On the test I did this morning the car was just idling ( I was doing a little refining on my cold start and warm-up enrichment tuning). The coolant temperature on the front of the head sensor came up to 81 - 82C and then sat at that temperature. The back of the head (dash gauge) temperature rose slightly higher and changed a bit when I opened and closed the heater valve. At idle, opening and closing the heater valve did not seem to have any effect on the front temperature sensor.

I have had this car since Gerald Ford was the US Pres and I used to drive it in really cold temperatures (-40 C). Of all my friends in university and post university my 142 was the winter go-to car because it had the best heat in the winter. I realize that the OEM temperature gauges tend to be desensitized in their normal operating range; but, in really cold weather my recollection was that the needle was pretty much where it was during warm weather and the needle pretty much always stabilized at that same spot when I opened up the heater valve on cold days.