Evaporative Emissions System: Description and Operation

Evaporative Emissions Control System Schematic:

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

PURGE SOLENOID
This normally closed solenoid is remotely mounted in the canister purge line. The ECM controls this solenoid which opens when the engine reaches operating temperature.

THERMAL BOWL VENT VALVE
The TBVV is located in the section of the hose that connects the carburetor bowl vent fitting to the bowl vent valve. It will close and not allow vapor movement at 32°C (90°F) and below. The TBVV will open at 49° (120°F) to allow vapor flow to the canister control valve.

Vapor Canister:

VAPOR CANISTER
This is a basic two tube canister. Gasoline vapors from the fuel tank flow into the tube labeled "Fuel Tank". 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. A timed vacuum source is applied to the tube labeled "Canister Purge" to draw in fresh air through the bottom of the canister (open bottom). The air mixes with the vapor and is drawn into the intake manifold to be burned.

Canister control Valve or Bowl Vent Valve (Type 1):

CANISTER CONTROL VALVE
This valve serves two purposes, one is to perform as a basic purge for the canister, the other is two act as a Vapor Vent Valve. It is connected through the Canister Control Valve to the carburetor bowl. Vapors are vented from the bowl through the Control Valve to the canister during periods when the engine is OFF. With the engine running, manifold vacuum from the PCV system pulls the diaphragm upward to shut off venting of the float bowl.