I've had the experience with two cars of adding coolant to the radiator and the system developing an air lock where the heater blew cool air afterwards. On most newer cars, there's little 'petcock' screws that need to be opened to allow air out of the system. Does the top and bottom radiator hoses feel about the same temp (so you know you don't have a thermostat problem)?
Assuming the thermostat is okay and it's an air lock, you can do more research to see the procedure for clearing any air lock OR if you took the car in for a radiator flush, they generally know what and where to look to return a good working heater to you.
Thursday, April 5th, 2018 AT 7:50 PM