To Stat or NOT to Stat...That is the Question?

New does not equal good. the new stat may be bad. Do not run without a thermostat

Is it in right side up? Could you have an air bubble in the system? Are you getting heat out of the heater?

Pull the heater hose that is UNDER the stat, if you can, from the heater. Fill the system til you get water out of the heater/ hose, reconnect. Warm up with cap off, watch level carefully.