I got a question for ya...when the thermostat was changed, was the o-ring type gasket installed with it?
For a while, mine would not pressurize either, but I WAS loosing coolant...slowly, but steadilly. I thought it was the waterpump gasket at first, then I replaced the cap, but it still had the same problem. Then I checked the last thing it could be...the o-rings for the coolant pipes. Sure enough, one had broken, causing the small leak. I am not sure how the S2'* are done, but it is worth checking into both these things I listed.
Other than that, I gots no other
Welcome, downwardly mobile yuppies, to the dawn of the Age of Nefarious.
1993 SSEi - FrankenJinx - dead and gone.
1994 SSE (136K strong) - 180* t-stat,HD alternator, most unequipped SSE on the club, XM radio, Kenwood CD/MP3.
2006 Chevy Malibu Maxx LTZ - Custom 2.25" manifold-back exhaust w/FM-80. Probably the only Maxx with this system.