Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

The evaporative emission control system used on this vehicle is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank and the carburetor to an activated carbon storage canister to retain the vapors when the vehicle is not operating. When the engine is running, and coolant temperature is above a predetermined level, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.




CHARCOAL CANISTER

Gasoline vapors from the fuel tank and the carburetor flow into the tubes labeled "Fuel Tank" and "Carb" respectively. Any liquid fuel goes into a reservoir in the bottom of the canister to protect the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running above idle speed. Ambient air is allowed into the canister through the air tube in the top. The air mixes with the vapor and is drawn into the intake manifold.



CANISTER CONTROL VALVE

The canister control valve is located on the top of the canister. Manifold vacuum opens this valve to allow purge of both the fuel tank and canister at the same time. The purge line is connected to a ported vacuum source on the throttle body. Purge occurs only when the throttle is above a certain opening, thus preventing purge operation under conditions of low ported vacuum, such as deceleration.


PRESSURE-VACUUM FILLER CAP

The fuel tank is sealed with a specially engineered pressure vacuum relief filler cap. The relief valves in the cap are a safety feature, and operate only to prevent excessive pressure or vacuum in the tank caused by malfunction in the system or damage to the vent lines.



ROLLOVER/VAPOR SEPARATOR VALVE

This valve provides a path for fuel vapors to travel from the fuel tank to the canister and provides fuel leakage protection when a vehicle rollover occurs. This valve also prevents raw fuel from entering the canister.




[1][2]BOWL VENT VALVE (4 bbl.)

The [1][2]bowl vent valve is connected between the carburetor fuel bowl, charcoal canister, and air pump outlet. When the engine is not operating, there is a direct connection between the fuel bowl and canister. When the engine is operating, air pressure closes the connection between the fuel bowl and canister. When the engine is switched off, air pressure in the valve bleeds off and the fuel bowl is allowed to vent into the canister.

Bowl Vent Solenoid:

BOWL VENT SOLENOID (2 bbl.)

The bowl vent solenoid is mounted on the carburetor. When the engine is not operating, there is a direct connection between the fuel bowl and canister. When the engine is operating the solenoid closes the connection between the fuel bowl and canister.

CVSCC Valve:

COOLANT VACUUM SWITCH COLD CLOSED (CVSCC)

This switch blocks manifold vacuum to the canister control valve until coolant temperature reaches a 98°F (36°C). After the coolant temperature reaches this level, the valve opens and vacuum is allowed to control canister purging.

Vapor Containing Air Cleaner Assembly (Some 4Bbl):

VAPOR CONTAINING AIR CLEANER ASSEMBLY

This system is used on Federal and Canadian 4 bbl. models. A trap door in the air cleaner snorkel is designed to control HC emissions when the engine is turned off. The trap door is controlled by an electric solenoid which is energized when the ignition switch is turned on. The solenoid will open the door about 20 degrees, and because the door is hinged at the top, it will open further when the engine is started and the air flow through the snorkel increases.