Evaporative Emissions System: Description and Operation

Fig. 63 Evaporative Emissions Control Wiring And Vacuum Hose Diagram:

The evaporative control system consists of a charcoal cannister, a vapor purge control system, a carburetor vapor control system, and a fuel tank vapor control system.
The evaporative emission 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 it can be purged from the canister into the engine and burned.

Vapor Purge Control System
The vapor purge control system controls when the canister is to be purged. When the coolant temperature is above 63°C (145°F) and the intake air temperature is below 65°C (149°F), manifold 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 fuel vapor from the canister.

Carburetor Vapor Control System
1. The air vent 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 vapor in the float bowls can be vented into the charcoal canister. When the engine speed is above 400 rpm, the air-vent solenoid valve opens the inner air vent passage, so that fuel vapor in the float bowls can be vented into the air cleaner.
When the engine is hot started, with the coolant temperature above 80°C (176°F), the opening of the inner air vent passage is delayed momentarily to ease engine start up.
2. 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 vent passage, but will only allow fuel vapor to be vented to the air cleaner.

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 seperator to prevent the buildup of vapor in the tank which might otherwise when the fuel filler cap is removed.
The two-way valve regulates both pressure and vacuum in the tank.
In addition there is a relief valve in the filler cap in case there is an excessive build-up of pressure or vacuum.