[submariner409]

Whether the ambient air temperature is 20 or 100 degrees the engine should run in the thermostat/fan operation band: 188 to 224 degrees, 20 mph or 120 mph.

As Ranger posted, these things have very healthy cooling systems. IF you are continually adding coolant to keep the level at halfway up the surge tank, COLD, then you have one of two problems: A system leak or failing head gasket(s).

If you cannot positively document a leak, then test the airspace in the surge tank for exhaust gas.