I’ve replaced the radiator, cap, coolant lines & the thermostat thus far but I’ve not solved this problem:
When the vehicle is driven about town at under 60-65 MPH, it never (months) runs out of fluid but take it on the freeway at 65+ MPH and it runs out of radiator fluid in 2-3 hours and after short cooling time, can run out very quickly if I return to freeway speeds right away.
No specific site of leakage obvious when I stop, overflow bottle is usually still full. The correlation to speed seems consistent but could be a chimera for heat from the transmission (automatic) or something.
This vehicle has the little 4 (I still think in CID, so I can never keep track of the engines measured only in Liters), no power steering, A/C (that hasn’t worked in a couple of decades), Automatic