I had exactly the same issue on my '01 last year. Replaced the filler neck and grommet and it solved the issue.Back from the dead with this post but I got the P0442 in 03558. Took it to the shop for a smoke test as replacing the gas cap didn't get rid of the code.
Root of my problem is the gas filler neck and grommet into gas tank. Getting it fixed up now with Ford original parts. Sad thing is that the car only has 20,000 miles on it so most likely the gasket cracked from being dry. At least that's my thought.