Flushing the Coolant System
On a cool engine (less than 140*F) remove the radiator cap. Now at the bottom of the radiator, there is a drain valve on the radiator. The drain turns 1/2 turn then pulls outward slightly then continues to turn to drain. To drain the overflow bottom it just easier to remove the hose from the radiator neck and then pull the overflow bottle upwards from the fan shroud.
Flushing Cooling System
Flushing the cooling system you'll need to remove the thermostat. There are three 8mm head bolts holding the cover in place. Once the cover is removed you can lift the thermostat out. Now replace the thermostat cover and the upper hose. The upper hose should only be hooked to the thermostat housing. This will direct the water away from the fan for you. Now using a garden hose at full flow you can place it in the radiator fill neck. Start the truck. Allow the water to flow till all solid debris and coolant color is gone. The water should come out clear. You can lightly rev the engine to pump water forceful for bursts. This will help push water through the heater core flushing it as well.
Take notice the radiator is removed in this flushing.
Filling With Coolant
Now as for coolant, our trucks came factory with green coolant. You may use green, yellow (universal) or orange (HOAT). Just remember "Universal Yellow" typically can be mixed with green or orange coolants in an emergency. NEVER mix green and orange coolants! If you are going to switch coolant make sure to flush the system thoroughly. I typically will park the truck on a nose down slope to attempt to get water out of the system. Cummins cooling system holds 6 gallons typically there is roughly a gallon trapped in the block. So the first gallon goes in full strength. All other coolant is mixed 50/50 before using after the first gallon. As for mixing coolant and water. Most suggest distilled water be used. Now back during the flushing, I had removed thermostat now you can fill the entire system till its right to the top of the block. This allows you to fill without having to burp the system later. Now once the block is full to the top you can install a new thermostat (suggested) or re-use your old thermostat. Re-assemble your thermostat housing. Make sure to use anti-seize on the thermostat bolts. Re-assemble your upper hose. Now finish topping off the cooling system with mixed coolant. Make sure to fill the overflow bottle to the FULL mark.
Coolant Color & Condition
I'm going to say don't open the radiator cap and look and say, "Yep it's still (whatever color) its still good." I've heard this a million times. If a customer is willing I drain their radiator and show them typically the first gallon comes out rusty brown, then may change back to coolant color slowly as it drains. What happens is every time you start your engine your passing high current flow through the block and tends to change the pH of the coolant just like changing a battery. So over time coolant will shift from pH of 7 and typically shift corrosive. Lime scale builds up typically is from not the water but from dissolving metals in the cooling system from the corrosive coolant. So don't get hung up on the idea of long interval coolants like HOAT or distilled water will protect you from scale build up. You can still get scale build up with distilled water or premixed coolants. Simply put if the coolant starts to darken in color it already failed.
As for my truck the first 150k miles was on green coolants. I was flushing the system every 30k miles or so. I will clearly state I've NEVER used distilled water in my truck. I've used either the irrigation water which is from a creek behind the house or from the well that is here on the property. No water here is treated or chlorinated. I'm now using "Universal Yellow Coolant" typically found as WalMart SuperTech, Prestone, and even Napa coolants. I've never had an overheat once. I've never had a heater core plug up. No limescale build up.
100k miles and 5 years....
Updated pictures... This is my cooling system after 9 years and 180K miles now...
2015 (13 years later) and 255k miles later.... Using Universal Yellow Coolants. Still no scale build up or cooling issues. This batch was extended to 70,000 roughly and 2 years.
350k miles 07/20/2018. Coolant change with NAPA Extended Life.
July 2019 - 385k miles and replaced the oil cooler gasket. As you can see after 17 years of service and using tap water (well water) or creek water like I have there is zero scale build up yet.