Cooling System Overpressurizes Under Heavy Boost

[SuperSevenRacer]

Hi Guys,

I am suffering from a very odd problem with my '97 V8TT.  I just had a full engine rebuild performed due to a cracked cylinder head that was causing an overheat condition, but now after breaking in the engine, the coolant temperature spikes to full overheat within seconds of boosting the car hard in 2nd and 3rd gear as the cooling system overpressurizes and pukes.  I can drive the vehicle normally with no overheating condition or overpressurization of the cooling system.  My car is modified with air/water intercoolers, ball-bearing turbos, large capacity radiator, etc.  Currently pushing around 12-14psi of boost.  My shop and I have checked every imaginable component of the cooling system and all is correct.  Water pump is brand new, thermostat, etc, etc.  We even obtained another V8TT for comparison purposes and both cars are set-up identically so there are no issues with coolant flow, coolant hose routing, etc.   What is interesting is that the coolant temperature drops very rapidly after the car runs hot and overpressurizes.  If we shut the car off, the temperature will drop back to normal within 3-5mins of sitting. We replaced the cylinder liners and are currently utilizing JAE's custom head gaskets as the OE units are no longer available.  When we sniff the coolant for hydrocarbons, there are no hydrocarbons present.  The only thing my shop and I can conclude is that we may be dealing with a head gasket problem.  We are figuring that the gasket may not be designed correctly and is not sealing against the liner sleeves well enough so when the combustion increases under boost, the gasket is separating enough from the top of the liner to allow combustion to escape into the coolant which is causing the rapid overheat and overpressurization condition.  The car performs flawlessly otherwise.  Curious to know if anyone else has suffered from this issue after rebuilding their motor?  It has become quite a pain for both myself and my shop!  Any input is greatly welcomed from all you experts!  :-)

Best Regards,

Matthew

[Yellow and Green]

Hi Matthew,

Apologies if this seems elementary, but I've seen similar symptoms in cars that did not have the cooling system bled properly.  Are you certain you have the air completely bled from both the radiator and the nipple near the thermostat?  What's leading me to that suggestion is how quickly you say it cools down.  I'm wondering if you are vapor locked just bad enough that you can drive along normally without overheating, but don't have the capacity to cool when beating on the engine.  It's probably super-heating the area near the coolant sensor, but perhaps not the rest of the system.

My technique for doing this is to use a coolant pressure tester to pump up the system when it's cold so you don't have to dump hot coolant on yourself.

It's a long shot, but I'm hoping that it is a $0 fix for you.

Jake

[jonwat]

13 hours ago, SuperSevenRacer said:

so when the combustion increases under boost, the gasket is separating enough from the top of the liner to allow combustion to escape into the coolant which is causing the rapid overheat and overpressurization condition.

Wouldn't that cause it to fail the sniff test, which you state it doesn't?