Diagnosing and successfully repairing evaporative emissions, or EVAP, system codes can be a challenge. Leak criterion has gotten smaller, so even a pinhole in a line can be enough to illuminate the Malfunction Indicator Lamp (MIL) and set a code. Over the past 15 years or so, Toyota has used three distinct EVAP systems designs, each with their own unique little quirks. This month, let’s take a look at these designs and offer a few tips on their repair.
Early (Non-Intrusive) Type EVAP Design
The early type Toyota system was used on a number of models from the mid-1990s up until the early years of the 21st century. It is a non-intrusive design, meaning that the PCM plays no active role in its testing, instead preferring to watch the system and observe it in action. A good way to identify an early system from a late system is to look for the canister fresh air line, attached to the intake air boot or air cleaner housing. If there is no solenoid controlling that line and it’s pre-2001 or so, it’s likely an early type design.
In this design, the ECM monitors a signal from the vapor pressure sensor. This sensor is not, as you might expect, always attached to the fuel tank but can be mounted in the engine compartment on some models or at the canister on others. Instead of electronic valves, vacuum operated valves called VSVs are used to switch the feed to the pressure sensor from the tank side to the canister side for testing.
During testing, the ECM switches the 3-Way VSV to the tank side. If all is normal, the ECM will expect the pressure sensor (VPS) to read anything but atmospheric. If it doesn’t, a code P0440 is stored. And on this design, the P0440 is always on the tank side of the system.
Next, the ECM switches over to the canister side. Problems on this side can set several codes. Code P0441 is a purge flow code and will set if the ECM detects either no flow or flow when there shouldn’t be. If this code sets all by itself, check the operation of the purge solenoid, and the lines leading from the solenoid to the canister. If a code P0446 is set along with the P0441, check the operation of the 3-Way VSV and the VPS. On these designs, canister failures would fill the lines leading to these two sensor with bits of charcoal and required replacement of all the related parts.
Late (Intrusive) Type EVAP Design
In an intrusive design, the ECM takes an active role in testing the system. It does so by closing the vent valve, opening the purge valve and drawing the system into a vacuum. It then closes the purge valve and monitors the pressure sensor for change. Most late type designs can be easily identified by the location of the CCV, or Canister Closed Valve (the vent valve), on the side of the air cleaner box. In fact, it is a common cause of multiple codes in the ECM.
If you see codes P0440, P0441 and P0446 stored together, check the line leading from the CCV to the metal tubing leading back to the canister in the engine compartment. Well meaning individuals remove the air box to inspect the air filter for the customer and accidentally pull this line off in the process, resulting in the codes.
A few notes on the late type system. There were TSBs for updated CCVs for some models, and in 2003 or so, Toyota moved the CCV to the canister. Access for testing these designs is awkward to say the least, and a better method is to use a capable scan tool to test the system. The procedure is outlined in Toyota TSB EG048-04.
Also, while code P0446 might share the same name as the early type design, ECM strategy for setting this code is not. In this case, a two-step test is performed. First, the ECM opens the CCV and monitors the system for pressure increase. No change or a change at a slower rate than expected is seen as a restriction by the ECM. This design also uses a Bypass Valve to join the two sides of the system for testing and its operation is the second part of this strategy. The ECM is closes the Bypass Valve with the CCV open, and again monitors the pressure signal. That rising pressure rate should come to a screeching halt if all is well. If not, the ECM concludes there’s a problem in the bypass circuit.
Leak Detection Pump EVAP Design
The increasing focus on reduced emissions and the fact that nearly 20 percent of all automotive hydrocarbon emissions comes from evaporative sources has led to the development of leak detection pump and natural vacuum decay methods for testing EVAP system integrity. These tests do not require the involved drive cycle earlier designs did, and most conduct their testing with the car parked.
In Toyota’s case, the leak detection pump draws the system into a vacuum after the vehicle has been sitting for five hours, providing the engine has cooled enough. If not, the ECM waits another two hours and tries again. If the engine temperature is still above 95 degrees Fahrenheit, it will wait another two and a half hours before making one more attempt.
If all is ready, the pump draws the system into a vacuum and monitors system pressure to test for leaks and the ability of the components to seal the system. This does not test the purge side of the system for operation, though. That is done with the engine running and a two-step test. The first step is run every key start. The ECM monitors purge flow by watching the system pressure change with the purge valve open. If a problem is detected, the ECM will open the vent valve. If pressure does not drop as expected, it will set a code P0441 for a purge flow restriction.
PAGE 3Toyota/Lexus EVAP designs, especially the older ones, might be a little different looking but the job is the same. Spend the time up front to properly identify the system you are troubleshooting, and learn how it works to speed up your diagnostic time.
A Few Basic Tips
The MIL is on, and an EVAP leak code is stored. A common response is to head back to the rear of the car, open the fuel door, and turn the cap to see if it’s on tight. If the cap is not on at all, there is nothing wrong with this approach. However, if there is a leak at the cap and you tighten it before you test, then you’ll never know if that was indeed the cause. So, leave the cap alone and use your EVAP test machine or pressure/vacuum pump to test the system properly.
Because I mentioned the tester, here are a few tips on using it efficiently — or at least, what works well for me. Most every tester on the market has a pressure function and a smoke function, and most every tech I’ve ever watched selects the smoke function first. Instead, use the pressure function and save the smoke juice for later. On the front of every machine I’ve seen is a check ball and gauge meant to be used as an indicator of how big a leak is in the system.
Personally, I’ve never used it for that specific purpose. Instead, I like to pressurize the system and look to see if the ball will make it all the way to the bottom. If there is little fuel in the tank, it can take a while to air up the entire system so be patient. There is no question there’s a leak if the ball never reaches the bottom, but there is still a question if it does and you can test for that simply enough. Once the check ball reaches the bottom of the gauge, stop adding air and wait 10 to 15 seconds. Then hit the switch again while watching the ball. If any pressure was lost, the ball will jump off its stop and drop down again. This indicates a small leak somewhere in the system that you’ll need to track down.
If you’ve determined there is a leak, the next best step is to break the system down into parts, starting with the engine side of the system (EVAP purge solenoid, line and part of the canister) and then the tank side of the system (everything else). Test each section just as you would when tracking down a break in an electrical circuit until you isolate the cause of the leak. If it’s not obvious, switch to smoke mode on the tester to help you locate it.
Lastly, if you have leak codes but no leaks, check the purge solenoid for intermittent operation (stuck open or closed) and the fuel tank pressure sensor or other feedback sensor for proper operation.
