Problem:

So my 99 Civic has been overheating every so often. The overflow reservoir has been overflowing itself with coolant rather than circulating back to the radiator. The upper and lower hoses become very stiff with pressure after running it for a while. The other day I ran the AC for a little while and that caused it to heat up.

What I've replaced:

-Radiator & radiator cap
-Reservoir & reservoir cap
-Upper and lower radiator hoses (original upper hose had ballooned and split)
-Thermostat

I've pressure tested it and combustion leak tested it.. both came out fine. I've checked the spark plug housing and there is no leak there. I've bled the coolant several times as well. No typical signs of a blown head gasket. I haven't replaced the water pump which may be the problem? Could it be just a small leak from the head gasket?

Help appreciated!

 

Does the radiator fan run when its hot? Approximately how many seconds (engine idling)?

 

I am afraid you have a blown head gasket. When the engine is cold remove the radiator cap. If the radiator isn't full and all the coolant is in the reserve bottle you have combustion gases going into the cooling system and displacing the coolant. All your hoses are pressurized with combustion gases. It should get worst. When you have white smoke going out the tail pipe there will be no doubt. I would fix it before then.

 

Fan kicks on after idling in about 10-15 seconds.

 

I mean how long is the fan staying on until it stops again while its idling. What is ambient temp. approximately?

 

Sounds like a small leak in the head gasket. Get the radiator completely full. Wait until the engine is cold. Feel the top radiator hose there should be no pressure inside. If there is pressure open the radiator cap to release it then put the cap back on. Start the engine (cold) and rev it while you squeeze the hose. Pressure should not build up before the engine has warmed up considerably. If you feel pressure immediately the head gasket is bad.

 

have your mechanic do a leak down test when the engine is cold, not hot due to expansion...good luck!

 

My mechanic performed the leak test and it came back fine. He's not even 100% what it could be now. Changing the water pump and timing belt this weekend along with a new thermostat.

 

Did you try mk378's suggestion? That guy knows Hondas backwards and forwards. Good advice to heed...