There's a small hole in the thermostat plate with a loose piece of metal in it. That should be situated so it is towards the upper part of the engine (in the case of B18 that's the front) to allow the air below the thermostat up and insure that the thermostat gets submerged in liguid and functions properly. It's also a good idea to test the thermostat in hot and cold water to make sure it opens and closes. I put them on a wire so they don't touch the pot while heating on the stove.
Another problem could be an air lock within the closed system. Once the thermostat is functioning and positioned properly then all the air in the system will be in the top part among those hoses. What I did was install a bleeder into the top part of the highest hose so I could let the air out once the thermostat opens. Now I only have to relieve the pressure if I open the system for some reason...change fluid, thermostat or water pump.
Works pretty good. No more overheating.