Pontiac FIREBIRD 1972 Service Manual page 756

EMISSION CON TROL SYSTEMS 6D- 13
Following is a breakdown of Evaporation control System
components, giving the functional description of each:
1. Fuel Tank
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An evaporation control system (E.C.S.)
which requires special features is included on all
models. This is a closed system, therefore, no tank
vents to the atmosphere are used. Fuel vapor nor-
mally escapes only through the evaporation control
system. This also means that the only way for air to
enter the tank as fuel is used by the engine is through
a vacuum valve in the gas cap and by reverse flow
through the canister at lower rates. Sedan and coupe
tanks have a fuel limiter inside. This provides room
for fuel expansion and prevents liquid fuel from being
forced through the system. The sedan and coupe tank
have three vents, one at each front corner and one at
the rear top center which exists at the front of the
tank. This three vent system always leaves one vent
open no matter on what angle hill the vehicle is
parked.
Liquid- Vapor Separator
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An external steel liquid-
vapor separator (standpipe design) is required with
all sedan type fuel tanks (Fig. 6D-19). Its purpose is
to stop fuel that has drained out of the tank through
the front vents when the car is parked heading down-
hill. This is accomplished by four vertical tubes open
at the upper end within a common chamber of the
standpipe.
If liquid fuel flows into the standpipe tubes from
either front corner tank vent when the car points
downhill, fuel will enter one or both of the tubes
mentioned above, and seek the new level of the fuel
in the tank. It will remain in the tubes until a normal
angle is resumed, then the fuel will drain back into
the tank.
The rear tank vent (Fig. 6D-19) will be submerged
when the car is pointed uphill, although no fuel will
pass into the standpipe in this attitude. The standpipe
tube connecting to the rear tank vent will act as a
return line from the common chamber of the stand
pipe if fuel overflows the corner vent tubes into the
common chamber (two small holes are provided at
the bottom of the rear vent tube in the common
chamber). This draining back to the tank occurs after
the car resumes level.
3. Canister
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The plastic canister is filled with charcoal
which adsorbs and stores fuel vapors (Fig. 6D-19).
When fuel is drawn from the tank during engine
operation, a fuel cap tank relief valve opens allowing
air to be drawn into the tank. When the engine is
running, air is drawn in through the bottom of the
canister. This air picks up vapors which are being
held in the charcoal and carries them through the
carburetor into the engine where they are burned.
The purge valve (three tube-6 cyl. and 307 V-8) canis-
ter operates basically the same as the two tube type.
However, a purge valve is added which is an integral
part of the canister. The purge valve controls the flow
of vapor from the carlister to the carburetor or intake
manifold.
The valve consists of a body (integral with the canis-
ter), a spring loaded diaphragm, a diaphragm cover
and metered purge restrictions.
The operation of the valve is such that it limits the
flow of vapor to the carburetor or manifold at idle but
allows maximum vapor purge during higher carbu-
retor air flows. This action is accomplished through
use of a vacuum signal from the carburetor spark port
which unseats the spring loaded diaphragm.
With this feature, a minimum amount of canister
purge can be maintained at idle because of the smaller
constant bleed restriction. At higher air flows where
more fuel vapors can be tolerated, the spark port in
the carburetor is uncovered and vacuum is applied to
the purge valve diaphragm. This lifts the diaphragm
off its seat and allows additional vapors to be pulled
through the larger restriction, thereby, completely
purging the vapor canister.
4 . Purge Valve(al1 except L-6 and 307 V-8)
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The purge
valve is located on the intake manifold next to the
water outlet housing. Its purpose is to prevent purge
of the canister until the engine water temperature
reaches 170°F. This is to improve exhaust emissions
during cold starts.
Carburetor
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Several modifications have been made
to the carburetor for the evaporation control system.
Vapor and idle vents are removed to seal the carbu-
retor. A purge port is provided to draw vapors from
the canister, and operates whenever the engine is run-
ning to handle any vapors previously stored in the
charcoal canister. With the engine running, vapor
from the bowl vents internally through a standpipe
into the air cleaner where it is drawn through the
carburetor into the engine. This standpipe also vents
bowl vapors to the air cleaner with the engine off,
allowing the air cleaner to store the vapors until the
engine is started.
6. Lines
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Conventional steel tubing, vapor resistant
rubber hose and hose clamps are used to connect the
various components of the system. It is extremely
important that all pipes and hoses are not kinked, are
properly connected, and that all connections are
tight. This 1s necessary to assure a vent through the
system for the fuel tank to prevent restriction of va-
pors to the canister. Thorough visual inspection is
and will remain one of the most important checks of
the system.
IMPORTANT: Use only hose marked "EVAP"
or "GM 6 1 0 7 M i f necessary to replace an
evaporation control system (ECS) hose. Gaso-
line vapors w i l l deteriorate other types of hose.