Detroit DD15 Bad Oil Cooler AGAIN??

I have a 2015 Freightliner Cascadia with a DD15.
A year ago I had oil in the coolant. I idled the truck, and stuck the radiator (surge tank) overflow line in a bottle of water. No bubbles.
I replaced the oil cooler myself. Now, 120,000 miles later (truck now has 715,000 miles) the coolant is black again. Still no bubbles from coolant overflow line. Oil on dipstick seems fine, no visual sign of coolant in the oil.
This truck doesn’t have a trans cooler. I have replaced the radiator and surge tank since replacing the oil cooler.
Where do I start? Thanks!

 

It’s not running hot at all. No check engine lights or anything. Hasn’t been going through coolant.

 

I’m starting to wonder if it might be fuel in the coolant? Leaking injector cups maybe? The coolant smells weird (although not really like sniffing a fuel tank) and if I stick a clean paper towel in the surge tank, it appears that the coolant and the oily foreign substance are more separated than blending together. Any experience in diagnosing this? Thanks!

 

Good advice! I have seen those O rings go bad too. In my experience, when they went bad they were old and dried out. These are only a year old.