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Morning,
I have a cm871 in a 2010 Pete 389. The motor was rebuilt and deleted by a shop in North Dakota. Since I have put the VGT back on, put in a reliable tune and got coolant flowing to the back of the head again. My problem now is that when doing my service I found my fuel in my filter was black and now my coolant reservoir is stained black. No blow-by or coolant pushing out the overflow. Any ideas? Is this a head again? I did run the truck for 10 months on the shitty tune and no coolant crossover pipe.
Thanks for your time ,
Bill

Maybe take a sample and sent it to a lab?.

Black stuff in the top of the coolant tank is usually a sign of oil getting in the coolant. Oil in the coolant is almost always from the oil cooler, not the head.

Also smell the oily substance. Sometimes it will be transmission fluid if the truck is equipped with a heat exchanger for the trans. Though the oil usually presents as a grey brown color, black is not out of the question. It will smell strong like gear oil.