Tell me how it can fail to open. Wax melts and expands when it gets hot. Maybe, if the wax was to find an escape route from the sealed brass capsule. Or using a higher temp t-stat. If the spring breaks it would stick open. Maybe, if you don't use anti-freeze or use the wrong product and let it sit for a few years it could corrode shut.
This is the 21st century.
I posted a relevant link earlier.

I just read a "certified mechanic" that claims that dirty oil can do it. The same guy says that if the engine seizes from overheating you need to replace the t-stat to fix it.
He shows a picture of a t-stat with transferred rust on it, evidently that proves that stainless steel and brass can rust together.
https://vehiclefreak.com/how-to-unst...stat-in-a-car/

 

All I can tell you is that I've replaced many failed stats that would not open and caused overheating. I do not believe I have ever seen it on an HHR and will need to see what the difference is. You mention wax. The stats I've seen all fail when the thermal spring has sprung for the last time. You can drop them into boiling water and they do not open.
Do you guys have the technical info form the stats used in the HHR?

 

What "thermal" spring. The spring holds the valve closed. The brass capsule has wax in it that melts at a given temperature. In days before anyone cared about poison they were filled with mercury.
It is just a standard 180F t-stat with no secondary valve, IIRC 50mm. The correct ACDelco 131-158 does not have the rubber seal that dislodges and holds the valve open in other designs, that is why we always specify. May people that think nothing ever changed since 1960 think that 190F or 210F is better. Just about any 50mm 180F t-stat will "fit", most will fail.

Seems you need to read something from this century: https://auto.howstuffworks.com/how-d...ystem-work.htm
For those too lazy to click, here is the pertinent excerpt:
They worked the opposite way back when the t-stat's job was to cool the engine.

 

Thanks, I was indeed too lazy!

 

I for one have seen a thermostat that leaked wax.

If I was designing a thermostat, I would try to make almost impossible to stick closed, and would be less worried about sticking open. So I'd rather see the spring break than the wax leak out.

Not sure I understand why a spring is even needed. Not saying it isn't, but as the wax cools and shrinks, I would think amdient pressure would push it closed, unless the wax contains some voiltiles with vapor pressure high enough to keep it open.

 

Here is another link that goes into a bit more detail about the operation of a t-stat.
https://www.theengineerspost.com/car-thermostat/

 

I was able to gleen a few interesting tidbits there but don't recommend. Seems to have been written by an AI bot or a jr. high student who cribbed everything and cut-and pasted it with no clue what they were writing about, resulting in factual misstatements and gibberish. Mishmosh, culminating with this nonsensical sentence: "Thermostats generally have a lot of advantages over the cooling system of vehicles."

 

We only need but 1 thermostat! God help us when that gets discontinued!

The OEM unit usually fails open , with the rubber seal stuck in the opening.

 

Some motors do have 2 t-stats; notably diesel truck engines.

 

I meant the 1 part, AC Delco 131-158 180 degrees thermostat.