2011 dd15 black overflow tank

[race7820000]

I have a 2011 Cascadia with a dd15, driver has to add a gal of antifreeze every 4-4500miles. When it started I assumed it was my egr cooler as it's went out before.

Truck runs fine, doesn't over heat. I dropped the pan to change the oil plug incert and there was no evidence or water in the oil. I can't say if it using oil as the incert would drip out about a half gal a week. Normally it doesn't use oil really.

I pressure tested the coolant system with pan off and didn't see any coolant. It held pressure well for a about 30min or so as I removed it. I can't tell if the loss of coolant was because of a loose clamp as I tightened all the hoses.

Today I took off the mixer pipe and the elbow that goes from the egr valve to the cooler and didn't see any evidence of coolant burning.

The truck starts super easy. Normal oil pressure. It's not building excess pressure in the coolant system

When you take cap off overflow it looks more like soot than oil and its sticking to the tank. It's not milky just dirty black whatever. You see some floating on top of the antifreeze but I can't tell if it's oil, diesel or what because it's so thin. Looks almost rainbow like swirl with a black outline.

Also I checked the trans cooler and there is no antifreeze in it.