2008 Chevy Equinox: no heat in idle

My 2008 Equinox had not been getting heat in idle, but heat would come on when driving. There was also a strong coolant smell when heat was on. Recently, I was getting overheating messages on the dash, but when turning the car off then on again pretty much right away, the message went away. Also, the temperature gauge on the dash appeared to “stick”. When the car is off, the gauge would only go back to about 50%.

So, I brought the car in to get it checked out. The mechanic’s first solution was to replace the thermostat and flush the system. They also wanted to replace the dash cluster gauge for a ridiculous amount of $ (I passed on that for the time being).

So, thermostat replaced. Same problem (no heat when idling and smell of coolant). No overheating and the gauge seems to be working properly again.

Brought the car back and they were able to find a leak at the heater core, so now that’s been replaced. The smell seems to be gone (possibly a little residual smell that will go away?). But, there’s still no heat in idle. On the highway, the heat is blasting.

I guess my question is.. is there a break-in period on a new heater core? Is there some other potential problem?

Thanks for any help.