Yes, I HAVE changed the thermostat!

History. Bought the 07 HHR in about 4 yrs ago with 40,000 miles. Within 2 weeks I got the CEL, flat temp reading and no AC. Replaced the thermostat with guidance of this site. Worked fine until this past winter, began flat lining again, but no big deal as I didn't need the AC. Last week replaced the thermostat again, now at 55,000 miles.

Ok, now the problem. The gauge still flat lines. It worked fine at first, did a whole day of running around with no problems. Past few days it keeps acting up. Have to keep pulling the battery fuse to reset it. The temp raises rapidly from 60 odd degrees if left overnight to 172/174 then drops back to 170/171 and stabilizes at 172. Often shortly thereafter the problem reoccurs.

It seems like the thermostat is opening early, I thought it should be 180? I plan to pull it and measure temp in a pot of water on stove in the morning, but what else might be going on. Faulty temp sensor? thanks.

 

A faulty temp sensor or air trapped in the cooling system, HHR's are notoriously difficult to "burp" after draining the coolant.

The high point of the entire cooling system is the heater core, hence the classic low coolant level telltale, which is a gurgling sound emanating from the dash.

Check your coolant level with the engine dead cold, and top up the coolant to the "Full Cold" marks on the coolant recovery bottle/surge tank.

You can have the codes pulled to see if a trouble code has been stored for the temp sensor, but when in doubt, take it to a professional radiator/coolant shop. HHR's are very susceptible to head gasket failure and cracked heads due to cooling system issues.

 

Or the dreaded, "a piece of the old thermostat rubber gasket is lodged in the new thermostat, keeping it partially open".

It has happened to several members on this site.

 

172F is too low! It is still stuck. It's a 180F t-stat.

 

True that - definitely a possibility, so give it a good flush and see how it goes.

After refilling keep an eye on the level in the expansion tank for any drop in the level, as it often works some of the air in the system out.

 

Removed the 'new' thermostat this morning, took it into the kitchen along with the old one I replaced last week.
Placed them in a pot of water and heated them both. The new stat opened visibly earlier than the old one. Around 10 degrees sooner based on the opinions of a bimetallic stem thermometer and my (admittedly inaccurate Harbor Freight) infrared thermometer.
I put the old stat back in the car, got it up to temp and then drove around the block. DIC showed a steady 180 degrees, so it seems the new part is defective. Will attempt to exchange it later. Hope I can find the receipt...

 

Ahhh cool (no pun intended) - sounds like you got it nailed, but what a pain, as swapping it in and out is no longer a 5 minute job.

Was it an aftermarket thermostat, or a GM one?

 

The defective new stat was a Toughguard from Advance. They exchanged it for me this morning, even without the reciept.
The previous one was from Autozone I believe, can't remember the brand, but it was made in China, and replaced what was probably the stock, made in USA, thermostat.

While testing the thermostats this morning I inspected the one I had taken out of the car and it did not look bad at all. The sealing rubber looked intact and I cleaned up the metal mating surface with a wooded dowel. Ok, it was a chopstick. I was in the kitchen after all.
This was the stat I reinstalled. I have run several errands today and it is working fine. Temp holds at 180-185 when up to temp.

I will keep the new exchanged stat in the glove box ready for the expected failure, but don't see a need to replace it right now. I change the thermostat without draining the system, just unbolting the housing and catching the coolant that comes out in a shallow pan and reusing it, so it really is a 5 minute job. It takes longer to get the car up to operating temp than it does to actually change the part.

 

Good deal - glad it's working good for you.

 

A note that is buried deep in the forum: The t-stat from GM does NOT have the inner rubber seal, it is better than the others with 2 rubber seals & the housing gasket. Cost is about the same.