[Jack21]

All engines have an optimum operating and coolant temperature range. Since about '72, this has been 180 - 210 deg. Engine clearances are designed to operate in this range. Too high, and the clearances tighten up resulting in accelerated wear. Too low, and clearances are too loose resulting in blow by, sludge formation, water and fuel contamination, and accelerated wear.

Need to keep coolant temp within operating range with correct, and functioning thermostat, and thermostatically controled fan with manual override switch.

The override switch lets you anticipate high coolant load when coming off highway into stop-n-go traffic on a hot day, or stuck in 0 - 10 MPH rush hour traffic.