On Mon, 21 Jun 2010 19:26:43 -0500, Steve Kraus
wrote:

I have the same cap as the OP on a power washer. I looked for a vent
hole but it is not to be found.

The best guess I can make is that it is allowed to vent along the threads.
The rubbery gasket on mine has a couple of notches so maybe those are to
keep it from being a total seal. And of course the metal plate is to
deflect away as much splashing as possible. Maybe under some circumstances
residue from evaporated gasoline can block this. On mine the rubber was
pretty flat to the interior of the plastic cap. Maybe it needs to be
looser.

Sure. Just back the cap off 1/8 turn after tight. That would break the
vacuum and allow fuel to flow.