Thanks for reading and sorry for my english , so i'll try to explain my problem.
My 996 TT k16/24 at 1.1 bar of boost with 60K miles leaks combustion gas in cooling system , but this happened only when i run the car at WOT and under hard loads in 3' , 4' , 5' and 6' gear .
So ,after i drive the car so hard , cooling system remain pressureized for days and days even when completely cold .
So When i check coolant level in reservoir , it is below Min , i remove the cap and big pressure is released (exhaust smell ) so the level rise at MAX again .
i have pressure test cooling system is OK , pressure test cylinders OK , no coolant loss , no oil in coolant and no coolant in the oil or milky oil .
I have checked in coolant reservoir without cap , at idle while car is warming up and no bubbles in coolant .
Again if i drive the car with light loads , at cruise speed or even spirited drive but not at full boost and not at red line in higher gears , this over (exhaust ) pressure in coolant not happens , and when i check the level at cold , level is ok between min and max marks , no pressure or very very small amount is released when remove the cap .
It seems the exhaust gas leaks from combustion chambers to coolant lines only when the pressure in cylinder in very high at peak power stroke , so i figure this is a cylinder heads lifting issue .
I think if it was a blown head gasket , pressure should escape with light engine load too and should the coolant leaks in the cylinders , so coolant in oil should be present.
Car runs great and never overheating .
Is possible this is really an head lifting problem ?
Wich could be the consequences of driving hard ( track days ) a car with head lifting problems ?
Thanks again

 

You lifted a head for sure. You most likely detonated on some bad gas or the bolts went weak on you. The worst thing that can happen is that it will blow water back into the cylinder and hydrolock your motor. That is what happened to me, there was nothing left of the bottom end. You should tear down and should not run this car any longer. Go with some rods and improved head studs. https://www.6speedonline.com/forums/...otor-pics.html

 

Check your oil .... Probably will have coolant intermixed

 

He did say that he doesnt have coolant in the oil. The head stays down at low boost enough to seem ok but it will get worse and you run the risk of destroying everything. Do not drive.

 

Sounds like head gasket failure between cylinders allowing it to push coolant into the cylinders causing exactly what you describe. Time to take it all apart.

 

Not much point in trying to drive that car any more. Rebuild!