Evaporative Emission Control: Notes

The Evaporative (EVAP) emission control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, open, fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking:

1. Large Leak Test  - This tests for large leaks and blockages in the EVAP system. The PCM will command the EVAP vent solenoid ON, closed, and command the EVAP purge solenoid ON, open, with the engine running, allowing engine vacuum into the EVAP system. The PCM monitors the Fuel Tank Pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time. The PCM then commands the EVAP purge solenoid OFF, closed, sealing the system and monitors the vacuum level for decay. If the PCM does not detect that the predetermined vacuum level was achieved, or the vacuum decay rate is more than a calibrated level on 2 consecutive tests, DTC P0440 will set.
2. Small Leak Test  - If the large leak test passes, the PCM will test for small leaks by continuing to monitor the FTP sensor for a change in voltage over a period of time. If the decay rate is more than a calibrated value, the PCM will rerun the test. If the test fails again, DTC P0442 will set.
3. Canister Vent Restriction Test  - If the EVAP vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The PCM tests this by commanding the EVAP purge solenoid ON, open; and commanding the EVAP vent solenoid OFF, open; and monitoring the FTP sensor for an increase in vacuum. If vacuum increases more than a calibrated value, DTC P0446 will set.
4. Purge Solenoid Leak Test  - If the EVAP purge solenoid does not seal properly, fuel vapors could enter the engine at an undesired time, causing driveability concerns. The PCM tests for this by commanding the EVAP purge solenoid OFF, closed; and vent solenoid ON, closed; sealing the system, and monitoring the FTP for an increase in vacuum. If the PCM detects that EVAP system vacuum increases above a calibrated value, DTC P1441 will set.
5. Check Gas Cap Message  - The PCM sends a class 2 message to the Driver Information Center (DIC) illuminating the CHECK GAS CAP message when a malfunction in the EVAP system and a large leak test fails or a malfunction in the EVAP system and a small leak test fails.