I have a 2003 Expedition which was throwing the P0455 - EVAP code. The first thing I did was change the gas cap and it did not fix the code. I had been able to smell some gas fumes at certain times and for the first time yesterday after the tank was filled up and we parked on a steep incline the tank was leaking out of the top near the back of the tank and truck. It only leaks while on steep incline.

I have read about the code and could not find any hoses disconnected. My question is could the evap purge valve being stuck open or closed cause the leak? Or is this an issue with the tank or vents from the tank? Has anyone had a similar problem?

 

Find where it leaks fuel. That's the same leak as is causing the P0455. That makes it really easy.

 

On an old car I used to have when it was filled with gas and parked with the nose pointed downhill on a steep hill, the tank would overflow through the tank vent to the charcoal canister. then it would create an over rich situation because the canister had a vacuum pulled on it from the intake manifold. I know your vehicle is much newer but I thought I would throw this out there for whatever it was worth. On the old car, the carb vent was switched to the charcoal canister when the engine was shut off and the pressure of the hot gas fumes in the canister would force the gas level in the carb down and gas would squirt through the jet into the carb venturi after the engine was shut off. I scratched my head over this a while before I figured out what was going on.