I have a 1998 Toyota Rav 4 with apporximately 140,000 miles. When I fill the gas tank the check engine light comes on, the engine runs rough (like a misfire). Diagnostic code comes back a “lean mixture,bank 1”. When the tank reaches approximately 3/4 of a tank the light goes out and the engine runs smooth and fine. Then, when the tank reaches just belwo hal a tank the light once again comes on and starts running rough again. Have drained the tank, used dry gas, Startron to counter the ethonal etc. Nothing seems to work.
The ECM may be testing the Rav 4’s fuel tank evap system based on the fuel tank level. Often these tests are done at transitions to 3/4 full and/or 1/4 full. If one of the evap valves isn’t working correctly, like the purge valve, it could cause such a symptom. You might want to take a look at this link.