I have a '98 Saturn SC2 (1.9 liter) 5 Speed. When I got home the other night I sat and chatted with my daughter for about 20 minutes when I realized the coolant was boiling over. I tested it again today and let it run for about 15 minutes, with my AutoXray attached, and it started boiling over when the Coolant Temp showed 186 degrees. The thermostat never opened, the cooling fan never came, and the temperature gauge on the dash was never elevated BUT according to my scanner the temperature never got to 195 degrees for those things to occur. Now why would it be boiling over at 186 degrees? Could the temperature switch be faulty and/or thermostat be bad?

If I recall, air and too much water (anything over 60/40) decreases the boiling point of coolant. How long has it been since the last coolant flush/fill?

Last week, and I replaced it with the no mix 50/50 solution.

Maybe a bad pressure cap?

i second the cap, the pressure is what raises the boiling point

The cap is pretty cheap to replace, I recommend AC DELCO Part # RC87, but the STANT Part # 10239 is a few bucks cheaper, and should work just as well, RockAuto has the cap for 2.89 right now.
Places like Advance Auto should have it for around 5.00, so you will probably get just as good of a deal from a local parts store as you would online.
Replace that and let us know how it goes.