Generation IV Internal Engine - Front mount turbo GTO spills coolant out of overflow after engine off

I have a home built s475 BW turbo build and I know the turbo exducer housing sits close to the radiator but I have twin 1400cfm pusher fans and the engine runs perfectly cool on the warmest days until the engine is shut off. I hear a hissing noise after engine is off and the overflow bottle slowly fills up until it starts to flow out of the small hole in the overflow bottle cap. I have recently replaced the radiator cap and that helped a lot, I also ceramic coated the down pipe and exducer housing but it still happens a bit. Thoughts? Suggestions?

is it lifting the heads by any chance?

No, checked that. No pressure bubbling back while running only after it's off.

I don't have an answer for you, but I've got a little info to add to the mix. I've got a Gen TTR kit on my car, and it builds pressure in the cooling system as well. Mine doesn't build enough to push past the radiator cap like that, but it's definitely more than stock.

In the process of heads/cam swap now, and just pulled the heads off. No signs of any issues at all fwiw. My turbos are ball bearing, they have coolant plumbed through them.

Cooling lines ran correctly? My car did this a while back and it's because I had some pressure lines running to a stock style no pressure holding resivour.

I have all of the hoses/lines run to stock locations. The only difference is a GM universal coolant overflow bottle that is quite a bit smaller than my stock one mounted on the passenger side of the firewall with the stock rubber hose from the radiator running into the bottom of the tank. Is there a right or wrong style overflow bottle?

So doing some looking through overflow tanks and there are vented, non vented, circulating and non circulating so maybe I have the wrong one. My old tank was the lowest point in the cooling system with a hose going into the top of the tank. The tank had a small vent type hose coming off of it. The new tank is the highest point and the hose from the radiator goes to the bottom of the tank and there is a small vent hole in the cap of the reservoir. I just also thought is there a difference in which of the 2 ports below the radiator cap goes to the coolant cross over and to the overflow? Just throwing that out there if someone knows.

Mate it sounds like a bad radiator cap. The engine builds heat as it runs right. When you shut down it has all that built up stored heat but no water flowing to cool it down. The coolant in the block will get hotter than the coolant in the radiator causing higher pressure. I'd bet your rad cap is crap or the wrong pressure.

Swap the overflow hose and steam hose at the radiator, they are backwards.

Did you find out what the problem was? Do you have any more pictures of your set up?

Swap the overflow hose and steam hose at the radiator, they are backwards.

this is what i was saying.. The two lines on my camaro are right next to each other on opposite side of the cap and can easily be swapped around.