1994 Toyota Corolla gas fumes

We recently purchased a 1994 Toyota corolla,
after driving the car several times, we noticed
after about 20 minutes a raw gas smell, we took
it to Toyota, and they suggested replacing all
these parts, they could not pin point the problem,
and our mechanic has tried new fuel lines, another
part that is like a holding tank? Not sure. He keeps
trying but we still have the fumes in the front of the car, especially when its warm out. If we drive for more than 40 minutes we have to roll up the windows and turn on the air conditioning which works fine. Any help would be greatly appreciated,
we really like the car, and it drives great except
for the gas fumes. Thanks much, J