Evaporative Emissions System: Description and Operation

Fig. 5 Fuel evaporation control system:

This system, Fig. 5, reduces the emission of fuel vapors to the atmosphere. This is accomplished by the vent valve unit maintaining an increased pressure in the fuel tank.
As the fuel cools down, a vacuum is created in the fuel tank which opens the vent valve. When the vent valve opens, air or fuel vapors are drawn into the fuel tank from the charcoal canister to replace the decreasing fuel volume. When vacuum equalizes, the vent valve closes.
The charcoal canister is connected to the throttle valve housing by a line containing the purge valve. When the engine is running with throttle valve slightly open and coolant temperature is approximately 122 degrees F, intake manifold vacuum flows through the thermo valve to the purge valve, forcing diaphragm in purge valve to open. This allows flow from charcoal canister to throttle valve housing. As the throttle valve opens wider, the two purge bores in the throttle valve housing are progressively exposed to venturi vacuum. This allows purging to be metered at partial load so driveability is not affected.




TESTING
1. Disconnect purge line between charcoal canister and throttle valve housing at charcoal canister.
2. Block hose with finger and slowly increase engine speed to 2000 RPM. There should be no vacuum at idle but vacuum should increase as RPM increases.
3. If vacuum does not increase, check purge connection at throttle valve housing for blockage and check hose for leaks, correcting as necessary.
4. If vacuum is still not available, disconnect white/purple/black vacuum line from purge valve and accelerate engine to approximately 2000 RPM.
5. If there is vacuum at line, replace purge valve.
6. If there is no vacuum at line, replace thermo valve.