I bought a Ford with an unknown amount of miles. A 94 Ford ranger 2.3. runs better then you would expect, accept the coolant temperature. The first thing I did to fix the problem was flush the system, put fresh coolant back in and change the thermostat. I thought that would fix the problem because the old thermostat was stuck open. But the coolant temperature still was not reaching normal temperature. I then tested the temperature sensor but both the wire leads and the sensor were working fine.
A few days ago a hose from the engine block to the heater core blow off spilling coolant all over the road. Fortunately I was only going to couple miles down the road so I pressed on. Now despite my coolant leaking pretty bad my coolant temp rose to about normal operating temperature and that was after stopping at red light, getting on the interstate, stopping at another light.
I fixed that problem by plugging the lines that ran to the heater core. And the I flushed the system and added more coolant.
And then yesterday my serpentine belt broke driving down the road obviously stop turning the water pump and my engine coolant temp shut up.
After all of that I know every is working. I've run out of answers. And with a Texas summer coming up I don't want a fidgety engine cooling system.