I am interested in why the manual recommends warming up at 50% rated. Why not warm up at idle, or slightly above? There must be some interesting practical experience or theory here. Is this the evil EPA at work again?
No, this has nothing to do with the EPA, believe it or not, they are not all evil or responsible for all things. Why certainly overreaching in certain areas, with out them we could all be living in love canals, watching our children be born with birth defects or die early deaths from cancer. And that is the plain truth that those who would abolish the EPA do not want to face!
As to why a above idle warm up, a diesel uses compression heat to fire the fuel off in the cylinder, and at idle, most of that heat generated is used to fire off the next fuel charge, very little extra heat to reject into the cooling system, so diesel take a long time to warm up at idle, if they will at all.
As to why you want to get the engine to warm as quickly as possible without over stressing a cold engine, it is because of incomplete fuel burn on a cold engine. That incomplete fuel burn tends to migrate past the piston rings and that negatively affects the engine oil, and it lubricating properties.
In all honesty, it would better to start the tractor and put it under a slight load, but since different people have a different idea of a slight load, Kubota just picked a particular percentage of full throttle for warm up, thereby negating random differences in different operating styles of people.
Me, I start and let it idle for anywhere from a minute to 5 depending upon the temperature of the ambient air, and then drive away under a slight load to help it get up to proper operating temperature just that much quicker. And that comes from 40+ years of being a gas and diesel certified mechanic. Letting anything, gas or diesel idle for extended periods when not up to operating temperature is plain hard on equipment!
David