Bjorn,
You keep making reference to running either the winter cover or your plywood cover for the radiator. Also, the reference to the engine temperature being poorly if not uncontrolled, especially the fact that your engine will not warm up or will cool below "normal" operating temperature when operated under light load. (Such as light crain work.)
This sounds like you have a leaking bypass or such in your cooling system. The thermostat should maintain temperature to within 5 degrees of the opening point up to the cooling capacity of the radiator.
Yes these engines are slow to warm, due to the mass and mostly to the air flow through the cylinders, but when warm should maintain the thermostat setting easily. The 4 units I have direct experience with all perform this way. The exception was the newest (1974), it was VERY slow to heat, and would cool excessivly under light load of cruising. Solution was replacement of the bypass seal in the thermostat cover. Now she warms in approximately 20 minutes, and will hold at 185 degrees regardless of idle, low load, cruise, even in 20 degree weather. It will climb to 200 degrees under long heavy pulls on 85 degree days with a total moving weight of 36,000 pounds. (Yes it was pulling my trailer with a substantial load..) It will also see 200 degrees when climbing such as MonEagle or running in the mountains and pushing it hard in 5th, but cools back to 185 at the top or coming down.
Any thoughts on why you seem to have a truck that has a thermo system that will NOT isolate the radiator? Your coolant filter is not the bypass path is it?
And by the way, the hotter you run the engine the better the fuel milage will be, and the more power it will make.......BUT it is diminishing returns due to not haveing an aftercooler on the turbocharger.
An after cooler is what we need to figure out how to retrofit...
Squirt Truck