How to Change your Antifreeze
Updated June 17, 2008
You need 3 gallons of antifreeze to do a complete change. for the Hummer. One major word of advice. Do not mix Extended Life Coolant (ELC) with regular antifreeze. The combination could cause a chemical reaction and damage the cooling system and engine. If you do change from one to the other flush the system out per the following directions.
What I do is open the bottom drain petcock and remove the radiator cap and drain it down. The picture on the left is from the AMG shop manual. It shows that the drain is lower then the engine so you should be able to drain most of the coolant from the truck. Put a hose in the reservoir and rinse out as much antifreeze as you can. If you are going to use a system cleaner such as Prestone now is the time to add it. Close the petcock drain, put in the cleaner, fill the radiator back up with clean water and start the engine with the heater on. Turning on the heater insures that antifreeze and cleaner is circulated in the heater core. Let the truck get up to temp so the thermostat(s) open. I then purge air by opening the small brass valve on the top of the thermostat housing and the other valve in the cooling line near the top right side of the radiator. Just crack open these valves enough to let the air out. Don't unscrew them all the way or you will loose the screw. Make sure there is always water in the reservoir. You don't want to overheat the engine. You will find that an idling diesel takes a long time to warm up so you might want to take it out for a short ride to bring it up to temperature.
Leave the truck running and open up the bottom petcock to relieve the pressure. I remove the radiator cap and stick a hose in the reservoir and let it rinse through until the water comes out clear. I close up the system and fill it up again, bring the truck up to temperature with the heater on. I then shut it off and drain it down as much as possible. I then repeat the process one more time. I think Prestone recommends 3 rinses. Water quality is important. If at all possible, make sure that the final rinse and the water you use to dilute the antifreeze is either distilled, Deionized or water that has been filtered through a R/O (reverse osmosis) system. Self-serve carwashes that have spot free water use R/O or DI systems. Another good idea which I haven't tried myself is the new (6/2004) Mr. Clean Auto Dry car wash unit that has a Pur water filter cartridge that gives you demineralized water for doing a final spot free rinse of your car. The cartridge is 10 bucks on average and will give you far more than 14 gallons of water.
The system coolant capacity is 6.5 gallons. Antifreeze comes either as a 50/50 premix or as a standard concentrate. The premix is nice because you don't have to get distilled water to dilute it. For a 50/50 mix I put in 3 gallons of straight antifreeze and top off with distilled water. If you really want to get technical you should mix the antifreeze with water before pouring it into the engine. Surprisingly enough, the water and antifreeze will circulate around for a long time without mixing which causes pockets of pure water that could freeze or turn into steam. If you use this method I would mix the 3 gallons of antifreeze with 2 gallons of water. This way you will be sure to get all 3 gallons of pure antifreeze in the engine. There will still be a considerable amount of water in the engine that didn't drain out. If you are filling the engine with a 50/50 premix such as Shell Rotella ELC (extended life coolant 300,000 miles) you will need 6 gallons. I like the premixed formula because you don't have to concern yourself with water purity. Don't worry about any left over 'old' antifreeze in the system, the new antifreeze will mix with it.
Bring the truck up to temp with the heater on and watch the level in the reservoir. Purge out the air by cracking open the bleeders while the engine is hot and running. Let the engine cool down. Then fill up the reservoir with water to the required level.
I check the level and purge air a couple of times over the next few days just to make sure. Get yourself an antifreeze tester which costs a few dollars at any auto supply. Make sure you have the proper freeze protection. If need be, drain some of the antifreeze out and replace it with water or antifreeze to bring it up to the proper 50/50 mix.
Too much antifreeze is not good either. Antifreeze itself is not a good conductor of heat. I've heard of trucks running high concentrations of antifreeze overheating.
I've been doing this at a self-serve carwash that has spot-free water. This water is typically processed through a RO membrane or a Deionizer which ensures it is very clean. Try not to rinse the engine out with really hard well water.
If you don't get all the air out of the system your truck may overheat and produce some steam. The high temperature steam will cause increased pressure in the system which will keep the thermostat open and blow antifreeze out the overflow.
Your radiator pressure cap is important too. The Maximum recommended pressure is 15 PSI. If you exceed that you run the risk of expanding the tubes of the heater core and thereby restricting air flow through the core and/ or causing it to leak ...don't think it can't happen ...it does!!!