I replaced spark plugs about 1,000 miles ago. They were all okay. I will do a compression test and see where that takes me.

Also, when I changed coolant about 3 years ago, I installed a slightly different thermostat. I don't see how this can be the cause of the problem, but who knows: Here is the thread: https://www.chevyhhr.net/forums/prob...e5/#post808782

Never had any issues with this thermostat, by the way.

You said back in 2016

As far as thermostat goes, it stay in there until it causes problems. I would rather follow time tested advice this time - if it is not broken, don't fix it. So far, it has been doing just fine according to the reading on the COOLANT display. No leaks, as well.

You might as well replace the thermostat with an AC Delco 131-158 and change the Dexcool, it is almost time to change it anyways, 5 years.

Three places coolant can go; into the oil pan, out the tail pipe, onto the ground or 3(b) as steam.

It can escape as steam from a worn out pressure cap or a crack in the rubber cap on the extra nipple on the t-stat cover or other rubber spots.

Couldn't it also go into the automatic transmission?

If the transmission cooler lines in the rad are rotted out , very possible.

The Coolant certainly should NOT be disappearing like that.

I topped of reservoir yesterday, and today it is back to being low. I will go down the list and check all of these.

1. Car is burning coolant
- blown head gasket, cracked head
- check engine oil
- white sweet smelling smoke from exhaust
- do a compression test

2. Bad radiator cap (rubber cap, pressure cap)

3. External Leak
- coolant residue
- Unproperly sealing hoses
- Cracked hoses

4. Did not completely fill the coolant system
- there MAY be a special fill procedure for your car. If you are not observing the fill procedure, you may not be putting enough coolant in, and OF COURSE the car will drain the tank, to fill the cooling loop, once the thermostat opens.

5. Water pump weeps coolant

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1. There is no noticeable smoke from exhaust. Engine oil is nice and brown. I just did an oil change not so long ago.

2. Rubber cap is in good condition. Pressure cap and the area around it shows significant deposits of evaporated coolant. What doesn't make any sense for me is that I added around 10 oz of coolant yesterday to the reservoir, and it is already gone. Coolant evaporating at such a fast rate seems a little bit odd.

https://cimg0.ibsrv.net/gimg/www.che...13ab29a57c.jpg
https://cimg1.ibsrv.net/gimg/www.che...565085fa5a.jpg

3. All the hoses good condition and show no signs of leaks.

4. By now, with all the coolant I added it over time, it should be fully filled.

5. Don't even know how to check that.

Check the front foot well carpets the heater core might be leaking, but at the rate you say it’s disappearing , they would be soaked.

What Blue said, & the cap is not the big leak, look around & below that area, no residue that I can see.

How about a pic of the tailpipe?