[robertgraber]

Multiple steps have been taken to resolve the initial over heating of my wife's G6. First we discovered a pinhole 8n the radiator and installed a new radiator, thermostat, and thermal sensor. Of course burped the system, ran it and everything was fine, for 3 days. Wife took it to work yesterday and lo and behold, temp shot up in seconds. I drove to where she was stopped on the side of the road and took over. It was at normal operating temp and said let's get to the mechanic quick. Well in the country that's relative. We were 10 miles from our mechanic. It of course heated rapidly.
Now here's the wired issue. my wife told me once that on a road trip to South Carolina that she had forgotten to put the cap on the overflow where you fill coolant. So I popped the hood again and thought well I'll just loosen it a bit. Took off the cap and put it back on but didn't tighten it down to what one would think it to be normal. Temp was at normal operating range so took off again. This time it never went above normal operating temp! Drove the entire way there with no overheat issues. Is there something with the cap that controls the heat issue??
Please help me understand what the heck is going on.

 

[MichaelWojo]

Must have been an air lock stopping the flow through. When you opened/ loosened the the overflow cap, you gave the system an opportunity to dislodge the air block. But that's just my theory.

 

[Goggles]

Not sure which engine you have but, On the V6 engines generally there is air bleed vent near the water outlet that must be vented when the engine gets warm after refilling. This is a higher location on the engine where an air bubble can collect potentially causing the coolant recovery tank to not work properly.
Just Sayin'