I think I am on to something
While checking out a Mountaineer I recently sold with myscangauge, I noticed it's coolant temp was as expected, from 192-196 in all conditions.
However Betty's temp runs at around 171, until a load is put on it, like 70 mph with ac, or towing a trailer. the temp will go over 200.
I now truly believe a genuine Ford thermostat might be the answer. After several different thermostats, I still see the same thing
Even the old engine did this.
"new" engine also has new ECT sensor, new radiator, new water pump, and new hoses. Everything has been gone though and cleaned out.
I suspect the "universal" 195 degree thermostats are faster to open and later to close. After 5 different brands I am going to bite it and pay 25.00 for one direct from Ford. A tech there told me he has seen this. These days "cheaper" thermostats are rated for when they open fully, not the rate at which they do so. So, I will let you guys know what happens once I get it installed.
Unless the scan gauge uses feedback from the temp gauge sender, I have not other explanation for this. It is the only part I haven't replaced, however I must add the gauge in the cluster coincides with what the scan-gauge says.