bleeding cooling system

[piersbc]

I've read about bleeding the cooling system by opening the screw on the thermostat housing. Is the engine supposed to be running or not when I do that and should the thermostat be open or closed (cool/hot engine)? How do I know when the air is out?

[JohnC]

If you drain the radiator you introduce air into the system. The air gets trapped in the thermostat housing because it is a high spot. Enough air will prevent the thermostat from sensing the water temperature correctly. Refill the system then start the engine and open the bleeder until water starts coming out. Close the bleeder and wait until the thermostat opens (top hose gets hot) then bleed it again. When air stops coming out, the air is out. (Yogi Berra)