EVAP Control System Operation

EVAP Control System Operation:

The fuel vapor generated in the fuel tank while driving or idling, passes through a fuel tank pressure control valve and enters the EVAP canister. The EVAP canister contains a charcoal element that absorbs and stores the fuel vapor. When the engine is able to use the stored fuel vapor, the fuel vapor is purged from the carbon element by intake air flow (low intake manifold pressure) and consumed in the normal combustion process.

The EVAP canister purge valve is a Powertrain Control Module (PCM) operated vacuum solenoid that controls the purging of the EVAP canister. The EVAP canister purge valve is mounted on the intake manifold plenum.

The fuel tank pressure control valve(s) is a pressure operated switching device. The fuel tank pressure control valve controls the vapor pressure in the fuel tank. The fuel tank pressure control valve is normally closed and blocks the fuel tank vapors from entering the EVAP canister unless the vapor pressure in the fuel tank is great. When the pressure of the vapor generated in the fuel tank becomes greater than a specified pressure, the vapor pushes past the fuel tank pressure control valve diaphragm and enters the EVAP canister. When the vapor pressure in the fuel tank is low (negative pressure), the fuel tank pressure control valve allows air to flow into the fuel tank. The fuel tank pressure control valves are located on top of the fuel tank.

The EVAP tank pressure control solenoid valve is a PCM operated vacuum solenoid that controls vapor flow into the EVAP canister. The EVAP tank pressure control solenoid valve opens a vapor passage to the EVAP canister allowing an equalization of pressure between the fuel tank and the EVAP canister. The EVAP tank pressure control solenoid valve is located on top of the fuel tank.

The EVAP tank pressure control solenoid valve is turned ON by the PCM under the following conditions:
^ The vehicle is stopped (the engine is running) and the fuel level is lower than the specified value.
^ The vehicle is moving and the fuel level is higher than the specified value.

The EVAP tank pressure control solenoid valve is turned OFF by the PCM under the following conditions:
^ The engine is OFF.
^ The engine is running but the vehicle is stopped and the fuel level is higher than the specified value.

The EVAP canister vent solenoid and the fuel tank pressure sensor are provided for the diagnosis of an EVAP control system leak. The EVAP canister vent solenoid is controlled by the PCM and allows air to enter the EVAP canister. The EVAP canister vent solenoid is normally open but is closed by the PCM when checking for EVAP control system leaks. The EVAP canister vent solenoid is mounted on the EVAP canister which is located on top of the fuel tank.

Additional information on the fuel tank pressure sensor can be found in the section titled Information Sensors/Switches Description.