Oct 30, 2009
- control module monitors the oxygen sensors during normal EVAP purging in order to determine if the EVAP canister is storing a sufficient amount of fuel vapors. If the test indicates that the EVAP canister is storing a sufficient amount of fuel vapors, the remaining diagnostic tests for the EVAP system will store a pass. If the Loaded Canister Test indicates to the control module that an insufficient amount of fuel vapors are being stored by the EVAP canister, the Weak Vacuum Tests will run.
Weak Vacuum Tests
The Weak Vacuum Test is performed by the control module in order to determine if the EVAP control system is capable of achieving a sufficient vacuum in the system. During the normal purging of the EVAP control system, the control module will perform the following:
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Command the EVAP vent valve ON or closed
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Monitor the fuel tank pressure sensor signal
If the control module determines that sufficient vacuum is not achieved, the control module will perform the following steps:
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Store a DTC P0440 after the first failed test
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Store the data in the Freeze Frame and Failure Records
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Turn the malfunction indicator lamp (MIL) ON after a second consecutive failed test
If the control module determines that sufficient vacuum is achieved, the control module will perform the following steps:
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While the EVAP vent valve is ON or closed, turn the EVAP purge solenoid OFF or closed, and run the Small EVAP Leak Test.
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Run the Purge Solenoid Leak Test if the first Weak Vacuum Test passes.
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Turn OFF the MIL, if the MIL is ON because of a previous failed Weak Vacuum Test.
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Clear the data stored in the Freeze Frame for the DTC P0440.
The Weak Vacuum Tests will not run unless the Loaded Canister Test has run and failed.
Small EVAP Leak Test
The control module performs the Small EVAP Leak Test in order to detect a small leak in the following components of the EVAP system:
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The fuel tank
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The fuel filler cap
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The fuel filler neck
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The EVAP canister
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The EVAP vent valve
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The EVAP vapor pipes and hoses
The Small EVAP Leak Test does not test the EVAP purge solenoid for leaks.
In order to run the Small EVAP Leak Test the control module performs the following steps:
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Turn OFF the EVAP purge valve.
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Following the passed Weak Vacuum Test and while the EVAP vent valve is still turned ON or closed, run the Small EVAP Leak Test .
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Monitor the fuel tank pressure sensor signal.
The control module monitors the vacuum trapped in the system. If the control module detects an excessive decrease in the vacuum that is trapped in the system, a DTC P0442 is set.
Purge Valve Leak test
The control module monitors the fuel tank pressure sensor signal during a cold start in order to test the EVAP purge valve. With the purge valve OFF or closed, the VCM expects to detect no vacuum in the system. If the control module detects a vacuum while the Purge Valve Leak Test is being performed, a DTC P1441 is set.
Evaporative (EVAP) System Canister
(1)
Purge Line
(2)
Vapor From Fuel Tank
(3)
Air
The evaporative system canister, filled with activated carbon pellets, stores the fuel vapors from the fuel tank. The engine vacuum purges the vapor canister during normal driving.
Fuel Tank Pressure Sensor
The fuel tank pressure (FTP) sensor is a 3 wire strain gauge sensor much like that of the manifold absolute pressure (MAP) sensor, however this sensor has a very different electrical characteristic due to its pressure differential design. The sensor measures the difference between the air pressure, or vacuum in the fuel tank and the outside air pressure.
The sensor mounts at the top of the fuel tank sending unit. A three wire electrical harness connects it to the control module. The control module supplies a 5 volt reference voltage and ground to the sensor. The sensor varies a voltage between 0.1-4.9 volts. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel fill cap is removed, the output voltage of the sensor will measure 1.3-1.7 volts.
When the air pressure in the tank is 4.5 inches Hg (1.25 kPa), the sensor output voltage should measure 0.3-0.7 volts.
The sensor voltage will be approximately 1.5 volts when the fuel tank pressure is equalized with ambient pressure. The sensor voltage increases to approximately 4.5 volts when a vacuum of 14 inches Hg (-3.75 kPa) is detected.
Evaporative Canister Purge Solenoid
When energized, the evaporative canister purge solenoid allows the fuel vapor to flow from the EVAP canister to the engine. The normally closed valve is pulse width modulated by the control module in order to precisely control the vapor flow. The valve opens during the Enhanced Evaporative Diagnostic Test in order to create a vacuum in the fuel tank and then closes in order to seal the system.
Evaporative System Service Port
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