes HEAT RANGE
0-1-2-3-4-5-6-7-8-9
COLD HOT
, G20200-6P
Fig 1C Spark Plug Coding
Normal service is assumed to be a mixture of
idling, slow speed, and high speed driving. Occasional
or intermittent high-speed driving is needed for good
spark plug performance. It gives increased combustion
heat, burning away carbon or oxides that have built up
from frequent idling, or continual stop-and-go driving.
Spark plugs are protected by an insulating nipple made
of special heat-resistant material, which covers the
spark plug terminal and extends downward over a
portion of the plug insulator. These nipples prevent
flash-over, which causes engine misfiring. Do not
mistake corona discharge for flash-over, or a shorted
insulator. Corona is a steady blue light appearing
around the insulator, just above the shell crimp. It is
the visible evidence of a high-tension field and has no
effect on ignition performance. Usually it can be
detected only in darkness. This discharge may repel
dust particles, leaving a clear ring on the insulator just
above the shell. This ring is sometimes mistakenly
regarded as evidence that combustion gases have blown
out between shell and insulator.
Ignition Switch
The mechanical switch is located in the steering
column on the right hand side just below the steering
wheel. The electrical switching portion of the assembly
is separate from the key and lock cylinder. However,
both are synchronized and work in conjunction with
each other through the action of the actuator rod
assembly.
For a complete explanation of the key and lock
cylinder, and the actuator rod assembly, see
STEERING, Section 31. See Section 8 for electrical
switching.
DIAGNOSIS
IGNITION SYSTEM
Spark Plugs
Worn or dirty plugs may give satisfactory
operation at idling speed, but at higher RPM they
frequently fail. Faulty plugs are indicated in a number
of ways: poor fuel economy, power loss, loss of speed,
hard starting and generally poor engine performance.
Spark plugs may also fail due to carbon fouling,
excessive gap, or a broken insulator.
Fouled plugs may be indicated by black carbon
deposits. The black deposits are usually the result of
slow-speed driving and short runs, where sufficient
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.
Excessive gap wear, on plugs of low mileage,
usually indicates 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. Electrode
wear may also be the result of plug overheating, caused
by combustion gases leaking past the threads due to
insufficient torquing of the spark plug. Excessively lean
carburetion will also result in accelerated electrode
wear.
Broken insulators are usually the result of
improper installation, 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 show up right away, but will as soon
as oil or moisture penetrates the crack. The crack 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.
This type of break may result from the plug operating
too "hot", which may happen in periods of high-speed
operation or under heavy loads. When regapping a
spark plug, always make the gap adjustment by
bending the ground (side) electrode. Spark plugs with
broken insulators should always be replaced.