Evaporative Emissions System: Description and Operation

Fuel EVAP. System Diagram:

PURPOSE
The Evaporative Emission Control (EVAP) system is intended to capture and store all fuel vapors from the fuel system and introduce then into the intake system to be burned by the engine. It is computer controlled and monitored to prevent the system from causing driveablity problems and to identify failures with in the system.

OPERATION
The fuel tank is sealed and venting is plumbed into the Evaporative Emission Control Canister where the vapors are stored until the engine is operating according to the parameters stored in the ECM memory, at which time the vapors are released into the engine. The ECM controls the release of the vapors by cycling the Canister Purge Regulator Valve which controls the flow of the fuel vapors to the intake. The duty cycle of the Canister Purge Regulator Valve is varied by the ECM to fit the current operating conditions. An electrical failure will be detected by the ECM and a Diagnostic Trouble Code will be stored.

SYSTEM COMPONENTS

Evaporative Emissions System Layout:

Evaporative Emission Control Canister
The Evaporative Emission Control (EVAP) 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.

Roll Over Valve
The roll-over (vent) valve is integral with fuel level sender, located on top of the fuel tank the valve stops any fuel from leaving the fuel tank in the event of a roll-over.

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.

Canister Purge Regulator Valve
The Canister Purge Regulator Valve (frequency valve or EVAP On/Off valve) is operated by the Engine 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 Canister Purge Regulator 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.