Hi everyone,

My Honda Civic 2002 (173,000) is all of the sudden overheating but in an inexplicable fashion. If I am driving in the city, traffic light after traffic light, stop and go, it will not overheat. Neither will it overheat in the highway. However, after highway speeds, once I enter city traffic, that's when it starts overheating. I brought it to the mechanic who changed the thermostat and otherwise checked it, but nothing is amiss. Of course when the little rascal is at the mechanic's, it works perfectly. The fans come on on time, the temperature stays exactly where it should be (a little below middle) and everything seems just fine. There is no visible leaks. The mechanic even said to me that he does not know what to do because everything seems to be fine. Any ideas what may cause this odd behavior? Another strange thing that I detected was that when the car was overheating and I pulled over (this happened 2-3 times), I turned the heat on full blast in the cabin so to release the pressure a little. However, while idling, no heat was coming from the vents but actually cold air. Once I started moving, the heat started coming on. I appreciate any thoughts on this and thank you in advance.