Somewhere under the hood, either on the hoses or the top of the rad, or by the firewall (on a hose), there my be a brass knob with a hole under it. This is the air bleeder. Renaults are famous for this feature. Why? Because their heater cores are normally located above the height of the radiator...You need to run the car at idle and open/close this valve until all of the air is bled out. You will see bubbles if air is in the system when you crack the bleeder. NOTE: the coolant coming out is going to be HOT. Take suitable precautions to not get burnt from the 200 degree F water that will come out of the bleeder. Hope this helps.
Ed