It's DIY time for changing the coolant. The Service Manual shows only one drain valve. It does appear to be lower than the engine block, so should drain the entire system. There isn't a drain valve in the block as with many "conventional" vehicles, is there?

...Bob

Bob:

I just did mine. What I did was drain all I could through the petcock in the lower radiator pipe. I then filled the system with water and ran the truck with the heater on till it had warmed up. I then drained the system. Next I filled the system with water and prestone radiator flush. I repeated the same thing twice to get all the remaining cleaner and antifreeze out of the system. Next I drained as much water from the system as I could. Put in 3 to 3.5 gallons of antifreeze (96 turbodiesel) to get either a 40 or 50% mix.

Bob--What I did was remove the caps from reservior and over flow. Warm up the engine a little, then remove the whole stop cock to let it drain the bigger debris (mine was very clean). When empty, replaced stopcock and added radiator flush with a topping up amount of water. Let run for 10 minutes (as recommended-Prestone). Stop Cock removed and drained again. Stopcock replaced, this time plain water added and run to temp or for a few hours. Then again stopcock and caps removed and drained. Replace stopcock and add antifreeze 8 qts and distilled water to level with caps open and to temp. Close cap and you are done. Check level in a few days.

ms

Mike:

Exactly right. Just note that the turbo diesel has a larger capacity cooling system then the gas engine. Bob should have added 3.5 gallons to get a 50/50 mixture. Antifreeze doesn't absorb heat as well as water and has a higher specific gravity. If you use too much antifreeze you cooling ability will be reduced and the water pump will have to pump harder to circulate the coolant. Being that our winter in Illinois is very mild, I have only added 3 gallons which gets me down to -10 f. If I lived in AZ this is the mix I would use all the time. It will give you better cooling when it's really hot. Note that the -10 is when the fluid starts to freeze. The actual burst temp (where the block would crack) is probably closer to -35.

For those that don't know you need the antifreeze year round because the A/C evaporator is next to the heater core. When the A/C is on in the summer you could freeze and split the heater core. You also want the rust inhibitors etc.

I also noticed when I drained my coolant for the first time that it was black. I was told that the manufacturers often add stop leak to the coolant to all new vehicles. This is another good reason to change your coolant early on.

Bob, Just got through with my coolant drain and fill. Unless you want to pull a hose off somewhere, it appears that the drain valve is the only access. Someone else with more intimate knowledge of Hummers may advise us further. A suggestion: leave about 2 hours to do the job right. The drain valve drains the coolant at approximately the speed and volume of taking a whiz. I did the following: initial warm-up, turn on heater, stop engine and drain completely. Refill with a radiator flush and water and run 10 minutes. Drain and refill with distilled water, especially if your house water is mineral-rich (hard). Run engine 10 minutes, stop, drain completely. Will take about 4 gallons of coolant (equal to about 60% coolant-to-water ratio assuming 26 quart total capacity). If higher ratio is required, drain off a gallon and add a gallon of coolant. Good luck, it's easy stuff but takes some time.

Bill Cole