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.

BOWL VENT VALVE
The bowl vent valve vents vapors from the carburetor bowl area to the canister when the engine is OFF. The bowl vent valve receives vacuum when the engine is running thus seating the valve preventing vapors from venting from the carb bowl. The vapors from the canister may still be purged.

PURGE SOLENOID
This solenoid is remotely mounted and is located in the canister purge line. The ECM controls this solenoid and monitors the air/fuel ratio from the canister. The solenoid will reduce purge when the mixture is rich (low air/fuel ratio).

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 (Type 3):

CANISTER CONTROL VALVE
This valve is located in the canister purge line between the purge solenoid valve and the intake manifold and connected to ported vacuum. Vacuum opens the valve to allow purge. When the engine is OFF the canister control valve prevents vapors from the bowl vent valve from venting to the atmosphere.

Fuel Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE
The fuel tank pressure control valve, located in the engine compartment, is a spring biased diaphragm valve, normally closed. When the vapor pressure in the fuel tank exceeds 6.0 kPa the valve will open allowing the vapors to vent to the canister and then be purged. When the tank pressure drops sufficiently the tank pressure control valve will close, thus keeping the vapors in the fuel tank. The control vacuum tube of the control valve is connected into the canister line to prevent contamination from entering the valve.