So, my wife has a '14 Malibu LTZ (2.5L). 66k miles, we bought it back in April with around 54k on it.

Yesterday it overheated on her (about 70 miles from home, also) displaying a dash message to the effect of "AC off engine overheating" and something like "idle engine, overheating." She said the needle was well into the "H" on the guage.

When I got to them, the upper radiator hose was warm, lower cold. And the dash vents were blowing cold. Stuck tstat, right? We get it towed home, and it took maybe a quart, quart and a half of water. I drove it like a 16 year old for 40 minutes never could get the temp above 205, as displayed on my scanner

Reading about those tstats, it seems they're electronically controlled? Before I just go throwing money at it, figured I would do some research
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Anybody have something like that happen, or have any knowledge to share about that engine, and it's cooling system?

Edit to add: the day before, I removed the 2" flap deal that hangs down under the front bumper. It was all torn up from catching on parking bumps, etc. Could that have made the difference? It drove 70 at 70mph fine, around town, and only overheated on the return trip

 

*Moved to 'The Lounge'*