I know lot of my friends out there have had this problem...
I was told that my 1994 gs300 had a blown head gasket, its not smoking change the water pump, thermostat and the rad-cap. all the fans works... it only bubbles if i drive it about 5 miles but , when idle its fine no problems temp is perfect... so now i had the head off and had it machine and check for leaks and warp, the head was fine. I replace the head gasket and it still warms up too fast...could it be the catalytic converter?