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 activated charcoal. A second vent line with control valves (if equipped) connects canister to air cleaner and intake system.
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 combustion chambers and is burned.
On 944 and 944 Turbo models, control valves connected 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 136°F (58°C), and is closed at full throttle and idle, or when coolant temperature is below 136°F (58°C). Control valve 2 is slightly open at light throttle, and fully open at wide open throttle.
On 928S models, 3 control valves in series connect intake manifold vacuum to purge line. The temperature valve opens when engine temperature reaches 136°F (58°C) allowing a vacuum signal to operate control valve. This vacuum signal is taken from a port immediately before throttle and allows control valve to start opening as throttle opens at least 1°.
A load sensitive shift valve receives a vacuum signal from a port located after the throttle and varies purge rate with engine load. Purge rate increases to maximum at wide open throttle.