DESCRIPTION
The Evaporative Emission Control System (EECS), prevents raw fuel vapors from entering the atmosphere. It consists of a venting system which allows only vaporous fuel to be drawn into the system.
During engine operation, vapors are drawn through system vent lines and into intake manifold. When engine is off, fuel vapors are stored in vapor storage canister charcoal. Vapors are drawn into intake manifold when engine is running again. (Scheme 173)
Scheme 173
AUXILIARY CANISTER
On models with dual fuel tanks, an auxiliary canister is included with the primary canister to increase canister capacity. Auxiliary canister is connected in-line to purge air inlet of primary canister.
Vapors overflowing from primary canister are stored in auxiliary canister. During purging, excess vapors are routed to intake manifold.
CANISTER CONTROL VALVE
Two types of canister control valve are used. One type acts as fuel bowl vent and canister purge. When engine is not running, spring tension holds valve open allowing venting of float bowl. When engine is running, manifold vacuum pulls plunger up to close valve.
The other type of valve acts as both vapor vent valve and purge valve. When engine is running, manifold vacuum from PCV system pulls lower diaphragm upward. This turns off venting of carburetor float bowl. When engine is operating above idle speed, control vacuum pulls upper diaphragm upward. This allows purging of canister through PCV system.
TWO-TUBE CANISTER
Gasoline vapors from fuel tank flow into canister and are absorbed by carbon. Canister is purged when engine operates above idle speed. A timed vacuum source is applied to canister to draw fresh air through bottom of canister. Fresh air mixes with fuel vapors and are drawn into intake manifold to be burned in engine.
Some models are equipped with a second canister purge hose. This hose is connected to a Canister Control Valve (CCV). The CCV is then connected to carburetor bowl vent line. This allows carburetor bowl to be vented to canister, through CCV, when engine is not running. These vapors are then drawn into intake manifold once engine is started.
FUEL TANK PRESSURE CONTROL VALVE
Fuel tank pressure control valve is a spring controlled valve. When engine is running, vacuum is applied to valve and valve is opened. This allows vapors from fuel tank to vent to canister.
When engine is not running, valve closes. This causes fuel tank vapors to be vented through restriction in valve. This restriction retains most of fuel tank vapors in fuel tank.
PURGE SOLENOID VALVE
Solenoid valve is controlled by Electronic Control Module (ECM). When energized, purge solenoid valve shuts off, which causes vapors to be drawn from canister through restriction and allowing high purge.
For high purge to occur the fuel control system must be operating in closed loop, a time of 3 minutes must pass after starting engine, and engine must be off idle.
FUEL BOWL VENT SOLENOID VALVE
Fuel bowl vent solenoid valve is a grounded solenoid energized by ignition voltage. Solenoid valve is off when solenoid is energized and open when de-energized.
THERMAL BOWL VENT VALVE
Thermal bowl vent valve, located in a section of hose from canister to carburetor, permits fuel bowl vapors to be directed to carbon-canister. Valve will close when engine temperature is below 90°F (32°C), and will open when engine temperature is above 120°F (49°C).
MAINTENANCE
No adjustments are required with this system. Replace air inlet filter (if equipped) in bottom of charcoal canister every 30,000 miles. Regular inspection should be made and defective components replaced as necessary.
VAPOR VENT CONTROL VALVE
- Attach a short piece of hose to carburetor bowl vapor tube of canister. Blow through hose to determine if air will pass vapor vent valve into canister. If not, valve is defective.
- Using a vacuum pump, apply 15 in. Hg to vacuum signal tube in diaphragm assembly cover. If diaphragm does not hold vacuum for at least 20 seconds, diaphragm is leaking and canister must be replaced.
- With vacuum still applied, again attempt to blow into the carburetor bowl vapor tube of the canister. If valve is sealing properly, air should pass vapor vent valve into canister. If air does not enter canister past vapor vent valve, valve is not functioning properly and canister must be replaced.
TANK PRESSURE CONTROL VALVE
Install a short piece of hose on valve inlet tube (fuel tank side) and blow into hose. You should feel diaphragm pop open and air should pass through valve. If valve does not open, it should be replaced.