Emission Control Systems: Description and Operation
EVAP Operational OverviewThe Enhanced Evaporative System Diagnostic is a precise series of eight operational sub-tests on the system. In some cases, the failure of only one sub-test will end all evaporative system testing; however, in many cases, the on-board system will conduct a series of sub-tests before storing any Diagnostic Trouble Codes (DTCs). A brief description of each of the sub-tests follows.
Excess Vacuum Test - Stage 1
During the normal purging of the Evaporative Emissions (EVAP) canister, the vent valve is commanded open. During this time, the Control Module monitors the Fuel Tank Pressure Sensor for an excess vacuum. The vacuum could build too quickly if the vent valve or vent valve hoses are restricted. If this test fails, the EVAP diagnostic is completed for that trip and a DTC P0446 is stored. A P0446 would indicate a plugged vent solenoid, plugged vent filter, pinched vent line, or a bad tank vacuum sensor.
Loaded Canister Test
Important: This test can only report a PASS.
The Loaded Canister Test uses the oxygen sensor data or the fuel trim information in order to determine if the engine is purging large amounts of fuel vapor from the EVAP canister. If large amounts of fuel vapors are being purged, this indicates that the canister is at least 60% full and that the system is working properly. When this is the case, the EVAP diagnostics P0440, P0442, P0446 and P1441 pass for that trip and no further testing is done. If this test does not indicate sufficient canister loading, the Weak Vacuum Test runs.
Weak Vacuum Test - Stage 1
The Weak Vacuum Test runs only if the previous test did not report a PASS. For this test, the vent valve is commanded closed during normal purge operation. If sufficient vacuum does not developed, the diagnostic fails and the Control Module stores a DTC P0440. If a sufficient vacuum develops, the Small EVAP Leak Test runs. If the proper driving conditions are not met, this test will be bypassed.
Small EVAP Leak Test
Notice: This test may run up to 3 times prior to reporting a failure. A P0442 indicates a small leak anywhere between the fuel fill cap and up to but not including the purge solenoid.
The Small EVAP Leak Test runs immediately following a passing Weak Vacuum Test. While the vacuum is still present in the fuel tank, the vent valve and purge valve are both command closed in order to seal the system. With the system closed, the Control Module monitors the trapped vacuum in the system through the Fuel Tank Pressure Sensor. If the vacuum decay rate is too fast, a small leak is indicated and a DTC P0442 is stored. Otherwise DTC P0442 is passed and the Excess Vacuum Test - Stage 2 is run.
Excess Vacuum Test - Stage 2
Important: This test is the same as the Stage 1 Excess Vacuum Test except for different driving conditions under which the test is conducted.
This test allows the normal purging and commands the vent valve open. If a high amount of vacuum occurs, this indicates that a restriction has occurred in the canister vent or the canister vent valve. If an excessive vacuum develops, a DTC P0446 stores. If an excessive Vacuum does not develop, a DTC P0446 passes and the Purge Valve Leak Test runs.
Weak Vacuum Test - Stage 2
Important: This test enables the Malfunction Indicator Lamp (MIL) on the next key up if a tail is reported and extinguishes the MIL on the next key up when a PASS is reported. When the MIL is extinguished, the executive also erases the freeze frame and the failure record buffer for this DTC (Check for a missing fuel fill cap). This test also runs on a hot start when all other are aborted if a prior failure exists.
This test is used in order to determine if the system can develop a sufficient vacuum. During a normal EVAP purging, the vent valve is commanded closed and the fuel tank pressure sensor is monitored by the Control Module. If sufficient vacuum is developed, a DTC P0440 passes; otherwise, the Enhanced Evaporative System diagnostic test ends.
Purge Valve Leak Test
The Purge Valve Leak Test runs only if there was a cold start this ignition, and the Weak Vacuum Test Stage 1 test did not tail. This test monitors the purge valve for leaks. During the test both the vent and the purge valves close. If a vacuum develops, the purge valve is leaking and a DTC P1441 is reported.