OK, I have a 91 350 tpi AUTO. My car runs great when it is cold or for the first ten minutes or so of driving. Also, it runs really smooth when it has below an 1/8 th of a tank! Ok, so when I floor the car (WOT) it pulls good,but after WOT runs it starts running really rough. A lot of hesitation between gears. OK, I have narrowed it down to a fuel issue. I have new injectors, fuel filter, fuel pressure regulator, diaphram. I have been told that I may have a bad gas cap.LOL. Maybe, I guess. What else could be causing this? It only runs bad after WOT or on a hot day. I am using a locking gas cap that I bought at Walmart. I know one thing, when I almost ready to run out of gas the car runs awesome. Feels really smooth, like a turbo car. Also, when I remove my gas cap to get gas the cap has a lot of pressure behind it. It goes bffffffffffhhhhhhh!!!!!!!! Any ideas?

 

Perhaps the fuel strainer is clogged in the tank, or fell off. Bits and pieces getting sucked up into the secondary screen.

 

maybe something wrong with evap system, or maybe the gas tube either dont have a relieve line or its clogged up building pressure in tank, releasing when you pull cap off, hense the reason car would run better at lower fuel levels the pressure is not as great when low on gas...... OR Maybe ther right........ gas cap is problem, some gas caps have releave holes in them instead of a hose running from the tank to a vent in tube..... if you have a cap without a hole and no tube int your tank thats the problem.....easy cheap check........gas cap

 

problem goes away when engine cools down. dont have the problem when below 1/8th of gas though.