Well been troubleshooting a friends car for awhile now. been through everything. Its a 96 Civic Single cam motor(STOCK) The fan is wired to always be on when the ignition is on. I took out the thermostat to make sure thats not the problem. I bled the system 1000 times. After warming up and driving for about 15 mins. If the car slows down (700-3000rpm)or is idling the temp gauge starts to rise. There is no evap of coolant. after letting the car cool down and opening the cap, there is no change in the coolant level in either the radiator or overflow. It seems that it has to be at RPMs above 3000 inorder for the temp gauge to stay normal. I have replaced radiator, CAP, Different Fan, Bled the system. Taken out the thermostat. THis motor was running fine in another car with no overheating problems but after transfering it to this car, it started to

 

It's possible the water pump impeller is corroded and is not creating enough flow at low rpm.

 

replace the water pump, and put a thermostat in. the thermostat helps with pressurizing the system due to it having a smaller hole for water to pass through than the rest of the system. my friends old car did exactly what you describe, he kept bsing, changing therm, bleeding, he was avoiding water pump cause its not a 10 min process. he finally changed it an fixed the problem.

 

i agree ^ make sure you put a NEW thermostat in

 

thanks for the input, i put a new thermostat and replace the waterpump and flushed the system. seemed to do the trick