Like Don, I use the 50/50 already mixed Dexcool.
No guessing , no mixing, no debate about tap or distilled water.

 

Below is the thermostat out of the old engine with a blown head gasket.

 

Below is the brand new ACDELCO (ACD#131-158, GM# 12622410) thermostat.

 

I can't make out the markings. Are the 180F? If they are the engine temp should hang around 190F while cruising.

 

The old one has a hole drilled in it? Old school but not great for a computer controlled Ecotec

 

The markings make absolutely no sense to me. I was hoping someone could shed some light on this. The new one is a 180F.

 

Usually the diameter in mm and the temp in C. Any other marking are only meaningful to the factory.

I would expect an engine with a 190F to run just over 200F. And the fan to run most of the time, it is set to come on at about 219F and turn of at 180F.

 

UPDATE: The 2.4L engine was replaced with a 33K mile 2.4L engine from a junkyard. The person that owned the HHR before me said it has a brand new radiator. I replaced the thermostat with a new (ACDelco #131-158, GM# 12622410) and the lower radiator Hose (also ACDELCO). I fill it till it spills out with my mix of 50/50 green antifreeze and distilled water. I take it for a spin and park it in the driveway. When the temp hit 217°, the fan turned on, the temp dropped a little then began to raise, it got up to 225° and I got scared and turned off the engine. After it cools I will check the antifreeze level again. The A/C was turned off and the outside temp today is 68°. What's your thoughts.

 

Green coolant?

 

Super Tech Antifreeze - Walmart.com