well, a lot of arfs tell you to clue the cap of the tank to the firewall with silicone glue and isolate the rear. i had a similar problem a long time ago with my tank foaming for that reason. don't pack as much foam as you can in there. if you are actually jamming hte foam in there it does no good. make sure that stuff is not sliding around but don't shove foam in there if it requires serious effort to do so