Everything you say is true. When I open my fuel cap, air rushes in for approximately 2-3 seconds, quite a long time! Even with the cap loose, the vacuum is causing the cap to seal. I checked it yesterday when I got home and pulled it away from the filler neck and heard a short hiss of air entering the tank ... not as bad as when it's screwed down. I don't know what is causing it, but I'm going to fix it!

 

FASS is in, all is good. I started the truck right before I did the conversion to make sure I was still having FP problems and was. Installed the FASS and now have 16psi at idle and 14psi WOT. Money well spent. Thanks for all the input.

 

there is a vent on top of our tanks, i remember seeing a post on this subject. but i don't know how to find it....the module has a vent that may have a plug or a cap on it, remove the plastic cap and you should be good to go