I have a 1994 pontiac sunbird and just replaced the heater core and inlet and outlet hoses, when I started the car up their was no pressure at the radiator cap and this was when the temp gauge was reading hot, it also seemed like it took an unusally long time for the heater core to warm up. On the second try it seemed that the hoses and heater core got very hot, and still no pressure at the cap and the hose before the thermostat was flat with no pressure, even the water in the resovoir was cool, it did warm up after awhile but not what I expected. Could I have hooked up the heater hoses backwards or do I have anther problem masking this one.
Do the heater or radiator hoses get warm/hot after driving? Does the radiator get hot? If the engine is all that is getting hot, either the thermostat is stuck closed or the water pump isn't pumping coolant throughout the system. Remove the thermostat and see if the heater core, radiator, and hoses get hot. If not, you need a new water pump.