Evaporative Emissions System: Description and Operation

Rollover/Gravity Valve (typical):

Fig. 40 Evaporative Emissions Control (EEC) System, 16V:

Fuel vapors from the fuel tank are passed through a rollover (gravity) valve, through a vent line to the charcoal canister, Fig. 40. The vapors are stored there when the engine is not running. The system used on this engine uses two solenoid valves to control the flow of fuel vapors from the charcoal canister.

Fig. 41 Evaporative Emissions Control (EEC) System Component Locations, 16V:

An ON/OFF solenoid valve is located below the intake air boot, Fig. 41. This valve is open when the engine is running to allow fuel vapors to be purged from the charcoal canister. The valve is closed when the engine is not running.
A frequency valve is located below the intake air boot and next to the ON/OFF solenoid. The frequency valve regulates the flow of vapors. The valve is supplied cycled DC voltage from the CIS-E Motronic control unit. The duty cycle supplied to the valve is determined by engine speed, engine load and engine temperature.
The frequency valve operates for 120 seconds and shuts off for 60 seconds. This cycle repeats as long as the engine is running and the coolant temperature is above a preset value.
Both the ON/OFF solenoid and the frequency valve are operated by the CIS-E Motronic control unit.

Fig. 42 Evaporative Emissions Control (EEC) System Frequency Valve, 16V:

When checking or adjusting the fuel mixture, make sure that the hose from the charcoal canister to the frequency valve is removed, Fig. 42. This will allow the system to draw fresh air.