Since the start of OBD-II diagnostics, Toyota has used different ways of testing the evaporative emissions system for leaks. The most recent system, starting with 2005 model year, is used on many Toyota, Lexus and Scion vehicles. This system tests for leaks using an evaporative leak detection pump module that is mounted on the charcoal canister under the vehicle. The leak detection pump module reduces the total number of parts by combining a vacuum pump motor, an evaporative system pressure sensor and a canister vent closed valve, all in one assembly.
As with the earlier systems, an evaporative purge solenoid is located in the engine compartment by the evaporative service port. A refueling vapor control valve is built into the charcoal canister.
The ECM usually tests the system for a leak approximately five hours after the ignition has been turned off, providing the altitude is less than 8000 feet above sea level, and the battery has more than 10.5 volts. When the engine coolant temperature and intake air temperature sensors indicate the temperature is between 40 to 95 degrees Fahrenheit, the engine control module (ECM) or the powertrain control module (PCM) will wake up and perform an evaporative emissions system leak test.
Data provided by Wayne Pasicznyk of Identifix Inc. For more information, call (800) 997-1674 or visit www.identifix.com.
The evaporative test sequence:
1. The ECM keeps the canister vent valve open so the evaporative pressure sensor gets an atmospheric pressure reading.
2. The vacuum pump activates and pumps vacuum through a 0.020 orifice built inside the evaporative leak detection pump module assembly. The ECM determines a voltage reading of the pressure sensor that represents a 0.020 leak in the system and stores this reading in memory to be used for comparison, later in the test.
3. The ECM continues the test by closing the canister vent valve to seal off the canister and then turns on the evaporative leak detection pump, to pump a vacuum into the complete evaporative system. This process may take up to 12 minutes to complete.
4. Once the vacuum stabilizes in the evaporative system, the ECM will determine if the reading is higher than the pressure sensor reading it stored during the first part of the test.
5. When the pressure sensor reading is higher (indicating less vacuum), the ECM will set a small or large leak code. The ECM will also set a code if the vacuum does not stabilize after 12 minutes of running the leak detection pump. A code P0455 indicates a large evaporative system leak and a P0456 indicates there is a small evaporative system leak.
6. If the pressure sensor does not indicate a leak, it will then perform a test to verify the evaporative purge solenoid operates correctly.
To test the evaporative emission system purge solenoid, the ECM will turn it on while there is vacuum in the system, and then will verify that the vacuum dropped rapidly as it vented out through the purge solenoid. If this does not happen, the ECM will set a P0441 code to indicate a problem with the purge solenoid. When the system passes the purge solenoid test, the ECM will then activate the leak detection pump again, drawing vacuum through the 0.020 orifice and looking for the reading of the evaporative pressure sensor to match the reading that was stored in memory during the first step of the testing. When the readings are the same, the evaporative emissions system monitor should indicate it has passed. The purge solenoid is also tested while driving. At this time, the ECM grounds or turns on the canister vent valve and then looks for a vacuum in the EVAP system while the purge solenoid is operating.
The ECM also has parameters for normal readings of the pressure sensor programmed in memory. This allows the ECM to determine if there is a problem with the leak detection pump internal orifice or a restricted evaporative vent filter.
Examples: If the pressure sensor voltage is below the standard voltage programmed in the ECM, after the leak detection pump has been turned on, the ECM will determine the internal orifice is restricted or plugged and will set code P043E. If the voltage measurement of the pressure sensor is above the standard value (indicating a high flow rate), the ECM will illuminate the malfunction indicator light and store codes P043F, P2401 and P2402. If any of these four codes are set, this indicates a failure of the evaporative leak detection pump, as long as the vent line and evaporative vent filter are not restricted.
If you have a vehicle in your shop with this type of evaporative system, the best way to determine a leak is to perform an evaporative leak test using a scan tool. If your scan tool cannot perform the test, you can use a smoke machine. Turn on (energize) the evaporative canister vent valve by commanding the canister vent valve through a scan tool or by manually back-probing and grounding the wire. Once the canister vent valve is energized, use the evaporative leak tester to look for a leak.
Just remember that most evaporative leak testers apply positive pressure to check for a leak and the ECM tests with negative pressure or vacuum. If a leak is not detected, the vehicle may still have a leak that only shows up under a negative pressure test.