I often drill a small hole in the therm. itself to vent the air out. You can vent it by taking the heater hose loose at the connection just under the therm. As soon as coolant comes out sock it back on. That will vent out the air under the therm. If it still overheats after that you have another problem. A lot of new therm. are not good. If it heats up right away that may be the problem or the rad. could be plugged up.
