could be a bad water pump. When i first picked up my 1996 monte carlo with a 3.1 liter in it for 300$, I was told by the mechanic that it had a blown head gasket because it was constantly overheating. Well luckily my dad is an ASE certified mechanic so I bought the car and was going to replace the head gasket with his help. After he looked the car over though, the head gasket was fine. Turns out, the water pump had stopped spinning. The car wasn't losing any fluid either. So threw in a water pump and was good to go till i replaced it two years later with my 2006! Hope this helps