Could you please elaborate on this?
In my experience, the heads are the first thing to heat up (and overheat) when there is no coolant flow.
It will take a while at cold start but once it gets hot, that's it.
Heads will not cool down without flow.
With stagnant coolant, it will heat up and the gauge will work fine.
Eventually the coolant will flash to steam.
But even with zero coolant in the system, a sensor in the head will still read hot from radiant heat or steam.
I guess it's possible for the gauge to fluctuate on the hot side in this state, but I can't imagine it ever moving into a normal range since the entire head will be heat soaked including the metal sensor itself ...
Same can't be said for a sensor in the intake or waterpump though.
Intake and WP will take much longer to heat soak than the heads ...
One of the dumbest things I've ever done - and documented here on the ISSF - was made much easier with an electric H2Opump.
On a day that peaked under 23F, I started my LT1 wagon with the HDMeziere disconnected and unpowered.
Wanted to see how long it'd take the LT1 to go from an ice cold start to close-to-overheat.
Once I started it, I drove conservatively (northbound on Woodhaven).
After 7min cruising 30MpH, the dash temp gauge needle showed hotter than I'd seen it in many years.
I reconnected the HDMeziere; my friend said the needle never touched / crossed the white line.
Important to note: H2Opump temp sensor did not heat up until after the HDMeziere was reconnected.
One of the ISSF elders admonished my folly; 'could've thermally shocked the engine', something like that.
Maybe he even remembers ...
Got lucky, it ran fine for several years after, despite my continuing to abuse it in more mundane ways, long enough to begin researching how to make LT1s run differently once 'hotter than necessary'.