My little brother has a 97 civic ex that is overheating while only at idle. We just replaced radiator and thermostat and still overheating only while at idle but when you give it gas the temp gauge goes back down. Also the overflow tank seems to be overflowing after car has been driving. We properly flushed system aswell. thinking its water pump or maybe temp sensor on thermostat? Any ideas?

 

air in the system

 

let car sit for about 5-6 mins with radiator cap open with car running with heater on while after we flushed it, so air should be in system

 

any other suggestions?

 

I think possible water pump.

 

Headgasket is a good possibility if there is coolant constantly filling the overflow.

While driving, the radiator is getting cool air, and it is trying to bring temps down...same thing when turning on the heater, it is evacuating heat.

Water pump could definitely be an issue, as well as a possible thermostat.

Start with the cheaper fixes, and work your way to the more expensive fixes.

 

Fan working? Usually overlooked.

 

Please don't just throw parts at it - troubleshoot!

You can take out the thermostat, put it in a pan of water with an thermometer, and observe the temperature at which it opens. This will rule out or identify the thermostat.

You will probably also want to do a leakdown test before changing the water pump - that would identify if the head gasket is leaking compression gasses into the coolant and causing it to overflow.