Probably an air pocket. T-stat is on the bottom hose so you'll have to work at it. They make vacuum fillers to pull a vacuum on the system to help remove the air. There may be a bleeder valve some where also.I've pulled the upper hose off the rad and fill the block/heads through that then reinstall and fill the rad, run it, open the bleeder to help remove the air. Good luck....it can be a pain!