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.

anthony_240

When you bled the system, did you have the car running and heater on at full blast?

Did you let the air come out of the nipple on top of the intake manifold?


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Zuke

I fill the funnel up and bleed the heater core as good as I can then put the hose back on that. I crack the bleeder valve on the IM until bubbles go away and it's flowing straight coolant , close the bleeder and squeeze both rad hoses, repeat cracking the bleeder until no more air bubbles form.

I then start my car, have my heater on and blowing, crack the bleeder again, squeeze the hoses and repeat until I can hear no more air in the system and the bleeder is flowing straight coolant. Sometimes i'll turn the car off after it warms up, let it settle and continue bleeding with the air bubbles that have accumulated. Also, yes the front of the car is elevated during this process.

I feel like I'm pretty thorough about the bleeding procedure.

After I've been driving and it starts to overheat, my whole upper rad hose will be empty with coolant and full of air. The level of coolant gets below the top of the radiator fins.

KJR50

Drive up onto a pair of oil change ramps i.e. "rhino ramps" or "race ramps". The idea is to raise the front end, leaving the rad fill opening at the highest point to allow all the air to escape. YMMV...worth a try.

anti tyler

You guys are all saying the same thing, which are good points, but you have to understand even a shotty bleed on the system won't give results like what he's encountering.

You've got an underlying problem going on somewhere.

You might have like some sort of Venturi effect (may be wrong context) where air is being drawn in thru a hole rather than water being pushed out of it, in your heater core.

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anthony_240

Wait dude wtf... why are you disconnecting the radiator hoses? Your confusing me, just put the hoses on, with the heater core connected , fill up with fluid (bleeder screw opened) , then close the screw when full... if you have a funnel that goes over the radiator put that in place and add coolant in there , if not cut up a 2 liter and put it in place if the funnel...

Turn the car on, heater full blast, car will heat up, when the temp goes up past half, turn it off, let the coolant go down, once it's cooled down, open up the bleeder screw, fill radiator until fluid comes out the screw hole, and start up again

The bleeder screw is the highest point in the system


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Zuke

Sorry I confused you. I'm not disconnecting the radiator hose, I simply removed the top heater core hose while filling the system with coolant in order to get any possible air out of the heater core, then connected it when the air was out. Yes, that is essentially what a spill free funnel is, it holds a large amount of coolant over the radiator so you can bleed the system without introducing air.

Alright ill try your method on bleeding by turning the car off once it gets warm, letting it cool then bleeding it again.

Zuke

I'm with you here, I honestly don't think bleeding the system better is my issue. I've taken this car to radiator shops just to have them bleed it and I've bled it multiple times both with and without the heater core hooked up.

It only has the issue when the heater core is hooked up though. I was saying the exact same thing about the Venturi effect causing a low pressure somewhere in my heater core, sucking air in. I would naturally think that it would be pushing coolant out of that same hole though but there are no leaks that I can find.

I guess I can exchange my heater core for another one and try it all again but I cant seem to find the plastic tubes with the water cock(heater control valve) anywhere and I feel like that is the issue.

anthony_240

Oh perfect yea since you have the spill free funnel that's perfect... just keep doing it that way man, it took me like 3 times, letting the coolant cool down , opening the bleeder screw filling it up till it comes out, but eventually it will let all the air out, patience is the key


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