absence of thermostat will take the engine quite a while longer to achieve the ideal operating temperature from a cold start, even in a hot climate like here in manila. this is not good for the engine.

the absence of thermostat may not even allow your engine to reach its peak operating temperature, because the coolant will be free flowing, this is especially true when you are on highway driving.

and when the engine is not operating in its ideal temperature, it will eat more fuel.

happened to me, so i put back the thermostat.