BLACK RESIDUE IN COOLANT RESERVOIR

[Waterloo]

That don't sound good, something is contaminating the coolant. Sounds almost like a head issue, have you checked the coolant system for exhaust gas? If not, NAPA has test kits, they call it their "Block Test kit", and it runs around $50. It consists of a bottle of blue liquid and a big turkey baster. You fill the turkey baster with the liquid and insert in the coolant bottle opening. If any exhaust gas is present, the fluid will turn green. Unfortunately, this test has worked every time I have tried it.

That is where I would start, it is an easy test to do, just make sure the motor is good and hot when you perform the test. When you do test, give it a good ten to fifteen minutes at a minimum while performing. Tape the baster to the bottle if it will not stay put.

If it passes, I would flush the coolant system, check the hoses too for any build up. That will be easy to do as the coolant will be drained out of the system.

And how is the heat working in the cab? Up front? If you are the second owner, maybe there was a head issue where oil and coolant mixed, could be residue in the heater core too.

The air compressor, and I am not familiar with the DD motors, so I am asking, does it have both oil and coolant running through the air compressor? Another place to inspect. Does the truck build air properly, or maybe there is a new compressor on there? Thinking the old compressor may have contaminated the coolant and no one bothered to address that, or change the coolant. I could see a dealership doing that and sending you down the road.

Just off the top of my head.