I have been running an open housing for 2.5 months. Just the outer housing of a thermostat, with the guts removed. Car has ran fine, just on the cold side. Now that it is closer to winter time I wanted to put a new oem thermostat in. this is where questions come in.

When I installed it the car over heated after 2 miles. I took it out and place it in water on the stove and it opened fine. I put it in again and same outcome.. overheated.

When it warms up and over heats even the heater doesnt have hot air coming out.

The bottom hose coming out of the radiator is cold as hell. One of the heater hoses is cold, the other is hot. When running it has water pressure coming from the water pump. Plus the water pump is less then 5k miles old.

This is all on a 1991 integra. It ran fine until today when all I did was put a new thermostat in.

Water pump is newer and fan turns on.... anyone know what to do?

 

Sounds like your either low on coolant or you have air in the system. Try adding water and bleeding the system, should have a bleeder on the thermostat housing.