what makes the oil go black so quickly in some diesel motors
while other others don't? I got a john deere 3 cyl tractor and a 96 f350 work truck that after changing the oil its black after a week. i also have a 03 f350 work truck and my personal truck a 02 dodge 5.9 cummins both of these motors can run 5000 miles before the oil turns dark.

 

Unless you have some strange things going on , black is carbon. Carbon would come from unburnt fuel, meaning rich running...OR lots of power. All depends on the engine. same source as the black buildup on your ricer dusting pipe I love filling up imports with Cummins leftovers, so much better if I interupt their cell phone call

 

The black, in the oil of diesel engines is soot. This is a product of burning fuel. The soot gets into the oil by way of blowby, which is compression gases that leak by the rings on the pistons.

 

It all depends on how much oil is left in the engine after draining it. If I leave the plug out of my Mack overnight, it takes longer to "get black" than if I drain & refill right away. When I drain & refill right away the new oil will be black as soon as I shut it down after checking for leaks.