Evaporative Emissions System: Description and Operation
SYSTEM DESCRIPTIONThese models use a manifold purge system. The charcoal canister media has been changed with a more efficient carbon, and a purge control valve situated atop the charcoal canister controls the evaporative system.
The purge signal (EGR port, spark port or manifold vacuum), actuates the purge valve to allow purging of the canister through the purge line. When the engine is off, the purge valve directs fuel vapors from the fuel tank and carburetor bowl to the canister. An exception to this would be when the engine compartment is below the temperature where sufficient gasoline vaporization occurs. During this time the thermal vent valve (on some models) closes, stopping flow in either direction in the bowl vent line.
The purpose of the thermal bowl vent valve is to prevent fuel tank vapors from migrating up the bowl vent line and out the internal vent of the carburetor when the fuel bowl is not vaporizing. When the fuel bowl is vaporizing, the thermal valve is open, allowing flow to the canister. Also, the internal fuel bowl vent valve must be open (at idle) and/or the solenoid vent valve must be open (ignition off) to allow flow into the cannister.
Fig. 53 Charcoal canister w/purge control valve:
Carbon Canister
The carbon canister, Fig. 54, is a carryover design of the one used in previous years, except that routing of the vapors has been change. The carbon has been improved to handle the increased demands which the system has placed on it. The vapors in previous years were fed to the carbon bed from the bottom up. On 1978, the vapors enter the top of the purge side of the canister and travel in a "U" shape pattern to the fresh air vent side.
On vehicles equipped with a dual canister system, two canisters are linked together in which the secondary canister is used for "spill over" fuel vapors. This system is called a "series purge". In a "split purge" system the primary canister collects carburetor bowl vent vapors, and the secondary canister collects fuel tank vapors.
Fig. 56 Purge control valves:
Purge Control Valve
The valve, Fig. 56, is installed on the carbon canister and controls the flow of fuel vapors during various engine operating modes. The control is provided by a vacuum signal from either the spark port, EGR port or intake manifold and opens or closes the valve accordingly.
When the engine is off, the vapors from the fuel tank and carburetor fuel bowl are routed through the purge control valve and into the carbon canister for storage.
During normal cruise conditions, spark port or EGR vacuum is strong enough to open the orifice in the purge control valve to allow fuel vapors to flow from the carburetor canister through the purge line to a connection in the PCV tube or into the carburetor spacer. At the same time, the vapors from the fuel tank are also directed into the purge line.
At idle and low speed cruise conditions, spark port or EGR port vacuum is not strong enough to open the orifice in the purge control valve so that the fuel vapors are then routed to the carbon canister.
NOTE: On some vehicles fuel vapors are not purged during low engine speeds because the additional fuel vapors will affect the fuel air mixture, resulting in a reduction of idle quality and an increase in exhaust emissions. On vehicles not affected by this purging, manifold vacuum is used to actuate the purge control valve and control the purging of fuel vapors since spark port or EGR port vacuum is too weak.
Fig. 57 Solenoid vent valve:
Purge Regulator Valve
On some vehicles the rate of purge flow must be closely controlled to prevent poor driveability caused by high purge flow. A purge regulator valve, Fig. 57, is installed in the purge line between the purge control valve and vacuum source to control the amount of air being drawn into the intake manifold through the canister. The valve must be mounted in an upright position.
The purge regulator valve contains a plunger and spring which functions in the same manner as a PCV valve. High vacuum draws the plunger up against the calibrated spring, closing the orifice to control the air flow. When vacuum drops, the spring pushes the plunger down to reduce air flow restriction.
Fig.57 - Air Cleaner Vapor Dam:
Air Cleaner Vapor Dam
A vapor dam, Fig. 57. is installed in the air cleaner at the zip tube opening. It traps most of the fuel vapors emitted by the carburetor while the vehicle is not in operation. The heavier than air vapors lie in the bottom of the air cleaner tray and are purged when the engine is started.
Solenoid Vent Valve
The purpose of this valve, Fig. 57, is to close off the fuel bowl vent line when the engine is operating. It is a normally open valve located in the fuel bowl vent to canister line. This valve is on carburetors which do not have a built-in fuel bowl vent valve.
When the ignition switch is turned on, the coil energizes and the plunger is pulled against the valve seat to the closed position to prevent purge vacuum from reaching the carburetor fuel bowl and upsetting the balanced air pressure.
When the ignition switch is turned off, the coil de-energizes and spring pressure unseats the plunger to allow fuel vapors to flow to the carbon canister where they are stored until purged when the engine is started.
NOTE: If vacuum from the purge control valve does reach the fuel bowl vent line, the resultant low air pressure in the bowl will cause a lean fuel mixture condition. When diagnosing a driveability problem associated with a lean fuel mixture, check the solenoid vent valve and/or the built-in fuel bowl vent. The vent valves must be closed when the engine is operating.
Fig. 59 Thermal vent valve:
Thermal Vent Valve
The valve, Fig. 59, is used on some vehicles and is located in the fuel bowl vent to canister line. This valve is normally closed.
When underhood temperatures are low, the bi-metal contracts to close the valve. This prevents fuel tank vapors from venting through the carburetor fuel bowl during periods of fuel tank heat build-up.
When underhood temperatures are high, the bi-metal expands to open the valve and allow fuel bowl vapors to flow through the thermal vent valve. Also, the engine must be turned off so that the solenoid vent valve is open to allow this passage of fuel vapors through the thermal vent valve.
PVS Valve
The PVS (ported vacuum switch) valve allows vacuum to open the purge control valve as the engine warms up. The purge control valve closes as soon as the engine is turned off and vacuum drops off.
The evaporative emission system may be equipped with either a 2 or 4 port PVS. The 4 port PVS valve is actually two vacuum valves in one, and performs the same function as two 2 port valves.
Fig. 44 Auxiliary fuel bowl vent tube:
Auxiliary Fuel Bowl Vent Tube
Used on some vehicles, this auxiliary vent tube, Fig. 60, is teed into the primary fuel bowl vent tube to vent the fuel bowl when the internal vent is closed (external vent is closed) and the solenoid or thermal vent valves are closed. This tube is vented to the air cleaner.
Thermactor Idle Vent Valve--TIV:
Thermactor Idle Vacuum Valve (TIV)
This valve, Fig. 58, is used on some vehicles to improve hot idle. When the engine is at normal operating temperature, the system is activated and purge air is drawn from the canister through the PCV line and into the induction system through the carburetor spacer. The purge air flow into the carburetor is taken into consideration when the idle air-fuel mixture is calibrated and set.
However, under warm start conditions the purge control valve is closed and the choke is off which creates an overly rich idle mixture condition, resulting in poor idle. This condition continues only for a few seconds after a warm engine start because actuation of the purge control valve is delayed by the retard delay valve and by the time required to evacuate the vacuum reservoir.
To overcome this condition, the TIV valve is used as an air bleed valve. The TIV valve bleeds air into the PCV line to lean out the idle fuel mixture until the purge control valve opens. The same vacuum that opens the purge control valve closes the TIV valve.
Fig. 47 Fuel tank vapor valve:
Fuel Tank Vapor Valve
All fuel tank vapor valves, Fig. 55, use a small orifice which allows only vapor and not fuel to pass into the line going to the vapor storage canister. This assembly is mounted on the fuel tank. Fuel vapors trapped in the sealed fuel tank are vented through the vapor valve assembly.
The vapors leave the valve assembly through a single vapor line and continue to the charcoal canister for storage, until they are purged into the engine. On vehicles equipped with fuel/vapor return lines, vapors created in the fuel line is continuously vented back to the fuel tank. This prevents engine surging from fuel enrichment.
Fig. 44 Auxiliary fuel bowl vent tube:
Auxiliary Fuel Bowl Vent Tube
Used on some vehicles, this auxiliary vent tube, Fig. 54, is teed into the primary fuel bowl vent tube to vent the fuel bowl when the internal vent is closed (external vent is closed) and the solenoid or thermal vent valves are closed. This tube is vented to the air cleaner.
In Tank Venting
This system provides a vapor space above the gasoline surface in the fuel tank. This area is enough to permit breathing space for the tank vapor valve assembly under all static and most dynamic conditions. Horizontally mounted tanks accomplish this by a raised mounting section for the valve assembly which is centrally located on the upper surface of the tank. Vertically mounted tanks use a centrally mounted vapor valve assembly on the uppermost surface of the fuel tank.