EVAP System Operation

The EVAP purge solenoid valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge solenoid valve (purge on). The EVAP purge solenoid control is Pulse Width Modulated (PWM) or turned on and off several times a second.




The EVAP canister purge PWM duty cycle varies according to the operating conditions determined by the mass air flow, the fuel trim, the engine coolant temperature, and the intake air temperature. For certain [1][2]EVAP tests, the diagnostic will be disabled if the TP angle increases to above 75 percent. The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring for vacuum decay.

The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the [1][2]EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15 and 85 percent.

The PCM monitors the fuel tank pressure/vacuum level via the fuel tank pressure sensor input.

Results Of Incorrect Operation
^ Poor idle, stalling and poor driveability can be caused by the following:
- Malfunctioning purge solenoid.
- Damaged canister.
- Hoses/lines split, cracked and/or not connected properly.
^ Evidence of fuel loss or fuel vapor odor can be caused by the following:
- Liquid fuel leaking from the fuel lines
- Cracked or damaged canister
- Inoperative canister control valve
- Vacuum hoses that are:
- Disconnected
- Mis-routed
- Kinked
- Deteriorated
- Damaged