In either case of a head gasket failure or cracked block the coolant can only go to 3 different places.
1. Leaks outside the engine along the seam between the block and the head, in which case you wouldn't be able to retain pressure in the cooling system and would be able to see traces of dried up coolant.
2. Coolant leaks into the engine and mixes with the oil. Checking the oil with a cold engine will give the oil a white milky appearance.
3. coolant leaks into the cylinder in which case there would be presence of exhaust gases into the coolant or coolant will enter the intake manifold and be burned with the gas.
Below is some ways of testing for this.
You can test for exhaust in the cooling system by using a combustion leak tester, which uses a fluid that changes color from blue to yellow when exhaust gases are present. Other methods include watching for a continuous stream of bubbles in the coolant reservoir with the radiator cap off or checking for the sweet smell of coolant mixed with exhaust fumes in the tailpipe.
I hope this helps you diagnose the problem.

 

I’m away for the week so I haven’t been looking at the car but I haven’t received a text or a message that the car has exploded so I’m hopeful I have found something that’s at least a temporary fix.

I decided that after all my testing and attempts that gasses are leaking into the coolant system as I had good pressure, no obvious hose problems… but my RELD test for gasses seemed negative, at least it wasn’t turning yellow. It may have gone a little green but being colour blind makes that difficult to see. Anyway, like I said it didn’t go yellow like I’ve seen on videos.

Regarding the oil, it had an oil change not too long ago but it looks pristine so it doesn’t seem to be leaking there.

Basically, going on the symptom of always having air pockets developed even after an extremely good bleed, and the coolant being forced into the reservoir but not returning had me decide that it has a gasket leak or a cracked head, the latter surmised because of the engine’s donor car having been in an accident.

So I tried what everyone says not to and used Rislone Head Seal to see if that stops the problem and if so the I take a look at the idea of replacing the head gasket.
At least after the first real drive with it the coolant in the reservoir was normal.

So far no phone calls or texts so I’m hopeful though I’m not going to ask and maybe jinx it. Hopefully this response doesn’t jinx it. Ha ha.

Im still going to look at the throttle body hoses when I get back and I’ll update what’s happened since I left.

 

I see that you have gone through several radiator caps and had not solved your problem. If coolant is expelled from the radiator cap into the reservoir when the engine is hot, as the engine and coolant in the system cool down the volume contracts and creates enough of a vacuum to suck the coolant back from the reservoir into the radiator. If the coolant is not being returned to the radiator from the reservoir, there are only two items that will cause that. One is a radiator cap, that the metal disc does not drop down allowing coolant to frow back into the radiator or two and most likely the small hose from the radiator neck to the reservoir is cracked or is allowing air to be sucked in due to a loose fit at the ends. Check the hose and perhaps use small hose clamps at each end. Look for easy fixes first, before you start tearing down the engine.

 

I know my responses have generally been long so subject to “tldr” but I did check the hose and replaced the hose, albeit with a fuel line but it’s tight with new hose clamps.
maybe the reservoir housing is cracked or there is a bad fit of the rad caps but due to the radiator itself as I’m now with a Mishimoto cap and owing to the pressure i had in the system the other day I think it’s holding fine.
its not easy to come by parts like a new reservoir.

 

Two weeks later. The car has not been overheating. Fluid in the reservoir has been mostly stable but I finally checked it today and it was a little high. There was pressure build up when I released the cap even thought it hadn’t been driven since the day before.
There was a little bit of “gunk” in the throat of the radiator that may have prevented some of the fluid coming back in. It looks like some of the Rislone Head Seal. That’s probably not good and I wonder if the higher pressure cap allowed the higher temperature that did that. I’m going to change the coolant and hope that it’s keeps running okay. I have a feeling this car isn’t going to last too long unfortunately. There’s a couple new sounds that don’t sound great.

Oh well…. I guess my son better start saving some money for something else.

 

I bought a 2015 Jaguar XFR-S from auto auction from Texas and it arrived with a very sparkly and flaky coolant.. Some moron dumped a can of 'leak-fix' into the coolant and the issue was a rotted away cheapo plastic Ford crossover pipe in the back of the engine (i have all that crap on video in my youtube channel). So what i did was buy a coolant filter system for a 6.0L Ford Power Stroke diesel and rigged it in there to clean the coolant. I bought it a while back and today, those go for around $50-$70. Filter is WIX 24070. So 2 filters and 250 miles later, no more sparkly flakes!


I did the same process in my G and my 05 Town and Country. Coolant does get fouled overtime and if the previous owner ran on water, then theres gonna be bunch of crap floating in there and most likely, stuck in the heater core.. It's always a good idea to back-flush the heater core using a garden hose, you'll be amazed what comes out of it..

 

As for the car itself, if it runs and don't overheat, then let him run it till it don't anymore. But in the meantime, save up for a newer car..
I know how awesome those G35 coupes looks, i always wanted one but those engines are obsolete, parts very hard to come by and the interior reminds me of my grandmas 1890s dresser..

 

Hey, did you just call me a moron.. ha ha

Really I only did that as nothing else was working and the engine having been swapped from an accident donor car I was starting to suspect a head gasket or cracked head.

I will try back flushing the core.

 

Thats basically the plan now. I drained the rad and filled with distilled water and ran it for a very short while then drained it again with new coolant. I ran the car with the no spill funnel which climbed almost to overflowing and high coolant temperature so I shut the car off and it sucked almost all the coolant in and the temperature dropped dramatically and immediately. Hopefully the “fix” didn’t damage the thermostat sticking it closed. I ran it again and things are running normally with modest heat at idle and really hot when I rev the engine. I’ll let it cool and bleed it again. Thermostat seems to be fine now and fans work fine.
Short of the flushing the core I’ll do like you suggested and I think I’ll just let it run if it’s not overheating. There are still some sparkly flakes in the coolant but hopefully it’s diluted enough.

 

And yeah, the interior is very dated, especially because it’s tan.. yuck