water out the exhaust

Smoke will either be coolant, oil or raw fuel. Enough fuel out the exhaust usually stings your eyes so bad you can even stand near it, hard to mistake it. If you are burning oil, or using coolant, then you definitely have severe problems.

 

I used to run a 2017 Volvo, I'd always keep the DEF topped off. In 1000 miles, every other day it would take about 6.5 gallons.
I don't know if that's normal.
But, we had other Volvos. Always about the same amount.

 

It is most likely water from condensation or left over rain water. I am assuming the Pete you are talking about is the the one in your picture with the miter cut stacks. I know the SCR Kenworth's my fleet has have drain holes in cross over exhaust pipe under cab to allow water to drain/ pipe inside of a pipe set-up. I uploaded a copy of your exhaust layout. With that much exhaust piping, I could see how water would condensate in your pipes and collect in the y pipe. Put some good heat in exhaust and then you have steam. With an ISX 600, I hope you are hauling close to gross most times. I am sure when empty it doesn't have to work hard and this keeps your exhaust temps down creating more water vapor/water @ y-pipe/ and steam. Just my theory.