Evaporative Emission Control System Advanced EVAPS

Component Location:

99MY Component Location Continued:

Identification:

The system was introduced on all North American specification vehicles from 1998 Model Year. Advanced EVAP vehicles can be recognized by the information contained in the EVAP. FAMILY entry on the underbonnet Emission label (mounted on the bonnet lock platform).

A - Vehicles without advanced EVAPS
VLR1095AYPBD

B - Vehicles with advanced EVAPS
WLRXEO124001

Evaporative emission control system - Advanced EVAPS.
The evaporation emission control system is used to reduce the level of hydrocarbons emitted into the atmosphere from the fuel system. The system comprises a vapor separator (B) and an anti-trickle valve (A), both located on the fuel filler neck (F), an Evaporative Emissions (EVAP) canister and an EVAP canister purge valve. A Canister vent solenoid (CVS) unit is mounted in front of the EVAP canister on vehicles up to 99MY. On vehicles from 99MY the CVS unit is mounted near the bulkhead on the RHS of the engine bay. The CVS unit is used by the ECM to control fresh air supply to the canister.

On NAS vehicles, the fuel pump and gauge sender unit incorporates a pressure sensor which is used by the ECM, in conjunction with the CVS unit, to determine the presence of leaks which may cause vapor to escape. This system is added for compliance with OBD measures.







During conditions of high ambient temperatures, fuel in the tank vaporizes, and pressure rises. Fuel vapor enters the vapor separator and any liquid fuel runs back to the tank. Two roll over valves are fitted in the fuel tank vapor lines. These valves prevent liquid fuel entering the vapor separator if the vehicle rolls over. The advanced EVAPS system has no two way valve, so vapor is free to flow to the EVAP canister, where it is stored in the canister's activated charcoal element. When the correct engine operating conditions are met, the Engine Control Module (ECM) opens the EVAP canister purge valve and vapor is drawn from the canister, into the plenum chamber to be burned in the engine. Fresh air is drawn into the canister through the EVAP canister vent solenoid to take up the volume of displaced vapor. During normal operating conditions, and when the engine is switched off, the vent solenoid remains open and the fuel tank is free to breath through the EVAP canister. If the vent solenoid should fail, or the main vapor line becomes blocked, excess pressure is vented to atmosphere through a valve in the fuel filler cap. Similarly, the cap vent valve will open to prevent the tank collapsing if excessive vacuum is present.







When the temperature of fuel in the tank reduces, pressure also reduces and vapor must be drawn back into the tank. Fresh air is drawn into the canister, through the open vent solenoid, to take up the displaced volume.

An anti-trickle fill valve is titled to the filler neck in the line between the tank and EVAP canister. The function of this valve is to prevent the user overfilling the tank by trickling fuel into the neck, thereby preserving the vapor space in the tank to allow for fuel expansion during hot weather.

The valve creates a blockage in the vent line during the fuel filling process. The valve is operated by the action of inserting the fuel filler gun. With the valve in the closed position, air displaced during filling exits the tank only through the internal fill breather. When the fuel level reaches the the level of the fill breather, the filler neck fills with fuel, shutting off the filler gun.

The breather ports from the charcoal canister are located high up in the engine bay (CVS unit on NAS vehicles, snorkel tubes on ROW vehicles), to prevent water ingress during vehicle wading.

The advanced evaporative loss control system used on NAS vehicles is similar to the standard system, but also includes a CVS unit and an in-tank pressure sensor to monitor the pressure build-up for determining whether leaks are present.

The function of the GVS unit is to block the air intake side of the EVAP canister to enable the ECM to carry out the EVAP system leak check. The leak check only occurs when pre-determined vehicle operating conditions are met. Failure of the leak check will result in illumination of the Malfunction Indicator Lamp (MIL). The system is included as part of the On-Board diagnostics (OBD) strategy and checks for leaks greater than 1 mm (0.04 in.) in diameter. During checking, the vent and purge lines are closed, exposing the fuel tank and vent lines to engine vacuum (through the engine mounted purge valve). The ECM then checks for any pressure rises at the in-tank pressure sensor which would indicate a leakage.