Well, since it's overflowing, you know it's flowing. The filler neck is not the highest point of the cooling system. You need to burp the system. See my prior post.

 

The overflow is caused by heat expansion, otherwise there would be no pressure.

The HHR has a odd cooling system, Us old folks are used to looking at an expansion tank and seeing a flow. That doesn't happen in the HHR, it does not flow when not under pressure.

The coolant is "sucked" out of the radiator through the lower hose, older engines it was "pushed" into the upper hose.

I worried about it, too. Yes there have been faulty water pumps.

My experience was a tiny crack in the cap on the t-stat housing was letting steam out, but it was not enough to leave evidence.

 

Well i noticed something interesting as I am monitoring the overflow bottle. I drove the car 20 min or so, then opened the hood and noticed the coolant was bubbling out of the overflow bottle vent hole or overfill hole. Then let it cool off and the coolant went back down to the normal level. Is that a sign of a bad head gasket?

 

Here is a link to show the proper "cold" level of the recovery tank.
https://www.chevyhhr.net/forums/main...ank-pic-55772/

If the coolant level is as shown when cold, and overflows after 20 minutes, you certainly have a problem. A compression test should help diagnose a bad head gasket. If you don't have one, your local auto parts store should have a 'loaner' program.

 

Overflow can also indicate a bad rad filler cap.

 

Over filling the bottle is bad. Coolant is supposed to go back and forth, however excessive amounts indicates exhaust gas pushing it out.

 

And if the cap is weak and won't hold 15 psi then it'll allow Dexcool past it! Here