dirty Coolant isx cm2250

[cjcool]

We have an ISX cm2250, it all of a sudden has dirty coolant, we did a coolant flush and it was dirty again within a day. Not sure if its oil, fuel, or according to Peterbitl ; soot from the EGR cooler ... is that even possible? never heard that before, that soot can leak into the Coolant from the EGR. Note : the system stays under pressure overnight. The Coolant is not overflowing or overfilling.
Overall runs decent, almost 800,000 miles. Did the usual DPF clean sensor clean/replaced does a burn every 19 hours like clockwork, max Hg 3.2 in last 10 burn history.
If we have to change the EGR cooler ... fine , i am just not convinced that is the problem... anyone ever had this symptom. Coolant is a brown color no smell, no oil separation in bottle test... we are lost