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Tech Update: A to VSV...Troubleshooting Toyota/Lelxus EVAP Systems

April 1, 2006

When I first had to diagnose a Toyota EVAP code, I found the available information a little lacking. So I decided to do a little homework…and the following article is the result of what I’ve learned. I hope it helps you diagnose and repair these systems a little easier.

First, a word on basic EVAP leak testing. I often see techs trying to find EVAP leaks by immediately using the smoke function on an EVAP tester. Personally, I prefer to first verify there is a leak, then treat it in the same way I would if I were looking for an open wire in an electrical system.

The first step is to seal the system for testing. This is usually accomplished by sealing the air vent line to the tank and any other normally open vent lines in the system. One trick I’ve learned is to first hook up to the PCM and enter via Global OBD II. Then I go to Mode $08-Request Control of Onboard Systems (see Photo 1). Currently, the only option listed here is the EVAP system, and by commanding control, you order the PCM to close the system for testing. If this option is available, you don’t have to do anything to seal the system; the PCM will do it for you. Then all that’s left is to connect to the service port and pressurize the system. (Note: On some makes, you may still have to seal the tank air vent.) You will have to cycle the key in order to return the EVAP system to its normal state.

Once the system is sealed, I use the pressure function of the tester to pressurize the system. I want the indicator ball to fall all the way to the bottom. Depending on the amount of fuel in the tank, it may take a minute or two. If the indicator won’t go down all the way, then I know there is a leak. My last step in checking the system is to release the pressure switch on the tester while it’s still connected and waiting about 30 seconds. Then I reapply pressure. If the ball jumps up and starts back down, I know I have a really small leak — typically weathered rubber lines or seals.

If a leak is indicated, the first thing I do is check the cap (don’t do this before testing or you won’t be able to verify you’ve found and fixed the problem). Even though the cap appears to be tight, that doesn’t mean it isn’t leaking. I like to remove the cap and seal the opening with one of the plugs supplied with the tester. If my leak test passes, I reinstall the cap and test once more. If it passes again, then I know the cap was just left loose and I’m done. If it fails, I inspect the cap and fuel neck for damage and repair as needed. Don’t forget to verify the repair. If the leak is not in the cap, then I break the system down into two parts, the purge side and the tank side. I isolate the two parts at the canister, and test each individually. Once I find which leg the leak is in, I can either break it down into smaller sections, or go straight to the smoke and look for the cause.

If the leak is in neither side, then it’s time to test the canister itself. Some canister designs are simple, and others, like Toyota’s, are a little more complex. Here I refer to the service information for detailed testing instructions.

Once the leak is repaired, I again pressure-test the entire system to verify the repair. This is an important step in customer satisfaction. Many EVAP codes are “2 trip” codes, meaning that the conditions that are needed to set the code must occur on two consecutive monitors in order for the MIL to be illuminated. This can take several drive cycles to accomplish and make many EVAP repairs hard to verify by running the monitors alone. And the average customer doesn’t understand or care; they only know the light is back on and they are back in your shop for the same thing.

TOYOTA/LEXUS EVAP SYSTEMS
There are two types of EVAP systems in use on Toyota and Lexus vehicles. The first is referred to as the “early” or “non-intrusive” type, and the second is referred to as the “late” or “intrusive” type. The PCM strategies for leak detection and system monitoring are different between the two and affect diagnostic procedures and code definitions.

The early system was developed to meet initial EPA and CARB requirements for leak detection. When leak detection standards became more stringent, the late-type system was implemented. The easiest way to ID which system is in use on the vehicle you’re testing is to look for the fresh air line connected to the intake boot or air box. If this line is connected directly, it’s an early system. If it’s routed through a solenoid, it’s a late system (see Photo 2). By the way, the solenoid in the late system is called the Canister Closed Valve or CCV for short.

Another important difference between the systems is located on or near the canister itself. On both systems, the PCM uses information from the Vapor Pressure Sensor (or VPS) to monitor for leaks. The important difference is in how this information is obtained.

On the early system, a solenoid called the 3-Way Vacuum Switching Valve (or VSV) is used to connect the VPS to either the canister side of the system or the tank side of the system. The PCM then compares pressure in the side being tested to its internal expectations. If these are not met, a leak code is set. We’ll be discussing that in more detail in a moment.

In the late system, the 3-Way VSV is replaced by a Bypass Vacuum Switching Valve. This valve connects the tank and canister side together for leak testing. The PCM closes the CCV mentioned earlier and opens the Bypass VSV, then commands the purge valve (called the EVAP VSV) on and draws a vacuum on the entire system until a specified pressure is met. It then closes the EVAP VSV and monitors the rate of vacuum loss to an expected value. Anything outside of this value will trigger a leak code.

It’s also important to note that EVAP codes are “2 trip” codes and monitors can take 20-30 minutes or longer to complete under very specific conditions. This makes verification of repairs in the bay more important, as test driving to run the monitors is impractical. Also, be aware while reviewing this information that restrictions in the system can also cause codes to be set as quickly as leaks can. Lastly, always diagnose and repair leak codes first before diagnosing component failures.

Early System Codes, Conditions and Diagnostic Tips

(see Fig. 1)

P0440 — EVAP System Malfunction
This code is set when the 3-Way VSV is switched to the tank side of the system, and the PCM sees atmospheric pressure instead of the values expected. This code is always a leak on the tank side of the system.

P0441 — Vapor Purge Flow Detection
This code can be set by two different conditions — either by a restriction in flow during purge or by purge occurring when it shouldn’t. It may also be caused by a leak in the purge side of the system. The PCM looks for these three conditions:

1. If the expected pressure drop in the canister is not seen by the PCM when the EVAP VSV is commanded open, the PCM suspects a restriction in the system.

2. If the PCM sees a pressure drop in the canister on initial start-up, the PCM suspects the EVAP VSV is not closed when it’s supposed to be.

3. If the PCM doesn’t see pulsations in pressure in the canister during normal purge, the PCM suspects a fault in the purge system.

If this code is set alone, check for leaks in the purge line and check the operation of the EVAP VSV. To check for restrictions in the purge system, disconnect the purge line from the canister and connect it to a vacuum gauge. With the engine running, command the EVAP VSV on — vacuum should equal manifold vacuum (see Photo 3). Then command the EVAP VSV closed — vacuum should be 0. If not, test the EVAP VSV and its related electrical wiring for proper operation.

This code test is dependent on readings from the VPS which, in turn, depends on proper operation of the 3-Way VSV. Should a problem exist in these components, expect a possible code P0446 to also be set (see below). And, as with any diagnostic, do a visual inspection of the lines for proper connection and condition.

P0446 — 3-Way VSV Fault
The PCM tests the operation of the 3-Way VSV by first looking for a pressure difference when it’s switched from the tank side to the canister side. Remember, the 3-Way VSV switches the input to the VPS (see Photo 4). If no difference is seen, two steps are taken:

1. If pressure pulsations (caused by normal pulsations in manifold vacuum) during purge are not seen by the VPS, the PCM concludes the 3-Way VSV did not switch. And/Or......

2. Pressure pulsations are detected in the tank side of the system, then the PCM judges that the 3-Way VSV did not switch.

It’s important to note that leaks in the system can affect these code criteria, and will also likely set either a P0440 or P0441. If so, diagnose and repair the leak conditions first before verifying operation of the 3-Way VSV. It’s also possible for an internal canister failure to set this code. If all other causes are ruled out, or freeze frame data shows that the code set at 0 mph, suspect a canister fault and see the model specific service information for canister testing procedures.

3-Way VSV failure is common, but placement of these valves can make bench testing difficult and time consuming. Another factor in diagnosing this code is the sensitivity of the VPS itself and the slow “refresh” rates of most scan tools. Here are some ideas on testing the valve without removal, and using a lab scope or DSO.

Using the appropriate wiring diagram, locate the signal wire from the VPS to the PCM. In some cases, connecting directly at the PCM is the easiest. On others, it may be more practical to connect in the harness or at the VPS itself. Connect the DSO to this signal, command the 3-Way VSV “on” with a scan tool, open the tank cap and note the reading. That’s atmospheric pressure, and what we don’t want to see during our testing.

Now reinstall the cap and let’s monitor the voltage. Connect the purge line from the canister directly to intake vacuum, either by commanding the EVAP VSV open, or using a length of vacuum line, and look for the pressure drop on the VPS signal line. You may also see the pulsations that should be present (see Photo 5). When the 3-Way VSV is “off” it’s connected to the canister side of the system, so make sure you command the VSV to this position.

Now, command the 3-Way VSV “on.” It should switch to the tank side of the system, and you should see a corresponding change in the VPS signal. The VPS voltage will increase with pressure increase and decrease with pressure loss.

If the results are reversed from what’s expected, check the hose routing between the 3-Way VSV and the VPS. If results on both sides are the same, check the electrical and mechanical operation of the 3-Way VSV and condition of the connecting lines. Inspect the valve for debris clogging the passages. If any is noted, then suspect internal failure of the canister as the root cause (charcoal granules escaping from the canister and clogging the valve).

P0450 or P0451 — Vapor Pressure Sensor Fault
Both of these codes relate directly to the VPS. They are set when the signal from the VPS is outside of specific parameters internal to the PCM. Use the DSO or scan PID to verify the signal fault and use basic electrical diagnostics to isolate the cause — typically a faulty sensor, open wiring or poor ground. Of course, verify that hose routing to the VPS and 3-Way VSV is correct. Check freeze-frame data as well; a run time of less than 200 seconds is a good indication of a fault in the sensor itself.

Late System Codes, Conditions and Diagnostic Tips

(see Fig. 2)

Remember the differences in the late system? The PCM uses a Bypass VSV and a Canister Closed Valve to seal the system for leak detection monitoring. The VPS is now mounted on the fuel tank and there is no 3-Way VSV. The PCM checks the system by closing the CCV and opening the Bypass VSV. It then opens the EVAP VSV and pulls a vacuum on the entire system until a specified pressure is met. It then closes the EVAP VSV and monitors the rate of vacuum decay in the system. Some decay is normal as tank pressure rises from fuel evaporation. Anything outside of the “normal” rate can trigger a code.

(Note: Some 2003 and later models have repositioned EVAP components to areas that make access more difficult. For these systems, the manufacturer has outlined a 6-step test using the factory scan tool to assist in diagnosis.)

P0441 — Purge (EVAP) VSV Operation
To test the EVAP VSV, the PCM seals the system as described earlier. If the specified pressure drop is not reached, or the pressure drop continues past the point that the PCM commanded the valve closed, this code will be set.

To diagnose this code, first make sure no leak codes (P0440 or P0442) are set. Any leaks in the system will not allow pressure to drop and can also set this code. If no other codes are set, you can monitor the VPS signal with either a scan tool PID or a DSO on the signal wire. Command the EVAP VSV to open and look for the pressure drop on the VPS signal. If no drop is seen, check the operation of the EVAP VSV itself and check the lines for restriction.

If pressure drop is normal, then command the EVAP VSV closed and look for continuing vacuum on the purge line or as indicated by the VPS signal. If continued drop is noted, test the mechanical condition of the EVAP VSV.

P0440 — EVAP System, Large Leak Detected & P0442 — EVAP System, Small Leak Detected
As described earlier, the PCM seals the system and applies a small vacuum. It then monitors vacuum decay. A rapid rate of decay indicates a large leak, and a rate of decay just above normal indicates a small leak. Use an EVAP tester as described in the beginning of this text to isolate the leak.

One important note: When testing Toyota/Lexus late systems for leaks via the service port, you will not be testing the fresh air inlet to the intake or the CCV valve. This line and valve need to be tested separately. The CCV is a common fault item, and some models have a TSB on an updated design.

P0446 — Vent Control-Canister Closed Valve & Bypass Valve Operation
This is a two-stage test of the system’s ability to be sealed for testing. It shares nothing in common with the early system code P0446. In the first part of the test, the PCM opens the CCV and monitors the pressure increase in the system via the VPS. No change or a change at a rate less than the expected values is read by the PCM as a restriction on the fresh air inlet side of the system. This is a test for restrictions in the air inlet line. A leak in this line will set a corresponding leak DTC as described earlier.

In the second part of the test, the Bypass VSV is closed, isolating the tank from the rest of the system (and the open CCV). The pressure increase rate should drop as a result. If no change is seen, the PCM concludes that the Bypass VSV did not close.

Testing is fairly simple. To test the CCV, command the valve open and ensure that there is free flow through the line all the way to the canister connection. Closing the valve should result in no flow (see Photo 6). Again, the CCV is a common fault item, so start here.

To test the Bypass VSV, disconnect the canister side line to the valve and monitor VPS signal while applying vacuum or low pressure from the EVAP tester to the line with the valve commanded open. The VPS should read the change. With the valve commanded closed, no change should be seen.

If either valve fails these tests, proceed with normal electrical-mechanical tests of the valves themselves. You should be able to hear them “click” when commanded on and off.

And, as always, once your repair is made, verify the repair and system function before returning to the customer.

Well, there you have it. Now you know what I know about diagnosing and repairing Toyota/Lexus EVAP systems. I’m sure we could both stand to learn more, but I think you’ll find this information will make it easier and more productive for you the next time you find a Toyota/Lexus in your bay with an EVAP failure.

 

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