I've recently noticed that my Civic overheats after driving on the highway. The car is fine when driving anywhere else in town. The heating problem isnt really an issue while on the highway, only when immediately exiting the highway onto local roads. The coolant gets to a boiling state and starts to overflow from the overflow tank. There does not appear to be any leaks elsewhere. I have already replaced the thermostat with no luck. Both fans on the radiator appear to be running properly. Someone recommended that I turn the heat on full blast if the problem occurred again in order to lower the engine temperature. I did that and noticed cool air coming through instead of hot. Anyone have any suggestions?

 

sounds like the system is empty
recheck your coolant level when it cools down or first thing in the am.

 

Have you properly bled the system? It could be air in the lines.