ORVR EVAP System Operation

The Evaporative Emission (EVAP) control system limits the fuel vapors from escaping into the atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon (charcoal) storage device (EVAP canister). The EVAP canister stores the vapors until the engine is able to use the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor. The system is required in order to detect the evaporative fuel system leaks as small as 0.040 inches between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative system integrity by applying a vacuum signal (ported or manifold) to the fuel tank in order to create a small vacuum.

The Powertrain Control Module (PCM) then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts sub-tests in order to detect the fault conditions. If the diagnostic fails a subtest, the PCM stores a Diagnostic Trouble Code (DTC) in order to indicate the type of fault detected.




The EVAP diagnostic tests include the following tests:

Weak Vacuum Test: This tests for large leaks and blockages in the EVAP system. The FTP sensor detects low fuel tank pressure resulting from a large vacuum leak or a restriction in the vacuum supply to the fuel tank. The FTP sensor value should increase over a period of time. If the PCM does not detect in increase, a malfunction is indicated by setting DTC P0440.

Weak Vacuum Follow-up Test: This tests the EVAP system after the Weak Vacuum Test has failed. The FTP sensor looks for high tank vacuum over a period of time. If this occurs the Weak Vacuum Test will Pass. If Weak Vacuum does not pass, the test continues through this ignition cycle.

Vacuum Decay Test: This tests for small leaks in the EVAP system. The decay rate is determined by measuring the change in fuel tank pressure (vacuum) over a period of time. If the decay rate is greater than a calibrated value, than another vacuum decay test will be initiated. If vacuum decay is still greater than a calibrated value, a malfunction is indicated by setting DTC P0442.

Fuel Vapor Build Pressure Test: This test is used to determine the vacuum leak size. When the vacuum has decayed to near zero, the PCM subtracts measured slope from the vacuum decay to calculate to predict leak size. If the calculation indicates that a leak is present, the small leak test will fail.

Canister Vent Restriction Test: This tests for a restriction in the EVAP vent system. The FTP sensor looks for sufficient vacuum from the engine intake manifold. After a period of time and conditions have been met, then this test will pass. If the vacuum value is to high, a malfunction is indicated by setting a DTC P0446.

Purge Valve Leak Test: This tests for small leaks to the intake manifold. This is accomplished by sealing the EVAP system purge valve and vent valve, and allowing the PCM to monitor the FTP sensor. The FTP sensor value should not increase. If the PCM detects an increase, a malfunction is indicated by setting DTC P1441.