You won't blow up leaving the cap loose driving to work. But just curious you say you smell gas and it gets crappy mpg. Along with a loss in power. Have you bothered to possibly check the fuel pressure?

The regulator could be bad if it is the original. That wouldn't explain your hissing. Cept that I believe there is a burp valve or some sorts back at the tank that vents the tank when the car isn't running. So it may have been normal.

 

i had the pressure tested a while back and it was great, but that might have been before the problem started. it only happens when i drive the car around then stop i will notice the hissing sound coming from the rear of the car then i open the gas cap to bleed the pressure and it stops