If you run with no thermostat, the system will never reach a correct temperature due to the radiator being able to get rid of more BTU's than the engine is transferring to the coolant.
I've had the opposite experience, running without a thermostat could cause you to overheat because the coolant is passing through the radiator faster than it can efficiently give up it's heat...by regulating the flow with a t-stat you give the radiator time to go it's job. It's one of the first things I ask when someone says they are overheating..."do you have a thermostat installed?"
Mike B
