Canister Purge Solenoid: Description and Operation

EVAP Vacuum Switch:

EVAP Control

The PCM controls the EVAP solenoid using a pulse width modulated (PWM) signal. By using this type of control, the PCM can vary the rate (duty cycle) at which it opens or closes the solenoid. As mentioned, a strict set of conditions must be met before the PCM will enable canister purge. This optimizes driveability and evaporative emission reduction.

Once the PCM decides to enable the EVAP solenoid, it monitors rear oxygen sensor data to determine if the air/fuel mixture is correct. If the PCM determines the air/fuel mixture is incorrect, it will adjust the EVAP solenoid duty cycle to provide the correct amount of fuel vapor from the EVAP canister.
This process is called duty cycle learning.

Enable/Disable Conditions

All of the following conditions must be met before the PCM will enable the EVAP solenoid. Once enabled, failure to meet any of these conditions will cause the PCM to disable the EVAP solenoid.

1. PCM fuel control must be in closed loop operation and commanded Air/Fuel ratio must be 14.7 to 1 for a certain period of time before the EVAP solenoid will be enabled. This delay will ensure that the engine is sufficiently warmed up and ready. The delay period is based on engine coolant temperature (ECT) at engine start-up.
2. Fuel is not being disabled to any cylinder due to traction control system operation or acceleration reduction fuel control.
3. The PCM determines that no engine overtemperature condition exists. The EVAP solenoid will not be enabled if an overtemperature condition exists in order to help reduce engine temperature.
4. Vehicle speed is present and MAF sensor input is at least 16.0 grams per second or Engine running for at least 17 minutes.

Duty Cycle Learning

When all of the conditions to enable the EVAP solenoid have been met, the PCM will adjust the duty cycle if closed loop fuel control is operating free of limitations (short and long term fuel trim not at maximum or minimum limits).