P0455 on 2003 Protoge ES, not gas cap or solenoid valves
I'm currently stumped on this issue I'm having with my 2003 Protege. I've had the P0455 code for the past 3 or 4 months, and haven't been able to narrow it down after replacing solenoid valves, gas caps, and testing a few components from the EVAP system with a multimeter. With my ODBII reader, I'm able to successfully pass an EVAP test with the engine off (I can even hear the gascap clicking from the tank being pressurized), but will fail with the engine on. I do hear a slight hissing when I shut the engine off, and it's coming from behind the engine somewhere between the catch tank and the master cylinder. The car will crank excellently even when I'm cold starting the engine, I don't smell gasoline, I'm still getting about 320 miles a tank with 180k miles on the engine, and the idle is just fine and sits around 700 RPM. Can anyone point me in some direction of what to check first before I go through each component? Thank y'all very much in advance!