Detroit 671 TIB Coolant Selection

Stick with the Powercool 3000

Distilled water, no ?

Your local Detroit Diesel dealer will have it. Also flush the motors before putting it in there.

https://www.carchex.com/blog/can-you-mix-tap-water-with-engine-coolant/

They may say tap water, but that's cause Detroits are old school and can almost run on spit. I think we've all learned in recent years that tap water is a wild card depending on where you live, with all sorts of minerals and pollutants. I'd stick with the good stuff that's the same jug to jug. As for coolant, I remember the days when if you didn't use the specified brand of oil in your car it could void your warranty cause you had some pretty questionable oils on the market and it was a great money maker for the car manuacturers. A few law suits and better quality control later and you can now use any oil you want (although the smart owner stays away from the no name brands). Pretty much the same thing with coolant. What's the difference between Detroit Power Cool and something like Prestone or a quality non-tox antifreeze other than price? Like with oils I'd stay away from no-name anti-freeze, but unless Detroit still gets away with it affecting their warranty I don't think you'll find much difference from one to the other.

+1 on the water quality.
Engine manufacturers also have learned that the acidity or alkalinity (PH) as well as salts, can vary widely in the wild. It's simple science to protect cooling systems and engines. Don't use salty acidic water in a system with moving fluid.
Coolant or coolant substitutes serve to control galvanic corrosion, or metal wasting. Years ago, I had to replace a thermostat housing and a water pump on a car engine because the aluminum was so wasted from plain water that the aluminum walls just disappeared until they opened to the outside. There are many dissimilar metals in a cooling system.
And, there is a lot of science for coolant types. There are different formulations for 1-2 year and 5-10 year coolants, as well as different products in the same categories for gasoline and diesel engines. There are test strips to determine the level of protection available in a cooling system that's in service. CAT and John Deere, to name two, call for measuring the effectiveness of the coolant at prescribed intervals using test strips. Some may dismiss the forward march of technology, but the thing to remember is that many of these developments came hand-in-hand with longer warranty coverage and recalls. When the manufacturers have to pay, there's a sudden leap forward in product serviceability and reliability.

Bring serial numbers with you and ask the parts guy to look it up. Since it's been so long, I would drop all of the coolant (easier to pump it out), add water and I think DD makes a flush solution......run the motors to temperature.......let them completely cool off, drain it, add straight water, drain it again, and then start with fresh........My guess would be 5-6 gallons per motor of coolant on 6-71