I have a 2003 rodeo with a 2.2l with a new head, head gasket, water pump. When it is running with the new thermostat it will run hot. So I used the old thermostat housing and remove the thermostat and it will run just fine, no overheating problem. My question is what is causing it to overheat when the thermostat is installed.

 

The thermostat can be put in upside-down, a common problem. It can also fit in backwards, a less common problem. Either of those can cause it to overheat. Also, if your particular thermostat is stuck closed, replace it with a new one.