Today when i filled up i noticed there was no hissing sound when i released took off the gas cap. Kinda bothered me so i drove the 20 miles home and then filled up again, no hiss and extremely poor gas mileage 16.3 MPG with 100% freeway driving (55-65 mph)

Only thing that has happened recently was i cleaned my cold start injector, cleaned the throttle body, and the douchesicles from Buckys exhaust blew my cat converter gaskets (and ive been driving around with one missing bolt from the cat to down pipe connection).

I would think the most obvious thing that messed it up would be the cold start injector, I took it out cleaned it and re-torqued the fuel delivery pipe to the rail at 22 ft lbs

 

Mine only hisses sometimes. For me, I think it is determined by the temperature outside. If it's really hot, it will do it. If it's cold, not so much. My dad's isuzu truck was the same way.