I have a 97 Honda Civic EX and it started overheaing about a month ago. It overheats usually just when it is sitting still or moving slow but when it first started this we were on the interstate. We have put a new radiator and fan in. It was not wanting to hold the coolant it had a slow leak in one of the hoses so we replaced the hose at the same time as the radiator and fan. It would still overheat so we thought ok maybe the thermostat. The termostat was just replaced not to long ago so instead of putting another in they just took it out to try it that way. So now it has a new fan new radiator and running without the thermostat. It is still having an issue holding the coolant but cant find where we are loosing it from. What else could cause it to overheat and where could the coolant be leaking from now?

 

One good possibility would be the head gasket. I can tell you how to test it but I'm not sure if you have an air compressor or not. The best way to check for a head gasket leak is called a CLT (cylinder leakage test and yes it's better than a compression test haha). Let me know if you have a compressor or not, because if not no need for me to spend 20 mins typing up a long post on performing a CLT test haha.

Easy test: check oil for any milky like substance, check coolant for any traces of oil, or white smoke out the tail pipe is and indication of coolant entering combustion chamber.