Typically when I see this concern it is usually air in the cooling system. This explains why it is over heating and why the heater is not working (or only works when driving but goes cold at idle). Make sure there is no air in the system by opening the bleed screw and allowing air to escape and it also sounds like you radiator cap is not opening to pull coolant back into the engine when cool. So your pressure cap or radiator is not function as designed.
Check for air and replace the pressure cap and get back to me
Thursday, January 31st, 2019 AT 6:46 PM