Could be that on the hot days, you're running the AC, which causes the cooling fan to run?

 

Good point RJ, are you running the A/C in both temperature days ? Michael_e

 

I have seen the tool in blister packs at Autozone.

 

But that ambient temperature differation is puzzling.
New rad, new pressure cap, Dexcool, AC Delco 131-158 thermostat, hmmm, you burped the Dexcool.
I wonder if you could park the HHR after a warm up drive , front bumper higher then the rear. Let it cool off over night, then burp it again, it could be some air still trapped in the system

 

To RJ & Oldblue - According to my wife who drives the car 95% of the time to work and back (app. 30 miles each way) with or without the AC, the temps are about the same as I posted above. That's also confusing me a lot.
Oldblue - I will try that burping procedure a couple more times this next week when she gets home from work.

And yes, i'm highly considering replacing the water pump. Thanks for the tool listing and i'll be checking that out today at the local parts store.

Mike