Cars produced for California after 1969, were equipped with fuel system Evaporative Emission Controls. For 1971, the system was modified somewhat and used on all models.
Changes in atmospheric temperature cause fuel tanks to breathe. Air within the tank expands and contracts according to how the outside air temperature is changing. As the temperature rises, air escapes through the tank vent tube or the vent in the tank cap. The air which escapes contains gasoline vapors. In a similar manner, the gasoline which fills the carburetor float bowl expands when the engine is stopped. Engine heat causes this expansion. The vapors escape through the carburetor and air cleaner.
The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. The system has three parts: a fill control vent system; a vapor vent and storage system; a pressure and vacuum relief system (special fill cap).
The fill control vent system is a modification to the fuel tank design. It consists of an air space within the tank which is 10-12 percent of the volume of the tank, when the tank is filled to capacity. The air space is sufficient to provide for the thermal expansion of the fuel.
The in-tank vent system consists of the air space previously described, plus a vapor separator assembly. This separator assembly is mounted to the top of the fuel tank, and is secured by a cam-lock ring similar to the one which secures the fuel sending unit. Foam material fills the vapor separator assembly. The foam material serves the purpose of separating raw fuel and vapors, thus retarding the entrance of raw fuel into the vapor line.
The sealed filler cap features a pressure-vacuum relief valve. Under normal operating conditions, the filler cap operates as a check valve, allowing air to enter the tank to replace the fuel which is consumed. At the same time, it works to prevent vapors from escaping through the cap. In the event of excessive pressure within the tank, the filler cap valve will open to relieve the pressure.
Because the filler cap is sealed, fuel vapors have but one place through which they may escape-the vapor separator assembly at the top of the fuel tank. The vapors pass through the foam material and continue through a single vapor line which leads to a canister in the engine compartment. The canister is filled with activated charcoal.
Another vapor line runs from the top of the carburetor float chamber to the charcoal canister.
As the fuel vapors (hydrocarbon molecules) enter the charcoal canister, they are absorbed by the charcoal. The air in which the hydrocarbon molecules were suspended is dispelled through the open bottom of the charcoal canister, leaving the molecules trapped within the charcoal. When the engine is started, the operation of the carburetor produces a vacuum within the line which runs from the float chamber to the charcoal canister. The vacuum causes fresh air to be drawn into the canister from its open bottom. The fresh air passes through the charcoal, picking up the hydrocarbon molecules which are trapped there, feeding them into the carburetor. From the carburetor, they proceed into the intake manifold, where they are fed into the engine and burned with the fuel mixture.