That hole in the t'stat gooseneck/housing is a vent for air to the radiator, it's supposed to have a ball bearing in it, it will stay open as long as any air is passing thru it, but will close when coolant tries to go thru it. If the ball is missing a lot of coolant will go thru it and delay the engine from coming up to temperature. also the t'stat will not function as it's suppose to.