If anyone lives near the DC,Md,Va arear I would love for you to come look at my car and help me find the problem. I have a bone stock 1991 Mustang lx 5.0. I am having a weird overheating problem. When I drive the car normal the temp gauge never leaves half way. when ever I get up on it for a little bit the temp. climbs all the way up. Once it goes all the way up it will not cool down until I turn the car off for a little while. Last week I drove it 2 hours and it never left half way. I was about 40 miles from home and I punched the gas for about a minute. The temp went all the way up. I then drove it normal with the heat on for the rest of the way. and the temp stayed close to over heating. The gauge does not go into the red it gets to the last mark before red. I replaced the thermostat and it still does the same thing. Let me know if anyone could have a clue as to whats causing this. I now it is getting close to overheating because when I parked it there was antifreeze bubbling out of the overflow (Barely dripping). It only does it when I accelerate hard for a little bit. It is weird.
Have you checked your temperature sending unit? Possibly a blockage. Check your sending unit found on the drivers side of the intake manifold near the front. Also make sure that your water pump is working fine.