Evo 8 Failed tk test but passes cylinder leakage test

I'm trying to diagnose my evo 8 that has failed a tk test and has constant air bubbles in the cooling system that can be seen with the engine idling while at operating temperature with the radiator cap off. The engine has passed a cylinder leakage test on all cylinders with no air bubbles entering the cooling system. The car does not overheat or push coolant or have excessive radiator pressure. I have so far replaced the thermostat, head gasket and cylinder head with the same air bubbles appearing once the engine is up to operating temperature. I'm unsure of what else could be the cause of the air bubbles in the cooling system and any advice would be appreciated.

It sounds like you may potentially have a cracked cylinder head or similar (I'd have said a head gasket leak but if you've replaced this then it eliminates that as a possibility). If the TK test is showing combustion gasses in the cooling system then it is getting there somehow. The problem with a leak down test is that it only uses 100 psi of air pressure which is much lower than normal cylinder pressure with the engine running. I've occasionally used a nitrogen bottle regulated at something like 1000 psi to test for head gasket leaks or cracked cylinder heads.

Thanks for the reply, I've just performed another tk test and have confirmed there are no more combustion gasses in the cooling system. Problem solved.