Engine block

Easiest way is to disconnect the lower radiator hose at the radiator, and disconnect a heater hose from the engine. Avoid pulling a heater hose off at the heater core. It is too easy to cause a leak there. Once the old coolant is drained, run water from a garden hose into the heater hose for a minute or two, then into the fitting on the engine. Reconnect the radiator hose without the clamp, then do the same steps. Plug the heater hose or port with your finger to force the water to come up out of the radiator or reservoir. The engineers could not be bothered to put a radiator cap on a lot of their vehicles, so you have to modify the procedure a little. The point is to get water flowing out of the various places.

Be aware that GM has a real lot of trouble with corroded heater cores and radiators, and flushing the system can remove debris that is blocking a leak. They also have trouble with the coolant becoming excessively acidic. Before you start, use a digital voltmeter with one probe on the battery's negative post, and the other probe touching the coolant in the radiator or reservoir, but without touching anything metal. You should not see a reading of more than 2.0 volts. If you do, the coolant has built up too much acid, and it will cause the same corrosion that takes place with battery acid. Flushing the system is the temporary fix, but new coolant has the additives in it to prevent this from happening again.

Once you have had water running out of as many places as possible, disconnect the radiator hose again to drain as much water out as possible. Run water into the reservoir to wash that out. If you have a radiator cap, install it and tighten it, then continue adding water to the reservoir. Eventually the siphoning action will pull most of the water out of the reservoir.

Reconnect the lower radiator hose and clamp, then start to fill the engine with fresh antifreeze. Please don't waste your money of the "premixed" antifreeze. You are buying a half gallon of water, and you will not have the ability to adjust the mixture. There is at least a half gallon of water in the system that will not drain out. Start by adding one gallon of new antifreeze. Next, add one gallon of water. If necessary, add another half gallon of antifreeze, then another half gallon of water. Keep alternating until the system is full. When you see coolant start to come out of the heater hose port on the engine, reconnect that hose and clamp. Leaving it off up to this point lets the air come out so the system can be filled easier. V-8 engines usually do not need special bleeding procedures. If there is a bleeder screw on the thermostat housing, open that as you fill the system. When needed, some engines have a threaded plug or sensor near the thermostat that can be removed for bleeding the air out.

Once you have the engine filled, run it until the temperature gauge comes up to normal, and the upper radiator hose is hot. That will mix the antifreeze and water. Hopefully the hot coolant will expand into the reservoir, or, if you are lucky enough to have a radiator cap, draw a sample to test the freeze point. Now you still have the empty reservoir to add antifreeze or water to bring the freeze point to -35 degrees. After a few drive cycles, recheck the freeze point again, and adjust it by adding a little water or antifreeze.

Do not try to get the freeze point lower than -35 degrees. As the concentration of antifreeze increases above about sixty percent, the freeze point actually starts to go back up. Antifreeze with water freezes at a lower temperature than does straight antifreeze. This is misleading with most testers because they simply measure the weight of the coolant in relation to the weight of water. Antifreeze is heavier than water, so one hundred percent antifreeze would show up as having a really low freeze point, but in reality the chemical mixture will freeze at round -10 degrees.