Evaporative Emissions System: Description and Operation

EVAP. System Diagram:

Fuel Tank:

This system prevents fuel vapors from entering the atmosphere. The fuel vapors are stored in the charcoal canister, located in the right front fender well, while the engine is OFF. When the engine is started, the combined frequency/on-off valve opens, which allows the vapors to be drawn into the intake manifold to be burned during normal combustion.

PRESSURE REGULATING VALVE
The pressure regulating (holding) valve is used to regulate the flow of fuel vapors from the expansion tank to the carbon canister.

ROLL OVER VALVE
The roll-over (vent) valve is integral with fuel level sender, located on top of the fuel tank.

SHUT-OFF VALVE
Vacuum operated on-off valve, located on the right fender is, combined with the frequency valve. Engine vacuum opens the valve, allowing fuel vapors in the canister to be drawn into the intake manifold through the frequency valve.

FREQUENCY VALVE
The frequency valve, combined with the on-off 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. The valve's duty cycle will vary depending on engine temperature, load and speed. Duty cycle operating range:

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