Fuel Evaporation System: Operation
Expanded fuel, caused by high ambient temperatures, is collected in the expansion tank. This fuel is returned to main tank by venting action as fuel is used from main tank.
Fuel vapors produced in main tank pass through a vent line to a carbon canister where they are stored in the activated charcoal in the canister. A second vent line connects canister to air cleaner. When engine is running, intake vacuum draws fresh air through carbon canister. This fresh air mixes with fuel vapors and is drawn into intake system where it enters the combustion chamber and is burned.
This action purges the activated charcoal and renews its storage capacity. On air-cooled models fresh air is blown through charcoal canister from blower housing to aid in purging action.
On 944 models, control valves hooked in series regulate canister purging. Control valve 1 is an on/off valve controlled by throttle vacuum and temperature switch. Control valve 2 is a variable valve which regulates amount of canister purging according to intake manifold vacuum. Control valve 1 opens at part throttle with coolant temperatures above 130°F (58°C), and is closed at full throttle and idle, or when coolant temperature is below 130°F (58°C). Control valve 2 is slightly open at light throttle, and fully open at full throttle.