Toyota Repair: check engine codes for emissions, 2005 toyota rav4, toyota rav4

Question
Hi,
I have a 2005 Toyota RAV4 with 74,000 miles on it. I have never had any prior issues with the car. I have been trying to get the check engine light issue fixed for months to no avail. I don't have the specific codes but they're all emissions related codes that seem to escalate in number over time. My first attempt to get this fixed was to take the car to the certified Toyota Service center. They kept the car for 3 days, and replaced the fuel cap. Three days later, before I had enough drive cycles to get it inspected the light came back on. Back to the Toyota dealer. This time they kept the car for 5 days and the certified mechanic and his supervisor went over every inch of the car and finally said a valve on gas tank 'might' be intermittently sticking so they replaced that. Three days later the check engine light came on again. I've had it to two other mechanics since and they can find absolutely no cause for the PM errors including an emissions error that indicates a gross leak# they have tried the full manufacturer diagnostics, Smoke test, dropping the fuel tank; everything. They can't find the cause of the problem. I have asked if it could be the computer but I've been assured that it is almost never the computer. Do you have any idea what the problem might be?

Answer
Hello, without knowing the specific code numbers it's a little difficult to make an assesment of what is going on with the evap system but if they tested it for leaks using the leak detector and found none then it's most likely a componenct failure such as a vacuum switching valve, the most common are the evap purge vsv and the canister closed valve. Other than that the computer may be the problem so I would not rule it out.