22 February 2016

DIAGNOSING TOYOTA’S KEY-OFF EVAP SYSTEM

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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 meth-ods 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 train-ing manual, “It is essential that the evap system be correctly identified before beginning diagnosis.” The quote has real meaning when you re-alize that Toyota has introduced four different evap systems in the last 10 years: 1996-2002 Early Type (Non-In-trusive), 2000-05 Late Type (Intru-sive), 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 oper-ates differently, so identifying which evap system is in the vehicle you’re working on is crucial.

Many domestic and Asian ve-hicle manufacturers adopted en-gine-off natural vacuum leak detec-tion (NVLD) methods using the Ideal Gas Law principle to check for small leaks. Many European manufactur-ers continue to use an electric pres-sure 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 pres-sure pump design. The patent sug-

gests 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 elimi-nates 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 can-ister 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 sys-tems, the key-off system has a work-horse side and a regulatory or compli-ance side. The workhorse side purges stored fuel vapors from the charcoal canister with the engine running. The engine control module (ECM) com-mands 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 anoth-er 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 in-side the pump module. As described earlier, the vapor storage and purging of the fuel vapors is fairly straightfor-

Evap.indd 1 1/21/16 11:24 AM

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.

DIAGNOSING TOYOTA’S KEY-OFF EVAP SYSTEM

Evap.indd 2 1/21/16 11:24 AM

24 February 2016

Diagnosing Toyota’s Key-Off Evap System

The calibrated orifi ce 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 in-consistent 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

pressure within the fuel vapor sys-tem 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 opera-

tion 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 pri-mary 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 at-mospheric pressure.

Phase 2 of the process uses the vacuum pump to set a reference val-ue 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. orifi ce, through the molded passage at the bottom of the pump module housing and vents above the vacuum pump and fi nally around the edge of the vent valve assembly. Ad-ditional fresh air can be pulled from the fresh air vent as needed. The tim-ing of phase 2 will depend on the vehicle application; somewhere be-tween 20 and 360 seconds is possible.

Phase 3 begins the fuel vapor sys-tem leak-check process. The vacu-um 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 effi ciently.

Phase 4 tests the condition of the purge valve. In phase 4, the vacu-um 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

ward. Fuel vapors from the fuel tank are captured in the charcoal canis-ter 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 fi lter.

The vent valve is incorporated in-to 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 clos-es and seals the upper portion of the calibrated orifi ce passage.

To Canister Air Filter

Vent Valve

Purge Line

Fuel Vapor Line

Charcoal Canister Fuel Tank

Vent Line

Pressure Sensor

Canister Air

Filter

VacuumPump

Purge Solenoid

Intake Manifold

.020-In. Orifi ce

This illustration shows the complete key-off evap system, with a detailed view of the pump module.

The key-off evap system is fairly simple, but to

properly test it, you'll need an enhanced scan tool.

Evap.indd 3 1/21/16 11:24 AM

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 atmos- pheric pressure is 732mm-Hg abso-lute (0mm-Hg gauge), which is within an acceptable range.

In phase 2 the vacuum pump is commanded on and the ECM mea-sures the negative pressure obtained as the vacuum pump pulls against the .020-in. orifi ce and records the value, which will be used as a reference for phase 3. In this example, the ECM re-corded 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 be-low atmospheric, which is expected as the seal above the calibrated ori-fi ce 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 tar-get, which indicates a sealed system with no leaks greater than .020 in. As the capture shows, the ECM con-tinues 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

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 accura-cy of the reference value set in phase 2. The vacuum pump is running, the

purge valve is closed and the vent valve is now open. The vacuum pump pulls against the calibrated orifi ce 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 record-ed in phase 1 and phase 6. Any sig-nifi cant differences between phases

Diagnosing Toyota’s Key-Off Evap System

This screen capture shows the individual phases of an evaporative system check on a known-good late-model Toyota Camry key-off system.

.020-In. Pressure

System Pressure

Atmospheric Pressure

ON (Open)

OFF (Close)

ON

ON

OFF (Vent)

OFF

Purge Valve

Vent Valve

Pump Motor

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)

702mm-Hg (A)−29mm-Hg (G) 1 2 3 4 5 6

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28 February 2016

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 oper-ation. In this example, the pressure sensor value goes to just below at-mospheric, 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 stat-ed previously, phases 2 and 5 values should be very similar—in this exam-ple, −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 prop-erly test it, you’ll need an enhanced scan tool. The values in each phase will help point you in the right diag-nostic 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 pur-poses 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

turn the evap test machine on, smoke will come out from the vent valve outlet. If the machine were fully con-nected 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 mold-ed 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 re-moved. 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 bot-tom 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 want-ed 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 con-necting 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.

Diagnosing Toyota’s Key-Off Evap System

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.

Vent ValveTo Canister

Fresh Air Vent

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.

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