Evaporative Emission Control Canister: Description and Operation

Several types of vapor storage canisters are used on General Motors Vehicles.

Fig. 55 Single stage type vapor storage canister:

Single Stage Type Vapor Canister

This canister, Fig. 55, absorbs fuel vapors from the fuel tank through a tube on the canister marked ``Fuel Tank'' and from an area just above the carburetor float bowl through a tube marked ``Carb Bowl.'' The storage of fuel vapors occurs when the engine is not running.
During engine operation, fuel vapors pass through a hose connected to the canister tube marked ``Purge'' and into a port on the carburetor, where they are mixed with carburetor inlet air and burned during the combustion process. Some single stage type vapor canisters are equipped with two rather than three tube ports. However, the principles of operation are identical for both models.

Fig. 56 Purge valve type vapor storage canister:

Purge Valve Type Vapor Canister

This canister, Fig. 56, absorbs fuel vapors from the fuel tank through a tube on the canister marked ``Fuel Tank'' and from the carburetor through a tube marked ``Carb Bowl.'' The fuel vapors enter the canister and are absorbed by activated charcoal when the engine is not running.
The canister is equipped with a purge valve which is an integral part of the vapor canister. The purge valve consists of a housing and tube molded into the canister cover, and a diaphragm, valve assembly, valve spring and diaphragm cover with an integral control vacuum signal tube.
When the engine is idling, spring tension holds the purge valve closed. A small amount of fuel vapor is then drawn from the canister through a calibrated bleed hole which leads from the charcoal bed to the tube marked ``PCV,'' enabling some purge of the canister through the PCV hose while the engine is at idle.
As engine speed increases, the carburetor throttle valve passes by a timed vacuum port and vacuum is supplied through a tube leading to the top of the purge valve diaphragm marked ``Control Vac.'' This vacuum lifts the purge valve off its seat, opening another channel leading to the charcoal bed. This enables more fuel vapors to be combined with the increased air/fuel ratio of the carburetor.
Both purge channels in the canister lead to a common ``PCV'' tube, which is directly connected to the PCV hose from the engine.

Fig. 57 Vent valve type vapor storage canister:

Vent Valve Type Vapor Canister

This canister, Fig. 57, has a vent valve system which is an integral part of the canister assembly. The canister collects fuel vapors from the carburetor float bowl through a hose to the vent valve. The vent valve assembly consists of a housing and tube molded into the canister cover, vent valve, valve spring, diaphragm, and diaphragm cover with integral vacuum signal line.
When the engine is off, no manifold vacuum is available to the vent valve diaphragm, thereby enabling the valve spring to keep the valve open. At this point, fuel vapors in the carburetor float bowl are pressurized and then directed from a tube in the float bowl, through a connecting hose, to the ``Carb Bowl'' tube in the canister cover. The vapors then flow past the open vent valve through the upper canister filter and into the charcoal bed, where they are stored until normal purging of the canister occurs.
When the engine is running, manifold vacuum is applied to the vent valve diaphragm through the signal tube marked ``Man. Vac.'' At this point, the vent valve closes against spring tension and seals the passage venting fuel vapors from the float bowl to the canister.
When the engine is stopped, the spring opens the valve to again allow venting of the float bowl.

Fig. 58 Vent & purge valve type vapor storage canister:

Vent and Purge Valve Type Vapor Canister

This canister, Fig. 58, is equipped with both a vent valve and a canister purge valve. The rate at which fuel vapor is drawn into the engine is controlled by a Thermal Vacuum Switch (TVS), which supplies ported vacuum to the purge valve when engine coolant temperature is above a pre-determined value.
With engine idling at normal operating temperature, the purge valve is forced closed by spring tension and no purging of the canister occurs. On some models, a small amount of fuel vapor is drawn from the canister through a bleed hole, allowing some canister purge through the PCV system at idle.
When the carburetor throttle plates are opened, the signal port located above the throttle plates is uncovered and a ported vacuum signal is applied to the purge valve. Vacuum then overcomes spring tension, opening the valve and allowing vapors to be drawn into the engine for burning.

Fig. 59 Inverted function vapor canister w/encapsulated purge solenoid:

Inverter Function Vapor Canister

In this canister, Fig. 59, gasoline vapors from fuel tank flow into tube labeled ``tank'' and are absorbed into the carbon. Any liquid fuel goes into a reservoir in bottom of canister to prevent damage of the carbon bed above. The canister is purged when engine is running above idle speed. Ambient air is allowed into canister through a tube. This air mixes with vapor which is drawn into the intake manifold.
The ECM operates a solenoid valve which controls vacuum to the purge valve in the charcoal canister.