[tsupranami]

Bad Cap. No O-Ring! Gotcha, not a turbo. Still high compression, just by piston design instead of forced induction. And machining of surfaces makes squish area smaller, which increases cylinder pressure (unless compensated by thicker gasket).

You could be onto something, though. I don't like aftermarket rad caps. New OEM Toyota has been the only thing that can hold the pressure when mine gets hot. Cheap fix to test the theory ~$10 at magauto.com, ~$15 at local dealer.

Same with the combustion gas test kit like CyFi6 mentioned. Cheap but effective test.

You could also have a good shop pressure test the system to test the cap/HG.

You say you replaced the head gasket. Did you pull the motor? Rebuild it? Did you disassemble the block, too? Because I don't know of any way to surface the block without pulling it out of the car and taking it apart completely.

I've done half a dozen Supra HG's, turbo's and N/A's. Every one had grooves worn into the block/head mating surface from the previous HG. How deeply grooved and corroded seemed to depend on if and how long the gasket was leaking prior to disassembly.

Iv'e done mine three times now, two times wrong. Once with OEM gaskets, no machine work, just cleaned extremely well, lasted a few months. Next was Titan MHG and ARP bolts with fully surfaced and lapped head only. I just cleaned the block again. That lasted until I went past 12psi boost. Then I lived with a mild BHG for 2 more years. Waterfalls and gurgling streams, ahhh....the memories! Good thing about the MHG's- even if they leak a little, they don't deform or deteriorate nearly as fast as the fiber style (OEM, FelPro, RockAuto, etc.).

I just finished a complete rebuild, fully machined and lapped surfaces, ARP studs, no leaks, no drips, no errors, @ 16psi so far.

I sure hope yours is just a bad cap! But if, heaven forbid, it's another BHG, go MHG w/ARP. It's money well spent.