I would say that the coolant isn't moving and the thermostat isn't opening. With my 216, if I don't remove the air after putting a thermostat in, I get an air block which won't go away by itself. I initially run it until the temperature gets above 180. I then turn it off and pull off the heater hose. Steam and hot air comes out then then the coolant starts to emerge. At that point I put the hose back on, wait for a few minutes and start it up. The thermostat then opens and it works correctly.
I would also try running it without the thermostat to see if it stays cool that way. That would give you an indication of what is going on.
