Could be a bad water pump.
Running a vehicle without a thermostat will cause it to take longer to warm up and your interior heater may not blow hot air.
Cracked.head
Without a temperature guage, the most acurrate way would be to use a infrared temp. gun and aim it at the thermostat housing/radiator/water pump.
Thermostat and radiator
That depends on the thermostat. If it's a 185 thermostat, the engine should run at 185. If it's running too hot, check the thermostat, radiator and water pump. If it's running too cold, replace the thermostat.
If water is coming out around the thermostat housing, the seal or gasket is bad.
Thermostat sticking. Replace thermostat and gasket.
Thermostat may be bad; or you may have a bad temperature sending unit You may have blown a head gasket when it overheated if you continued to drive the truck.
It's likely the thermostat. That's assuming the coolant is full. Thermostat runs $11 for the 3.5.
Is you Geo Tracker overheating or the temperature is running almost hot then you need to change you thermostat
IT should have a 195 degree thermostat in it so the computer will work right And the heat will be hot. THAT IS WHAT THE FACTORY USES
This is likely a stuck thermostat.