in1995 civic. 130,000 miles. A/T. Temperature guage never gets above about 1/4 of the way to hot. Can't find a leak, but every morning it takes about 1/2 gallon of water to fill up the radiator. Heater never gets really warm. Water in radiator gets hot, but doesn't seem to be circulating at all.
I've replaced the radiator it had a crack below the filler neck when I bought the car a few months ago. I've removed the thermostat and tested it several times. It opens consistantly a pot of water when it reaches 180 degrees. I don't think I have water circulating. Water pump makes no noise and doesn't seem to leak, but even when it warms up and the upper and lower hoses are warm, the temperature gauge never goes very high. The fan never comes on. I've tested the fan and it works. I just replaced the temperature switch for the fan, but the fan still does not come on. I don't think the temperature is getting high enough to make the fan come on. I've flushed the system out and the water flows good through the heater core with a garden hose.
Why doesn't my heater work? Heater control valve works perfectly and the door under the dash opens and closes when you turn the heat on. Just no warm air.

 

It's losing water somewhere but you can't find a leak. Sounds like a head gasket issue. Check the engine oil for coolant or any kind of foamy oil, also check to see if there's alot blue/white smoke coming out the tailpipe when you step on the gas(you can do this in your driveway). If all is good then do a compression test. all cylinders should read the same readings, if not..than that's your clue.

 

The only reason for the engine not to warm up is a bad thermostat. Regardless of what your test did, replace it.

But the leak is a bigger issue. Usually the first symptom of being low on coolant is the heater stops working even though the engine is hot. Rev the engine with the radiator cap off and see if any air bubbles out due to a head gasket or other leak in the engine.

 

Bad water pump and bad head gasket. When I bought the car it had a cracked radiator. Someone tried every stop leak in the world and it actually damaged the water pump. With no water circulating, the heater doesn't get hot. With no water circulation, the temperature gauge isn't very accurate and the car runs hot at lower temperatures. With no water circulating the cooling fans don't come on until it's too late.
Again, you test a thermostat by putting it in a pot of water with a thermometer and heating it up. You can see the thermostat open up and you can see what the water temperature is. It's an easy test and pretty cool to watch.
Some folks start by replacing the thermostat and see what happens. I prefer to check the thermostat to see if it's working instead of simply asuming it's the problem. And yes, the headgasket is bad too. Probably caused from the guy before me constantly running it hot with the cracked radiator.