Zuke

First off I apologize for the length of this post, I just want to be thorough and this is driving me crazy.

Cliff Notes:
[Overheating, air builds up in the cooling system when heater core is hooked up, thermostat doesn't open. Runs fine and no issues when the heater core is bypassed. No leaks, head gasket is good, system holds pressure.]

I've been battling an overheating issue with my 91 with a ka24de. I have replaced nearly the whole cooling system less than 100 miles ago with a new oem thermostat, new oem rad cap, new radiator, new heater core (used old plastic elbows), all new coolant lines. My circuit sport overflow tank and my water pump have about 5k miles on it.

My issue only happens when I have my heater core hooked up. My car will start to overheat within 10-20 minutes of driving. The needle shoots up to H and I can hear air being pushed into my overflow tank. If I turn my car off the velocity of the air increases greatly. Once it stops I'll turn the car back on and the needle will usually go back to the normal temp. My theory is that for some reason air is getting trapped behind the thermostat causing it to stay closed.

I have troubleshot the idea of me not bleeding the system well enough by using a spill free funnel. I have tried taking the upper heater core hose off and let coolant flow through both the core and the line from the block until there are no bubbles, then reconnected them and bled the motor like usual.

There are no leaks that I can find in the system, a shop pressure tested and block tested my system. It holds pressure and there aren't combustion gasses present.

The strangest part is that when I bypass and loop the heater core hoses I don't have any issues. I've tried bleeding my system multiple times with the core hooked up and it always ends up with the same result. I even let my car run with the spill free funnel on it and after about 20 minutes air bubbles started coming up through the funnel which continued for at least 10 minutes. Then I'd shut the car off, there would be a burst of bubbles then the coolant would get sucked back into the radiator. When the core is bypassed these bubbles don't appear. I used a block tester with the dye and the bubbles aren't from combustion. My coolant is clean as well.

I'm thinking the water cock on the heater core is somehow introducing air into the system but isn't leaking under pressure. The shop I took my car to thinks it's an air bubble but I don't see how that's possible considering the testing I've done. I have no clue though, any opinions and ideas are appreciated.