Operation CHARM: Car repair manuals for everyone.

EVAP System Operation

The evaporative emission (EVAP) 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 PCM will command the EVAP purge valve open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent valve open, fresh air will be drawn through the valve 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 valve into the intake manifold to be consumed during normal combustion. The EVAP system requires the PCM be able to detect a leak as small as 0.04 inch in the EVAP system. The PCM uses several tests to determine if the EVAP system is sealed.

Weak Vacuum Test
This tests for large leaks and blockages in the EVAP system. The PCM will command ON (close) the EVAP vent valve and command ON (open) the EVAP purge valve with the engine running, allowing engine vacuum into the EVAP system. The PCM monitors the Fuel Tank Pressure (FTP) sensor to verify that the system is able to reach a predetermined level of vacuum within a set amount of time. If the PCM does not detect the predetermined vacuum level on 2 consecutive tests, a DTC P0440 will set.

Vacuum Decay Test
If the weak vacuum test passes, the PCM will command OFF the EVAP purge valve, sealing the EVAP system. The PCM tests for vacuum decay in the EVAP system by monitoring the fuel tank pressure (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, a DTC P0442 will set.

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 valve CLOSED and commanding the EVAP vent valve OPEN and monitoring the FTP sensor for an increase in vacuum. If an increase in vacuum is detected, DTC P0446 will set.

Purge Valve Leak Test
If the EVAP purge valve 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 and vent valves closed, sealing the system, and monitoring the FTP for an increase in vacuum. If the PCM detects an increase in vacuum, DTC P1441 will set.

EVAP Service Bay Test
The EVAP service bay test is accessed with a scan tool, and allows EVAP diagnostic tests to be run at higher Engine Coolant Temperatures (ECT) than are allowed during normal testing. The EVAP service bay test allows all of the above tests to be run on demand. When the EVAP service bay tests are run, the scan tool will indicate the tests have passed, or which specific DTC has failed. If a EVAP service bay test fails, no DTCs will be recorded in the PCM Freeze Frame/Failure Records, they will only be displayed on the scan tool. The EVAP service bay test is useful in determining if a fault is present, and for verifying a repair.