My father in law's Honda Civic overheats when the water reservoir is full or to the top. He is wondering what could be the problem if the radiator needs water when the reservoir is full. The temporary solution we found was to use a sponge or something to suck out the water from the reservoir to leave it leveled and pour some in the radiator. But it would overheat after a while due to the same problem.

We thought it could be a sensor but he changed it a while back. Also the thermostat is new the radiator as well and the hoses.

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    YOu could test the thermostat on the stove, with a thermometer. Makes sure it opens at the 'set' temperature...Perhaps 80 degrees C. Quite a few 'new' thermostats fail. – Grantly Oct 20 '16 at 20:51
  • We will do that. Another thing. If it's overheating it's because the radiator isn't getting enough water because the reservoir is full. Could this be a problem too? – OB1gamerxD Oct 20 '16 at 21:37
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    Overheating is a combination of incorrect flow, pressure and amount of water. You do not really need a 'full' radiator to cool adequately, although it helps. The reservior (TTBOMK) filling from the radiator too fast is not a good sign, it seems the water is not flowing properly...Smaller hoses? Blockage? Dirt? Have you tried without the thermostat? And leave the radiatir cap off as another test (if you can) to see if the water flows when warm... – Grantly Oct 20 '16 at 21:40
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    You have "burped" the system? (get all trapped air out) Honda 05 CIVICs are sensitive to air in the cooling system. – spicetraders Oct 20 '16 at 22:30

Most common cause of these two symptoms together on this model is a failure of the head gasket. Testing for a failed head gasket is the next step I would take on this issue.

What happens is that the gasses entering the cooling system from the head gasket leak displace coolant into the reservoir. The overheating occurs because the large volume of gas disrupts coolant flow through the system.


the head gasket has failed, allowing combustion gasses into the water jacket. Easily checked with a special tool / device to detect carbon monoxide in the cooling system. This device is placed over the radiator fill point WITH THE ENGINE RUNNING and the blue liquid inside changes colour to yellow in the presence of carbon monoxide (exhaust gas) confirming the fault. To localise, if you have a radiator pressure tester, switch the engine off, remove all spark plugs and then pressurise the system to 15 pounds per square inch overnight. The next morning crank the engine and you will see a jet of water from the offending cylinder erupt from its spark plug hole. Sad and costly, but at least you wont be ripped off by some losers poor diagnostic skills as he removes cylinder heads or your entire engine for total welch plugs replacement!

Cheers Hutch

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