Rep Power: 0

Hello all!

I'm in the process of getting a new (to me) 1981 Civic 1500 Wagon back on the road. It's sat for years. Currently I am fighting an overheating issue. The car runs and idles fine but after about 10 minutes the temperature gauge will get hot and the radiator overflow tank will boil. Obviously I turn off the car as soon as I notice it starting to get to that point. I have ruled out thermostat (replaced with new), water pump (replaced), new radiator. I did a compression test on the engine and all 4 spark plug holes read 180 on the nose, so compression is perfect.
Couple of things...I have seen the radiator fan work, but I suspect it is intermittent. The previous owner told me that the heat didn't work right. I checked the hoses to the heater core and lo and behold one is hot and one is cold. Would this be enough to overheat the engine? Can I just bypass the heater core and test it that way?
I also suspect that it might have a head gasket leak. A compression test on the valves came back perfect, so I tried a block test. Coolant contaminated the sample though as the coolant level would rise up the radiator fill neck as it got hotter. I'm going to try again possibly from the overflow container.
Even as I would drain off coolant to make room for the tester as the engine heated up the level of coolant would rise by the radiator cap.
Any thoughts?

 
Rep Power: 228

I don't believe the heator core would cause overheating.

I'd be looking at
-air pockets in coolant
-thermostat (pull the new one test it to make sure it opens)
-fans/fan switch

Even if the head gasket is blown, the vehicle shouldn't over heat if there is fluid in the system. Usually the head gasket will either leak coolant into the cylinders or pump exhaust gas into the coolant causing the system to overpressurize and dump fluid out of the reservoir.

 
Rep Power: 0

Thank you for replying! I'm happy to report that I have stopped the overheating issue. I did a gentle flush of the heater core as well as bled the coolant system. I also swapped to a stock radiator cap. Now both heater core hoses get hot and the cabin blows hot air, something that wasn't happening before. I agree that it should not have contributed to the overheating issue though.

I rented a block test kit from AutoZone, but am unable to use it as the coolant is frothy and continues to overflow if I run the engine without the cap on. I have read that this can be either a head gasket or sometimes debris in the system frothing the coolant. I have also read to double check the torque on the head bolts to see if any of them are loose.

EDIT: I should also add that the radiator fan is operating properly.