If you have a Durametric and want to test the valves see this post #6 and #7
http://986forum.com/forums/boxster-general-discussions/75684-hard-restart-after-fueling.html
I temporarly fixed the issue on mine by putting a small piece of masking tape to partially cover the vent hole(half of it). The one located a 3 oclock in the housing.
A foam would work as well !