Check fuel cap warning came on[

get that cap from the dealership?

This obsession with not putting unburned petrochemicals into the atomsphere has us putting a device that makes it breathe it's own output...the EGR..of all tarnation, look-here, have a source of failure! The cap on my toyota seals completely, and don't know if this is right, kinda obsessive, doncha think? The Japanese will stop at nothing to comply with regulations. seems like it should 'breathe' a little, seeing as this is an American buggy, cut us some slack...to answer your question, IT SHOULDN'T. The fuel pump is designed to draw the gas no matter what, pulling the tank into a vacuum. It is designed to!...have to admit turning the cap and hearing ssssshhhhsss for more than a couple of seconds is a bit disconcerting.

my MINI just has an ordinary vented cap. bully for the sportscars!

NO new car (and that means built after 1970, by the way) has a self venting fuel cap. They should all seal tightly. That is why gas tanks have vent lines connected to a charcoal canister under the hood. Vapors are trapped in the charcoal and are then sucked into the engine to be burned off. Over filling the tank can flood the canister, and then you have to replace it. Fuel injected cars NEED to have the system under slightly positive pressure. The pump, regulator and venting system are calibrated to work together. The 'hiss' should last no more than about 1 second when you take the cap off. Of course, that is not applicable if the car is not running. The vent system should release the pressure when the engine is turned off. In this case, I would guess that you have the incorrect cap or the venting system is not working correctly. Make sure you get the proper replacement cap.