Cummins isx 15 coolant loss no external leak

How long have you ran your truck with coolant consumption ? I really don’t want to spend 10k on this truck pulling the head off and who knows what else will have to changed. Most likely will run it like this till dues truck is 2011…

 

Update:
First i have to thank to everyone for trying to help and solve this issue. Today i did the pressure test on the coolant, i did 14 psi not sure if that was enough but i think it did the job because i finally found the mysterious leak.
Main leak was coming from the small plastic splitter that goes into the heater core. In 45 min i lost 2 psi from 14 to 12.

 

I know it has been a while but finally i found the solution for this issue. In the sleeper heater core one of the o rings failed not sure which one, there are 2 lines. I just wanted to be sure that the heater core is not the issue as well and i replaced the heater core while i was there and new o rings. This was replaced couple of months ago and i have not added any coolant ever since. If somebody have this is issue and your heater core is actually bad and need replacement, on volvo it is PITA to replace because you have to pull out the whole unit, I found a way to cut the little trim on top of the radiator i guess it is part of the bed just so you can slide the heater core. Hope this is helpful to somebody out there