GM actually recommends treating the cooling system every year, and what a stroke of luck, they even sell cooling system treatment tablets that you crush up and put in the reservoir.
Correct: Do not mix coolant types
If it started with dex-cool and was converted to green... switch it back.
Not sure how long it took to lose that gallon, but if it took a long time, you may want to look for signs of leakage on your heater hose connections. I have seen several of those with seepage that adds up over time.

Bars Leaks does work, just be sure to follow the instructions.... there's nothing wrong with your thermostat or fans.

 

Headache... thank you and thank all of you for the help and advice.

After putting the gallon of coolant in yesterday, it was full to the cold mark.

My wife and I drove yesterday to work (yes we work together) and we were running late so she was driving it pretty hard. Last night I noted that the coolant level had dropped about an inch or two.

I think I will go ahead and put the Bar's Leak into it and see what happens.

Is this along with the mild oil usage show that the engine is getting worn out?

 

You should hold off on the Bar's for a little bit !!
That drop in level, may have just been a little air space being taken up by the Coolant.

Let it run a day or two, you may not need it.

IMHO I just don't like to put anything in the system, that's not absolutely needed.
That's just me though.

 

Very good advice! Thank you

 

Just a side comment on this...

Once/if you get everything working right (pump/coolant/stat/cap) GM has gotten the Cooling Systems down almost perfect on the recent (10 or so years) models. I've gone over a year, without having to add any Coolant to the reservoir. Chemistry and System components in general are really reliable/consistent.

HA, I've got some other issues with GM, but, the Cooling Systems aren't one of them.

 

1 year? haven't bothered with ours till this April when went to FL to see family. Brother-in-law has his own shop. We drained the system, pulled a complete vacuum and refilled. The level was still perfect after 5 years and 121K miles...

 

LOL.. My Cavalier is an 03 with 235,XXX miles. It had 51 miles on it when I bought it, and I have never, EVER opened the cooling system. Flirting with disaster I know, but as soon as it's open, it's contaminated.
BTW, ditto what SWH said about waiting a day or two.

 

You're quite right about that, but...
I suggest that the expansion and contraction of the Coolant via the Reservoir (given that it's open to air) causes some contamination anyway.

HA, it's nice to be able to brag on how much service we can get out of our stuff, BUT, Coolant like anything else, has a life expectancy. I'm not sure that GM may be "buttering our bread" to a point, in recommending their change frequency to some extent, but at some point, all Coolant regardless of who made it, should be replaced.

Now, beyond that, there's a limit as to how many times the System Components (hoses/TStat/Cap) can be expected to operate efficiently, so...
That should be a consideration on how often one should do maintenance, which should include a change of Coolant, and possibly a Flush.

BTW:
I think that Prestone has come out with a Extended Life Coolant, possibly others as well ?