[Tapani]

You do have all the bypass lines connected as they should? Heater and the TB too? And the hoses are not kinked? These provide the signal for the thermostat to open.

And the thermostat you have is an OEM Subaru part, yes?

Wen the engine starts to overheat is the lower rad hose cold or almost as hot as the upper one?

And your fans do come on as they sould when the ECU tells them to - even w/o the AC on?

It is funny that the engine boils at low loads (only?).

My HG leak was so small it could be noticed only on track days.

I could hear slight bubbling in the reservoir way before the flash boil. I could do two laps and then take it easy and it would just bubble :-).

The rad gas test went green only after a few uphill WOT spurts in first gear. Nothing after "normal" driving.

Proved to be HGs and all is well after the engine overhaul.

If you have the Ecutuner or Evoscan you can monitor what's going on. If I recall correctly the temp gauge needle started to move from the "normal" position at 103 degrees C. That's high :-). The highs during flash boiling were 112-115 degrees C.

The Subaru cooling system is carefully balanced and all needs to be in good condition for it to work as designed (including the rad-to-body seals). Same applies for any and all modifications too ;-).

Tapani

PS What are the ambient conditions in Sacramento, btw?