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Necessary equipment:

Your vehicle
Radiator anti-freeze (either ethylene glycol or propylene glycol based)
Ratchet wrench with properly sized socket
Soft water or distilled water
Pail or other liquid container
Service or owner's manual for your vehicle (preferred, but not required)
Funnel (preferred, but not required)

To keep running smoothly, your car or motorcycle needs to have its radiator fluid changed from time to time. If you let your radiator go neglected, you might end up over-heating your engine, resulting in a shutdown and possible engine damage. While you know that your engine needs a little TLC, you might want to save yourself a few dollars by changing the radiator fluid yourself. While all automotive repair has been portrayed as a skill akin to conjuring, changing your coolant will be far easier than you might otherwise have expected.

Understand the workings

A radiator is designed to keep your engine cool when it's running. To do so, it provides a constant flow of water around the engine parts, which will 'take' the heat from the engine. Though water is about as good of an engine coolant as you can find, it has a particular weakness: it can freeze up and it can boil off. To keep the water in your engine from boiling or freezing, you will mix an anti-freeze into the water. Anti-freeze doesn't cool your engine as well as water, which is why you don't just poor it straight into the engine. The mixture of anti-freeze and water is called coolant.

Locate the relevant parts

Using either your service or owner's manual (if you've got one), locate the fill cap for your radiator. Here is where you will pour in your coolant mixture into the engine. Also, find the drain bolt, the pipe from which the coolant will pour out. Finally, check to see if your vehicle has a bypass valve. A bypass valve allows coolant to skip the thermostatic valve in your engine (that's what determines your coolant temperature) and flow straight to the drain bolt. There will be a bolt with an 'on' and an 'off' position; the 'on' position will allow coolant to skip the thermostatic valve.

Drain your old coolant

If necessary, switch the bypass valve bolt to the 'on' position (usually a 180 degree revolution). Open your radiator fill cap to allow for air to press the coolant from the engine. Finally, place your bucket under the drain bolt and unscrew it. Don't use an adjustable wrench to remove the six-sided bolt; you will strip it and make it irremovable. Use a ratchet wrench with the proper socket (probably, but not always, a 19mm). When you've got it most of the way out, be prepared to get that bucket directly underneath the pipe, otherwise, you'll end up spilling a lot of fluid. Let the fluid drain out completely. Some engines have drain plugs in the engine block as well; check your service manual to find out if you should open these. If you don't have the manual and aren't sure, call a professional and ask. Even if you can't find the answer and you neglect to do this, it won't be the end of the world; so, don't stress out too much.

Close it up

When the old coolant has been drained from your engine, retighten the drain bolt, but leave your bypass valve in the 'on' position.

Mix your new coolant

It is not actually necessary to mix the anti-freeze and the water before pouring into the engine, but this might help you to get the ratios correct. First off, find out how much fluid you need by consulting your manual or by measuring the drained coolant. Next, decide upon the proper ratio of anti-freeze to water. Never use tap water or drinking water in your engine-the salt and minerals will harm your pipes.
Most vehicles will use a one-half anti-freeze, one-half water mixture, but your manual will provide you with specific information. If you live in extremely cold weather, you should increase the amount of anti-freeze in your mixture, in order to help prevent freeze-ups. Only in the most extreme of weather, however, should you consider a mixture made up of more than 70% anti-freeze.

Add the coolant

Either one liquid at a time, or with them already mixed together, pour coolant into your radiator until you have filled it up to within an inch or so of the opening itself. You can determine how high to go by pouring until it fills the radiator, but not the pipe, which juts out with the fill cap. Turn on the engine and let it run. The coolant level will drop, so start adding more to the radiator while the engine is running. When the fluid levels off, stop adding coolant and turn off the engine. Fill the radiator to the proper level.

Flush the engine

This step is essentially optional. Because a conventional draining often leaves some of your old coolant mixture in the engine, you may wish to repeat the above process once or twice. Unless your coolant is extremely old, however, this probably won't be necessary. If you're uncertain, measure the amount of coolant, which you drained out and compare that to the radiator's capacity. If there is a significant difference (greater than 20%), you may wish to flush the engine.

Finish it off

Replace the fill cap for your radiator and turn the switch on your bypass valve to the 'off' position. You should probably expect to change your radiator fluid every three to five years, depending upon the type of anti-freeze you purchase and how many miles you put on the car.
Your car is ready to roll. Now that you're done, you realize that the task was much easier than you'd anticipated. Your engine is safe for another 100,000 miles and you've learned a little about the way it works. Don't be afraid to try working on other aspects of your car also, just make sure that you've got a good set of instructions!

Let's recap:
1) Understand the radiator set-up
2) Locate the parts of the radiator on your engine
3) Drain the old coolant
4) Figure out your new coolant mix
5) Add your new coolant
6) Flush the engine
7) Reset your radiator to go
8) Drive safely