Evaporative Emissions System: Description and Operation

Evaporative System Component Layout:

Fuel EVAP. System Diagram:

CHARCOAL CANISTER
The charcoal canister collects and stores fuel vapors emitted from the fuel tank when the vehicle is not running. When the engine is running, the stored fuel vapors are sent to the engine via the frequency valve to be burned.

Fuel Supply Layout:

GRAVITY VALVE
The Gravity valve is integral with fuel tank assembly, located on top of the fuel tank filler neck the valve stops any fuel from leaving the fuel tank in the event of a roll-over.

Evaporative Emissions Breather Valve:

BREATHER/VENT VALVE
The breather/vent valve is used to prevent fuel from reaching the charcoal canister when filling the fuel tank, as well as regulate the flow of fuel vapors from the expansion tank to the carbon canister.

FREQUENCY VALVE
The frequency valve, which is located on the right fender, is operated by the Motronic control module. It regulates the flow of fuel vapors from the charcoal canister to the intake manifold. The valve's duty cycle will vary depending on engine temperature, load and speed. Duty cycle operating range:

^ Coolant temperature below 60°C (140°F) valve off, no vacuum, or very little at connections.

^ Coolant temperature above 60°C (140°F) valve must cycle for 120 seconds with very noticeable vacuum to the connection.

NOTE: If the frequency valve is operating properly it will repeat this pattern of 120 seconds ON and 90 seconds OFF for as long as the engine is running.