Early evap monitors checked for leaks greater than.040 in. (1mm), but starting in model year 2000, the allowable leak was reduced to.020 in. (.5mm), which was phased in over several years. The reduced allowable leakage created the need for more accurate testing methods to adapt to changes in regulations and current technology. Toyota has evolved and refined its evap systems to locate smaller leaks. According to Toyota s technical training manual, It is essential that the evap system be correctly identified before beginning diagnosis. The quote has real meaning when you realize that Toyota has introduced four different evap systems in the last 10 years: 1996-2002 Early Type (Non-Intrusive), 2000-05 Late Type (Intrusive), 2000-06 LEV II and, beginning in 2004, the LEV-II with Vacuum Pump (Key-Off) Evap System. The 2004 Toyota Highlander was the first model to have this key-off system, and by 2007 all models were equipped with it. Each of these systems operates differently, so identifying which evap system is in the vehicle you re working on is crucial. Many domestic and Asian vehicle manufacturers adopted engine-off natural vacuum leak detection (NVLD) methods using the Ideal Gas Law principle to check for small leaks. Many European manufacturers continue to use an electric pressure pump identified as a diagnosis module-tank leakage (DM-TL), but changed the calibrated orifice from.040 in. (1mm) to.020 in. (.5mm). Toyota/Lexus/Scion are the only makes I know of currently using the vacuum pump key-off system. Why did Toyota decide to use a vacuum pump key-off system? A U.S. patent (US 7004013 B2) filed by the Denso Corporation might provide some insight into key-off vs. a pressure pump design. The patent suggests that a pump pressurizing a fuel vapor system with a leak will release fuel vapors into the atmosphere every time a leak test is performed. Using a vacuum pump positioned on the vent side of the fuel vapor system eliminates this issue. If the evap system has a leak, fuel vapors would escape only when pressure exists in the fuel vapor system and not during a vacuum leak test. This will make more sense when we describe the key-off system. In a typical Toyota key-off system, the key-off pump module is normally bolted directly to the charcoal canister and is positioned on the fresh air side of the fuel vapor system. The pump module contains the vent valve, pressure sensor, calibrated orifice and vacuum pump and is replaced as a complete unit. Similar to all enhanced evap systems, the key-off system has a workhorse side and a regulatory or compliance side. The workhorse side purges stored fuel vapors from the charcoal canister with the engine running. The engine control module (ECM) commands the purge solenoid open and fuel vapors are extracted from the charcoal canister. Fresh air is pulled from a passage just under the gas cap filler neck area through the canister air filter, around the vent valve and through the charcoal canister. The regulatory side of the system checks system integrity. The enabling conditions are similar to systems on other vehicles, but will be performed only after the engine has been turned off for five hours and the engine coolant temperature (ECT) is below 95 F. If after five hours the ECT is above 95 F, the ECM will wait another two hours and check again. If the ECT is still too high, the cool-down time is extended another 2½ hours. The illustration on page 24 shows the complete key-off evap system with a detailed view of the components inside the pump module. As described earlier, the vapor storage and purging of the fuel vapors is fairly straightfor- Photo courtesy Toyota Motor Sales, U.S.A., Inc.; diagram courtesy University of Toyota 22 February 2016
DIAGNOSING TOYOTA S KEY-OFF EVAP SYSTEM BY BOB PATTENGALE Evaporative emissions regulations continue to become more stringent, which requires vehicle manufacturers to develop new methods to test the effectiveness of their evap systems.
To Canister Air Filter Vent Line.020-In. Orifice Vent Valve Pressure Sensor Charcoal Canister ward. Fuel vapors from the fuel tank are captured in the charcoal canister and extracted when the engine is running. If vapor pressure increases in the fuel tank or during refueling events, excess pressure is released from the normally open vent valve via the canister air filter. The vent valve is incorporated into the pump module assembly and is normally open and closed only for leak-testing. In the diagram, the vent valve is positioned in the left side of the pump module. When deenergized, a spring pushes the plunger downward, which opens the vent passage and closes and seals the upper portion of the calibrated orifice passage. Vacuum Pump Purge Line Fuel Vapor Line Fuel Tank Purge Solenoid Intake Manifold Canister Air Filter This illustration shows the complete key-off evap system, with a detailed view of the pump module. The calibrated orifice is below the vent valve and just above the charcoal canister. Pressure from the vent valve spring keeps the top of the orifice channel sealed. Intermittent or inconsistent reference value readings might be the result of dirt and/or loose charcoal preventing the bottom valve from sealing. The pressure sensor (PS) in the center of the pump module is sealed with an O-ring and measures the The key-off evap system is fairly simple, but to properly test it, you'll need an enhanced scan tool. pressure within the fuel vapor system through a passage molded into the pump housing. The PS is on the vent side of the charcoal canister and not on the fuel tank side of the fuel vapor system. During normal operation with the vent valve deenergized and the purge valve closed, the PS will measure atmospheric pressure. A slight negative pressure might be seen during purge operation. The primary purpose of the PS is to provide accurate measurements during the leak-testing process. The leak test timing chart on page 26 shows the different phases of the leak-testing process. Phase 1 of the process begins after the five-hour waiting period has expired and all other enabling criteria have been met. In phase 1 the purge valve is closed, the vent valve is open, the pump motor is off and the ECM checks the PS, which should read atmospheric pressure. Phase 2 of the process uses the vacuum pump to set a reference value for a.020-in. leak test. It shows that the purge valve is closed, the vent valve is open and the ECM has commanded the vacuum pump on. Fresh air is pulled through the.020-in. orifice, through the molded passage at the bottom of the pump module housing and vents above the vacuum pump and finally around the edge of the vent valve assembly. Additional fresh air can be pulled from the fresh air vent as needed. The timing of phase 2 will depend on the vehicle application; somewhere between 20 and 360 seconds is possible. Phase 3 begins the fuel vapor system leak-check process. The vacuum pump continues to operate, the purge valve remains closed and the ECM has commanded the vent valve closed. The fuel vapor system is now sealed and the larger opening above the calibrated orifi ce will provide a larger passage for the vacuum pump to operate more efficiently. Phase 4 tests the condition of the purge valve. In phase 4, the vacuum pump is still on, the vent valve is closed and, for a brief moment, the ECM has commanded the purge valve open. This action opens the fuel 24 February 2016
Purge Valve Vent Valve Pump Motor ON (Open) OFF (Close) ON OFF (Vent) ON OFF Atmospheric Pressure System Pressure.020-In. Pressure vapor system to the atmosphere by pulling fresh air in from the intake manifold. The pressure sensor should quickly transition from a negative pressure to atmospheric, which tells the ECM the purge valve is operating and relieves the negative pressure from the fuel vapor system. Phase 5 is used to verify the accuracy of the reference value set in phase 2. The vacuum pump is running, the 1 2 3 4 5 6 This timing chart shows the phases and component actuation of the evaporative system monitor. The average test should take approximately 20 minutes to complete. 732mm-Hg (A) 0mm-Hg (G) 711mm-Hg (A) 21mm-Hg (G) purge valve is closed and the vent valve is now open. The vacuum pump pulls against the calibrated orifice and expects to see a value close to what was recorded in phase 2. In phase 6, the vacuum pump is commanded off, the purge valve is closed, the vent valve is open and the ECM compares the PS values recorded in phase 1 and phase 6. Any significant differences between phases 702mm-Hg (A) 29mm-Hg (G) 1 2 3 4 5 6 This screen capture shows the individual phases of an evaporative system check on a known-good late-model Toyota Camry key-off system. 1 and 6 or phases 2 and 5 will abort the test. Now let s put the theory described above into practice. The screen capture below is from a late-model Toyota Camry with no leaks present. In phase 1, the atmospheric pressure is 732mm-Hg absolute (0mm-Hg gauge), which is within an acceptable range. In phase 2 the vacuum pump is commanded on and the ECM measures the negative pressure obtained as the vacuum pump pulls against the.020-in. orifice and records the value, which will be used as a reference for phase 3. In this example, the ECM recorded 711mm-Hg absolute ( 21mm- Hg gauge). The ECM ran the test for approximately 360 seconds. Phase 3 begins as soon as the vent valve is commanded closed. In this example, the pressure sensor shows a quick pressure increase to just below atmospheric, which is expected as the seal above the calibrated orifice was opened to the larger volume area. Within a few seconds the fuel vapor system is being pulled into a negative pressure. The time of the leak test depends primarily on how much fuel is available in the tank. A fuel tank that s three-quarters full will pull into a vacuum sooner than a tank that s only one-quarter full. Approximately 15 minutes will be the maximum time allowed; the capture from the late-model Camry with 80% fuel volume took approximately three minutes. The PS shows a steady negative pressure buildup, which eventually pulls below the 711mm-Hg absolute ( 21mm-Hg gauge) reference target, which indicates a sealed system with no leaks greater than.020 in. As the capture shows, the ECM continues to test the fuel vapor system until it reaches 702mm-Hg absolute ( 29mm-Hg gauge). Pop quiz: You have a leak in the fuel vapor system. How does the 26 February 2016
Vent Valve Fresh Air Vent To Canister There are many challenges associated with testing Toyota key-off systems. For one thing, commanding the vent valve closed will not fully seal the system. ECM determine which code to set a P0455 (gross leak) or a P0456 (very small leak)? The answer is fairly simple. If the vent valve closes and the PS jumps from 21mm-Hg and continues to read atmospheric or only a slight negative pressure, this would indicate a gross leak is present. If the pressure sensor jumps from 21mm- Hg to atmospheric and then pulls to 17mm-Hg, this would indicate there s a very small leak. Phase 4 checks purge valve operation. In this example, the pressure sensor value goes to just below atmospheric, which indicates that the purge valve is open. This test is very quick approximately 10 seconds. Phase 5 is a recheck of the phase 2 PS.020-in. reference value. As stated previously, phases 2 and 5 values should be very similar in this example, 21mm-Hg for phase 2 and the same for phase 5. Phase 6 is a recheck of the phase 1 PS recorded value, and the values are right on the money. As you can see, the key-off system is fairly simple, but in order to properly test it, you ll need an enhanced scan tool. The values in each phase will help point you in the right diagnostic direction. Once you ve confirmed the leak code, how do you test the system for leaks? The evap test port is no longer available, so you must find a place to connect the evap test machine to look for leaks. The most logical point would be the purge valve typically found in the engine compartment, but on the key-off system, this will not work. For demonstration purposes we have connected a vacuum tee to the purge line, as shown in the photo below. If you follow the tee downward you ll see that the evap test machine outlet hose is connected to an extra hose off the tee. If you command the vent valve closed and When testing the Toyota key-off system, the best option is to connect the evap test machine to the fresh air side of the system at the air filter hose fitting on the pump module. Photo courtesy Star EnviroTech turn the evap test machine on, smoke will come out from the vent valve outlet. If the machine were fully connected to a vehicle, the smoke would exit from the gas cap area. We briefly touched on this earlier, but with the vacuum pump off and the vent valve commanded closed, the smoke will travel through the charcoal canister, through the molded passage at the bottom of the pump module, through the vacuum pump and finally around the vent valve to the canister air filter. The photo on the far left shows the pump module with the cover removed. The red and black jumper leads are commanding the vent valve closed. Smoke is being introduced into the fresh air vent and you can see smoke is coming out around the gap between where the vent valve is inserted into the pump housing, but not from the canister side of the pump module. An O-ring at the bottom of the vent valve provides a seal for the fuel vapor system, but not for the fresh air vent side. This is all nice to know, but I wanted you to clearly understand why you need to test for leaks from the fresh air side of the system. For testing key-off systems, this requires connecting a hose at the pump module vent. On some applications there s a hose connection between the pump module and canister filter, which can be used for testing. Toyota does have special adapters for the job. Toyota s Special Service Tools Bulletin SS002-05 describes the process and the part numbers for the adapters. Evap systems are going to continue to evolve, so it s important to know which system is on the vehicle you re testing and what it takes to properly diagnose and test that system. A few minutes spent on research might save you a ton of time on the job. This article can be found online at www.motormagazine.com. 28 February 2016