Dark brown / black coolant?

[NotEnufGarage]

I drained my radiator yesterday and what came out was very dark brown, almost black yet it had a coolant smell. I drained about 6 quarts and added back that amount of distilled water. I’m gonna drive it about 50 miles today and then I’ll drain it again, and probably repeat the process until what I drain is clear, clean water at which time I’ll add full strength Zerex G05.

Any idea what this black stuff is?

[meder24]

My guess would be the previous owner used different coolants at the same time. The recommended HOAT coolant is orange/yellow in color. This mixed with the standard green color could result in a darker brown color.
This is purely a guess though.

[rcktpwrd]

I agree, probably mixed incompatible coolants.