Toyota Repair: closed fuel system, vent system, computer inputs

Question
I have a 1990 V6 Toyota 4-Runner. Milage when problems started (76,000).  I believe that my fuel system is a closed system.  If I tighten my gas cap as it should be the car has very limited power and the mileage is reduced to very very poor. Also after I've been on the road, when I loosen  the cap to gas up it releases pressure for up to a minute or more.   If I leave the cap loose it
runs normal and does not build up pressure. I know I could continue to leave the cap loose but I know this isn't right and I would like to see if I could have it repaired.  I took it to my Toyota dealer and they had no luck.  Said all computer inputs normal, compression normal and fuel pressure normal. I've replaced the fuel cap.
If you have any ideas I would sure appreciate it.  Thank you for your time--Jim Hesser

Answer
I'm not sure what is going on here but the problem seems to be in the vent control system of the gas tank, this is also called the fuel evap system, there may be a blockage in the vent system causing excessive vacuum in the tank, make sure that you use a factory OEM replacement fuel cap.