First off, the temp sender/guage (as a working system) can be off kilter and you really don't know what the accurate temp is until you put a digital laser thermometer on the sending unit. If you have a relationship with a mechanic most will have one of these and it takes all of 1 minute to diagnois the accuracy of the readings.

Once you determine the accuracy of the system, then proceed. You probably have a bad or "tinkered with" thermostat. My experience is that the Whaler brand from Autozone has a larger opening at "full open" than the dealer or Behr brand. The frame construction is different. I have checked them on the stove.

Check your thermostat in the kitchen on a pot of boiling water with a meat thermo before installing.

I currently a thermo that opens too soon, the engine takes too long to heat up and I need to replace. Only had the car 2 months. Yes I checked the temp system and its reading accurately.

My previous 300D temp reading system was off by 17 degrees C. As long as you know where you start, then you can properly diagnois.

BTW there is an approx. 10 to 15 degree C temp difference between the 80 degree thermostat and the temp at the sending unit (after the coolant has circulated through the head).

__________________
84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)