Got a minor issue with my coolant turning black. This was a problem once before that seemed to have been remedied by replacing the heat exchanger (probably oil cooler leak). But now it's back. I'm not sure what this is. In the tank it looks gritty or powdery. It's like a sediment that settles at the bottom and when shaken acts like kool-aid powder turning the water black. Combustion gasses? It doesn't smell like oil and its not slimey or slick. I have a sample from draining the tank I'm thinking about getting tested somewhere.

 

Petroleum products don't mix with water so they rule themselves out pretty quickly there. I'd agree with you on it being carbon but don't know where it would get into the coolant. Head gasket?

 

I'm thinking head gasket leak too. But I hold some disbelief because I did a dry compression test earlier this year troubleshooting a different issue and all 6 cylinders hold 176 psi - 184 psi just fine. I figured if there was a gasket leak, I'd see a bigger delta between two of the cylinders. I'll have to see about getting this sample tested.

Definitely not intake gaskets. I just changed those out this past Spring. I did find some carnage around the water channels on the old gaskets, so there could be residual matter left over from that. But this is the third time I flushed the reservoir tank, so I'd think it clear up by now if this is residual matter from a leaking intake gasket.

 

I had that too. You can drain it out of the reservoir and put clean water in but in a few days the black will be back. The cure for me was a new radiator and a flush.