Then you must have a bleed screw on the thermostat housing (4x4), if so undo it 3 or 4 turns (no need to take it all the way out) then run the engine up to temp and while doing so, look for a flow of water from the base of the screw. When you see a flow of water/or steam, tighten up the bleed screw.

Thats it!

BTW good idea to set the heater controls to HOT (red zone) during the bleeding process, should prevent an air lock in the heater loop (circuit)..........