Evaporative Emissions System: Description and Operation
EVAPORATIVE SYSTEM OPERATION
The evaporative emission system used on this vehicle is of the charcoal canister design. This system is designed to prevent gasoline vapor emissions, from the tank or the carburetor, from escaping to the atmosphere. When the vehicle is not running, fuel in the fuel tank evaporates, and the vapors travel through vent hoses or tubes to be stored in the charcoal canister. Once the vehicle is running, these vapors are drawn into the engine and consumed with combustion gases.
The system consists of a Charcoal Canister Purge Solenoid, a Charcoal Canister, an External Bowl Vent Valve, a Pressure Vacuum Filler Cap, a Thermal Bowl Vent Valve and a Vacuum Controlled Orifice Tank Vapor Valve.
CANISTER PURGE SOLENOID
This solenoid is controlled by the Spark Control Computer (SCC). The SCC controls this solenoid by providing or removing the ground. When the engine temperature is below a temperature of 90°F the SCC grounds the solenoid energizing it. Once the Purge Solenoid is grounded, no vacuum is allowed to flow to the charcoal canister control valve. If this valve has no vacuum, the canister cannot vent to the carburetor. When the engine reaches 90°F, the SCC removes the ground from the solenoid which de-energizes it, allowing vacuum to be applied to the charcoal canister control valve. Once vacuum is applied to the valve, the charcoal canister vapors are then purged through the carburetor.
The SCC module de-energizes the Canister Purge Solenoid after these conditions have occurred:
1. Engine temperature is above 90°F.
2. A predetermined time has elapsed (about 100 seconds).
3. Vehicle Speed is above 5 mph.
4. Engine rpm is above 800 rpm.
5. Vacuum is below 30 inches Hg.
6. Carburetor switch is open.
CHARCOAL CANISTER
Gasoline vapors from the fuel tank flow into the tube on the canister 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 at idle and off idle by utilizing a dual source of vacuum. Ambient air is allowed to enter the canister through the air tube in the top. The air mixes with the vapor and is drawn into the intake manifold.
EXTERNAL BOWL VENT VALVE
The external bowl vent valve's purpose is to allow for carburetor fuel bowl vapors to vent to the charcoal canister. This valve is connected to the carburetor fuel bowl, the charcoal canister, and the air injection system (air/switch relief valve). When the engine is not running, there is a direct connection between the fuel bowl and the canister. When the vehicle is running, air pressure from the air switch/relief valve closes the external bowl vent valve, which in turn shuts off the connection to the canister. When the engine is turned off, air pressure in the valve bleeds off and the carburetor bowl is allowed to vent to the canister.
Pressure-Vacuum Filler Cap:
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.
THERMAL BOWL VENT VALVE
The thermal bowl vent valve is located in between the carburetor and the charcoal canister. The purpose of this valve is to improve cold startability in cold ambient temperatures.
The thermal bowl vent valve is designed to remain closed until a specified temperature has been reached. While this valve is closed, fuel vapors are trapped within the carburetor bowl. These vapors will condense to liquid in the bowl which provides more fuel for the next cold start. Once above the specified temperature, the thermal bowl vent valve opens allowing fuel vapor to flow from the carburetor bowl to the charcoal canister.
VACUUM CONTROLLED ORIFICED TANK VAPOR VALVE (VCOTVV)
The purpose of the vacuum controlled orificed tank vapor valve, is to retain fuel vapor within the fuel tank until the fuel tank pressure reaches a calibrated level. This valve is located in between the fuel tank and the charcoal canister in the fuel tank vent line.
If the engine is not running and the fuel tank pressure raises above the specified pressure, the valve will open and allow fuel vapors to vent to the charcoal canister. When the engine is running, manifold vacuum to the valve keeps it in the open position, thus allowing fuel vapor to flow from the fuel tank to the charcoal canister.