cooling nightmare continues

[82eye]

ok, i'm getting to the end of my rope on this. every time i think i've got it handled the car screws up again. this thing just won't bleed correct and stay working without eventually getting too hot.

symptoms :

overheat. car will run normally for a time - days - then the temp begins creeping up. has gotten over the half mark to as hot as the 3/4 mark. has never gotten to red.

leak. car will not leak when hot, but seems to lose a little coolant as it cools or sits when cold. haven't been able to find or trace the leak, it shows up in random spots under the car, on the passenger side of the engine opposite to the res. leak drops behind the rad but in front of the engine. no leaks from hoses under there.

losing coolant. the res will drop about 1/3 to 1/2 when sitting cold but leaks very little to the ground. tested with cardboard under the car.

res holds pressure even when cold.

car blew return hose right off the therm housing once recently. has not done it again since.

what has been replaced:

everything except radiator and fan clutch. car has a bit under 2000 km on a rebuild. been trying to find the issue since it first fired up.
all new hoses, new thermostat, new therm housing, new res, new cap, new heater core, new heater core o-rings, rebuilt water valve, new water pump and belt.

what's been tested / looked at :

compression and leakdown are good. no issues.
no sign of coolant in oil or oil in the coolant. both changed multiple times to check. oil sample tested local with no contamination.
head gasket combustion gas check in the coolant passes. combustion gas test liquid remains pristine blue.
coolant system pressure test - seems to pass but the res adapter for the test sucks. only air we could find escaping was coming from the res itself. coolant does not escape from anywhere on a pressure test.
fan clutch tests indicate clutch is good. shreds rolled up newspaper, does not stall fan out.

what we are guessing :

sticking or bad thermostat? these can be shitty from new now. no idea if it matches the symptoms.
failing rad? rad seems to act normally and bleed correctly back to the res off overflow hose.
bad / leaking or loosely installed water pump? what leak there is appears to be coming from the front of the engine, but does not account for amount missing in res.
air lock? some kinda air lock that will not bleed out and does not affect the heater. heater returns hot air.

has anyone have any ideas or had anything similar happen? i really sick of this over complicated cooling system that does not want to work.

[It's Soda Not Pop]

Years ago I had an issue with my m20 overheating even after I replaced the water pump and bled, bled, bled, bled, bled, and bled some more. I just had to keep raising the front up and bleeding it. It eventually stopped overheating and starting acting right. That was the only time I had a huge problem. I have drained the coolant a multitude of times and I haven't had a very hard time since then. I always try and fill up everything with coolant that I can (fill voids where I can and then install) and have the front in the air as high I as I can.
Check your clutch fan as well.

[nadodude111]

You definitely have a leak somewhere. As the car cools down, there’s less pressure to push water against the hole and eventually gets to an equilibrium that allows air to enter in thru the hole. As air enters the system, coolant escapes and leaks onto the ground. This is causing you to have to bleed repeatedly to release the air that keeps coming in as the car cools down. Something says it’s the radiator.

[TobyB]

I forget- do you have an aftermarket temp gauge?

I have fixed a couple of overheating problems with an accurate gauge...

That said, if you're losing coolant, I too might be suspicious of the radiator.
They're not E36 fragile, but the seals on the end tanks don't last forever...

Also, in the for what it's worth category, I put a coolant pressure gauge on
a car once, when I was having similarly frustrating problems. A turbo boost
gauge works well. That helped me find a crack in a radiator neck...

spitballing,
t

[82eye]

thanks guys. pretty much any and all of the suggestions could be plausible.

[BadDays]

There might be a air leak coming from the radiator So I'd suggest checking it out to rule it out, On My Radiator, It was always letting air into the system before I'd figure out that there was some pinholes coupled with coolant leaking out of the bottom fins, that is what led to me throwing in a Chevy radiator in the first place.

[reelizmpro]

I went through the same thing with my S52 swap and replaced everything in the coolant system. After a while, it would always overheat expelling coolant out of the cap when it boiled over. I tested for combustion gases but since it was my first time I read the results wrong. I was looking for a dramatic change in color but since the blue fluid barely turned into a turquoise color...I thought it passed the test. A month later, another block test with fresh blue fluid showed even more of a color change and a shop confirmed it was indeed a bad head gasket. Perhaps it was just starting to go bad when I did the first test and got worse later. I would do another test before replacing a bunch of parts.