Ford thought it was the flapper valve but now we find out it was a quality control problem with the supplier. The complete fuel filler pipe along with sometimes the roll over valve on top of the tank would stick and not allow air out of the tank that the fuel is displacing.
I have a letter from June 2001 program "01M04" 10 years or 100k miles. It says flapper valve but soon after the letter was sent out I heard that dealers ordered a flapper valve and got the whole filler-pipe to replace.
I usually have trouble with mine on hot days and have been on the road for several hours (heat soak). Then even my dingus I have (that worked intermittently) where it bumps open the flapper it still makes no difference.
Ah the EPA and there evaporative emissions.
The chance of going to the junk yard and going thru all the work to swap the filler pipe might get with the problem also.
The last time it happened I was out of town it happen I got mad I open the trunk lid and shook rear side as hard as I could. I helped a little.
Two days later hotter on the road --not one problem??????
Also check hose/lines back to the canister and from filler down to the tank none are pinched or restricted. Also the rollover valve mounted on top of the tank can stick also. There is also vent hole to the right side of the filler cap that has a hose that run down to the canister. At certain times air is drawn in while engine is running--I only mention this several winters back while spraying the salt off with the engine running the water hit the gas door and this loud sucking noise. Could figure out what it was oops found it I had to remove the canister a dumb out the water not to mention using many cans of dry gas. Latter I found out the mode the evap was purging the canister.
I could have had filler up replaced at no cost but decided that old just to leave well enough along. Something else would break in the process and might not be covered.