Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

The evaporative control system consists of a charcoal canister, a vapor purge control system, a carburetor vapor control system, and a fuel tank vapor control system. The evaporative control system minimizes the amount of fuel vapor escaping into the atmosphere.

CHARCOAL CANISTER:
The charcoal canister is a container for the temporary storage of fuel vapor until is can be purged from the canister into the engine and burned. The canister is filled with activated charcoal which absorbs the fuel vapors until they are drawn in by the engine.

VAPOR PURGE CONTROL SYSTEM:
The vapor purge control system determines when the stored fuel vapors are released to the engine. When the coolant temperature is above 63°C (145°F) and the intake air temperature is below 65°C (149°F), manifold vacuum (Federal models use ported vacuum) is directed to the purge control valve, which allows venturi vacuum to draw fresh air through the bottom of the canister into the engine, purging the stored fuel vapor.

CARBURETOR VAPOR CONTROL SYSTEM:
The air cut-off solenoid valve regulates air flow to the carburetor float bowls. When the engine is not running, the outer air vent passage opens so that fuel vapors in the float bowls can be vented into the charcoal canister. When the engine speed is above 400 rpm, the air vent cut-off solenoid valve opens the inner air vent passage so that fuel vapors in the float bowls can be vented into the air cleaner.
There is also an inner vent solenoid valve to control air flow to the carburetor float bowls. When the engine speed is above 400 rpm, the inner vent solenoid valve normally opens the sub-inner air vent passage, but will only allow fuel vapor to be vented to the air cleaner.
When the engine is started hot, (coolant temp above 80°C/176°F), the opening of the inner air vent passage is delayed momentarily to ease engine staring.

FUEL TANK VAPOR CONTROL SYSTEM:
The fuel tank vapor control system consists of a fuel cut-off valve, liquid/vapor separator, a two-way valve, and fuel filler cap. All fuel vapor inside the fuel tank is directed to the charcoal canister through the fuel cut-off valve and the liquid/vapor separator to prevent build up of vapor in the tank which might otherwise escape when the fuel filler cap is removed.
Normal pressures in the fuel tank are regulated by the two-way valve. A secondary relief valve for pressure or vacuum is incorporated in the fuel filler cap in the event the two-way valve fails.