coolant in the radiator goes back into the reservoir but coolant in the reservoir doesn't go into the engine. I have to keep pouring coolant into the radiator to keep the temps down. bad thing is the reservoir overflows with the new coolant. I've changed the thermostat already, doesn't work. I wonder if the radiator is clogged or something. Anyone know of a solution? Thanks.

 

flush the radiator out .look for debris floating in coolant

 

If the problem started when you changed the thermostat, I would say your water pump has lost prime and is not circulating coolant. But if it started at random, I would check your water pump.

 

It could be a blown head gasket forcing exhaust gas into the coolant, causing the radiator to spew into the overflow.

How/when did the problem start?

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I agree with ChrisKatana about the head gasket.

Before you do anything, change the radiator cap first, since it plays a role in letting coolant go backwards and forwards into the expansion tank.

Before you start on a wild goose chase, simply ask your mechanic to do a compression test on all 4 cylinders.
It it indicates, a blown or failing gasket, then that's the cause of your overheating problem.
My civic's overheating problem caused me to spends lots of time and money to diagnose. At last, it was caused by a failing/partially blown head gasket.

 

Compression tests are easy, go buy a guage and do it yourself. The mechanic will prolly charge like $50 jus to run the test. If you can change spark plugs you can do a compression test.

 

Hey this is an old thread but any help would be appreciated. I have had a compression test, it indicated head gasket is good. I replaced the radiator and the cap, hoses,cooling fan sensor,thermo,water pump and timing belt. The problem is every 4 days the radiator is bone dry and overflow tank is full and car runs hot. I have to add approx. 2 qts of coolant to the rad. Is this a slow leaking head gasket that is undetected by the compression test?
Please help, this is driving me insane. This is on a 1995 Honda Civic LX, 144,000 miles. Thanks in advance for your help.