Greetings fellow Toyota owners! Is your check engine light on? Do you have the famous p0440, p0441 or p0446 stored in your computer? Well you're in luck! I am going to talk a little about this problem Toyota has been having, how to fix it, and how to prevent it from coming again.
Did you know that Toyota issued a Technical Service Bulletin issued for this problem? The Technical Service Bulletin simply goes into explaining that one or more of the above codes stored in the computer are due to an inoperative Canister Closed Vacuum Switching Valve. This TSB from Toyota allows the technician to quickly identify the problem so the tech can assign the correct fix. Or does it? Some of these Diagnostic trouble codes are fixed by replacing the Canister Closed Valve Vacuum Switching Valve (VSV valve) located on the air cleaner box, however most are not.
You see, all these codes are saying that there is a leak in the Evaporative emission system. The evaporative emission control system advances fuel vapors generated in the fuel tank from escaping into the atmosphere. These vapors are stored temporarily in the EVAP canister. When the vehicle is in operation, fuel vapors stored in the EVAP canister flow through the EVAP purge solenoid, purge port and take manifold plenum to the combustion chamber and are burned in the combustion process. This increases gas mileage and decrements the emissions as well. The leak in the evap system could be anything from a loose or missing gas cap, to a stuck open purge valve.
This is where the diagnostics come into play. Many shops will just replace the vacuum switching valve as proposed in the TSB. But this may not take care of the problem. What many Toyota owners do not know, is that by topping of their tank after the gas pump has already stopped can cause serious problems in the evap system. When the gas tank is overfilled, the gas will actually back up and go into the evap system. Because the Evaporative Emission system is only designed to work with vapors, as soon as liquid gas enters the evap system, it starts to create problems immediately.
Another cause is when the charcoal in the Charcoal canister breaks apart and contaminates the whole system. This can happen with age as well as by overfilling your gas tank. The little charcoal bits will actually make their way from the rear of the vehicle by where the canister sets, to the front of the vehicle to the purge valve and the canister closed vacuum switching valve. When the computer opens and closes these valves, the charcoal bits will lodge into the all valves in the system causing them to stick open when they are supposedly to be closed. This is where the computer will recognize the leak in the evaporative emission system and flag your famous evap codes.
The fix: VSV valve. If the charcoal is breaking up in the canister, it will need to be replaced. It would be important to also replace the purge valve and canister vent shut valve as these are contaminated as well. Blowing compressed air through the lines will clean out any access derbies that will contaminate the system in the future.
So as you can see, this is a huge problem and Toyota is aware of it. Toyota has taken the right steps to fix this problem for the later generations models.