Evaporative Emission System Monitor

PURPOSE
The Evaporative (EVAP) Emission System Monitor is an on-board strategy designed to test the proper operation of the EVAP system by checking the function of its components and ability to flow fuel vapor (hydrocarbons) to the engine.

OPERATION
The monitor relies on intake manifold vacuum supplied to the EVAP Canister Purge solenoid to pull vapors from the carbon canister when the system is enabled at idle, after certain entry conditions are satisfied. The entry conditions are as follows:

- In closed-loop fuel control,

- RPM below 900,

- engine coolant temperature between 104 and 215 degrees F,

- vehicle speed below 3.76 MPH.

During the monitor testing, the transient load has to be steady for at least 5 seconds. Once the entry conditions are met, lambse is sampled and averaged before vapor is flowed. The EVAP Canister Purge solenoid is progressively opened in steps (from a 10% to 40% duty cycle, then from a 40% to 100% duty cycle) while the resulting lambse value is compared to the averaged, no purge flow value of lambse calculated previously. If there was not an adequate lambse shift while purging, the system has failed the first execution (Test 1) of the EVAP monitor logic. Failure could occur either due to insufficient or excessive vapor in the carbon canister. After Test 1, the monitor will allow normal control to run while determining when most of the vapor should have been removed from the canister. The monitor will then take over and the second execution (Test 2) will run to check for enough vapor flow. If the Test 2 fails, the appropriate fault will be set. Input from the Intake Air Temperature (IAT), Engine Coolant Temperature (ECT) and Vehicle Speed Sensor (VSS) is required to enable the EVAP Monitor.

DTCs and MIL Operation






1 The EVAP Canister Purge solenoid is a normally closed solenoid that when opened is used to control the flow of fuel vapors into the engine.

The EVAP Canister Purge solenoid and circuit are tested for failures. The DTC associated with this test is DTC P0443.

2 A general EVAP emission control system test is performed. The DTC set as a result of the EVAP system's failure of both Test 1 and Test 2 on a drive cycle.

The DTC associated with this test is DTC P0440.

3 The MIL is activated after one of the two tests fail on two consecutive drive cycles.