I have an 86 6.9 idi new to me by about two months. The truck has never gotten up to the middle of the temperature gauge as long as I’ve had it. I drove it home from Massachusetts’s to NJ on a hot July day and the gauge never got above the C mark on the dash. I figured it was a faulty sender so I flushed the coolant put in a new motorcraft thermostat and changed the sender but the truck is still reading on the C mark on the dash. I took a temp gun and shot the thermostat housing after driving the truck around and it was reading in the 190 range give or take a couple degrees. The radiator cap is a 13lb motorcraft. Any reason why the gauge isn’t reading in the middle of the normal range? What should the thermostat housing be reading? The PO changed out the radiator to an aluminum one with plastic ends is that the culprit or just a bad gauge on the dash?