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 vapor passes through a vent line to the charcoal canister where it is stored. A second vent line, with control valve (if equipped), connects canister to air cleaner and intake system.
When engine is running, intake vacuum draws fresh air through charcoal canister. This fresh air mixes with fuel vapor and is drawn into intake system, where it enters combustion chamber to be burned.
On all models except 911 Carrera and 911 Turbo, the thermo valve, switching valve and control valve are connected in series to control amount of vapor purge.
The thermo valve opens at coolant temperatures greater than 136°F (58°C) to apply vacuum to the switching valve. The switching valve receives vacuum when coolant temperature is greater than 136°F (58°C) and when throttle valve angle is at least one degree.
The control valve receives vacuum from a vacuum line that is connected to a port below the throttle valve. This allows the control valve to vary amount of purging depending upon throttle angle.