Having problem with 1999 civic over heating, it's not leaking any fluid, the thermostat seems to be working fine, I believe the water pump is working fine, let the temp come up, when it reached normal and the fan kicked on one time, but would not come on as the temp increased. I suspect some sort of temp switch, but the gauge is showing the temp correctly. So I'm not familiar enough to diagnose. Any ideas would be greatly appreciated.....

 

Could be a head gasket issue or possibly a flakey rad fan relay.

 

Is the radiator staying full (not just the tank, the radiator itself)? If not and you don't see coolant leaking out anywhere, it could be the head gasket(*). If the radiator is staying full, likely you have a balky thermostat getting stuck sometimes. Just replace it. Its not expensive at all so it's not worth trying to test it.

(*) Simple head gasket test: Engine must be cold. Take cap off and fill up radiator if necessary. Start the engine and rev it with the cap off. If bubbles blow out constantly, gasket is leaking. You can make the test more sensitive by holding your hand over the radiator neck (do this only right after starting the engine so everything is still cold) to direct gas into the reservoir tank and look for bubbles there.

 

tend to rule out head gasket, no coolant in oil, or steam/white smoke but i will try that head gasket test, what was confusing was the fan kicked in one time when engine reached normal temp, but when it continued to heat up the fan would not kick in.

 

If the thermostat does not open or the system is very low on coolant, the fan will not come on. The fan is activated by the temperature of the coolant in the lower radiator hose, not the engine itself.

 

The one thing to check, are the rad hoses firm not mussy when the engine is warm. I say this because if there is air in the system it can cause this issue, with air in the system the hoses are not hard they are mussy when you grab & squeeze them. I just ran into a thermostat & air in the system problem. Someone replaced a heater hose & didn't bled the system, it lead to a thermo failure.