My '05 Civic has had no heat at idle. When the engine is reved at 2k rpm the heat begins. This had been happening for a few weeks. Now the car is starting to run hot. Not overheating, but hot. Temp gage is between normal (middle ) and hot (H).

Changed the thermostat and drove the car for 20 minutes. No heat for about 10 minutes, then the temperature in the car got nice and warm. Shortly after, the temp gage crept toward overheating (H) and air turned cold. The gage never reached "H" and then slowly came down, but still hotter than normal. The lower radiator hose is not that hot. Antifreeze levels seem fine, no sign of leaks and engine oil seems clean with no water or coolant mixed in.

Anyone have any tips on what this is? Thanks

 

Is the radiator staying full? Don't just check the plastic bottle-- when the car is cold open up the radiator itself. A leaking head gasket will push coolant out of the engine and radiator into the bottle and it will not return. Heater not working though the engine is hot means there is a bubble of air in the system.

 

i post this constantly but ill post it again cuz its good info watch it ok. the issue your saying usually happenes when there is a air bubble trapped in your coolant system.