Ok, here is a new one. After almost a 1,000 mile trip, after the last fuel stop before I came home. All of a sudden on my acceleration to get back on the interstate, I hear this hissing that increases with throttle input. Also seems like there is a drop in power. It sounds like it’s coming from behind the seat! It idles fine, no code, low speed is fine, but as soon as I get on it past about 1/2 throttle, or if it is under a load like going up hill, the hissing starts and increases with the throttle input. It feels/sounds just like a turbo leak on a big turbo diesel! Except it is just a stock ‘91 5.7 Tbi with a 5- speed, regular cab, short bed. I know the fuel tank is just below and behind the cab. Could it be the evap. system is leaking on top of the fuel tank? Exhaust leak? I would think an exhaust leak would just be loud and not a hiss. I dunno, anybody heard of this? Thanks.