Hy Sabrina, With no thermostat water is constantly circulating through engine and radiator so much less likely to get too hot. With thermostat water is basically held in the engine until it reaches opening temparature of the particular thermostat,then the water is allowed to circulate from the upper water hose to the rad down through the rad rejecting heat as it drops down until it reaches the bottom of the rad where it is picked up by the water pump and ciculated through the engine again. If your getting steam from the rad and it is new I would suggest one of two problems may be the culprit, an air lock in the rear of the engine block or head, or an actual obstruction [crud buildup] in either of those two areas. You could try loosening your sending unit at the back of the head and making sure water comes out, if thats o.k. I think I would reverse flush the block, take the thermostat out and put your garden hose there, take the bottom rad hose off and direct it towards the ground, now flush water through the engine with the garden hose and see what comes out, this is done with the engine cold and not running, hope this helps.
