Nope on the 100% vs. 50/50. I always use 50/50 out of the bottle because I can never get the ratio correct while filling.
Well, I pulled the water neck off this evening as well as the thermostat. Nothing seemed amiss there. I'll test the guage sender unit with a pot of boiling water (should read 212F here at sea level) to see if that is the issue. Might as well replace the thermostat, but test it for opening with that same pot of water before I install it.
In the past (yeah, like 20 years ago...) I've installed a new thermostat only to find it isn't working correctly. Good idea to toss it into boiling water to make sure it opens.
The only thing I hate working with more than
antifreeze is brake fluid! Yuck!
DD