Welcome, An 85 300TD, great car. Mine was 904 blue with palomino, it was nice but the NE tin worm reclaimed her @ 307k.

Couple of things...

1 I wouldn't be comfortable with what the dash gauge shows.

Put black electrical tape on whatever you aim the IR thermometer at to get more accurate readings.

Treat it with CRC ThermoCure, it works wonders.

Don't exceed 50% coolant.

RedLine WaterWetter emulsifier helps.

On a hot engine just after shutdown does the fan clutch offer resistance or freewheel? It should freewheel cold and build resistance hot.

If you have a used thermostat, gut it and retest. I used only Behr or Wahler stats.

Remove the gauge sensor, clean both ends, tighten the contacts and retest.

The temp should be a degree or two above the thermostat rated temp. You should be able to see it open by watching the gauge for a hiccup near the rated temp.

Air pockets are a possibility, fill the system with the nose raised, slowly and with the heater on full temp.

Again, CRC ThermoCure has worked very well for me and actually saved the original block in a valuable collectible car.

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