The liquid in your radiator not only stops your engine from overheating, it also protects against corrosion and prevent calcium build up inside your cooling system. Often the cooling liquid specified by manufacturers has some other beneficial properties, like absorbing heat even better than plain water.
Not all cooling liquids are the same, and it’s unwise, and in some extreme instances even dangerous, to mix different cooling liquids together. It’s therefore important to topping it up with the right liquid, but replacing it with fresh cooling liquid is a good insurance. You should also consider the properties of the cooling liquid, as the differences in engine cooling can be up to 20 degree Celsius.
To avoid risk of burns, replace the cooling liquid with a cold engine. With the motorcycle on a stand, put a container that’s large enough to hold all the fluid underneath the drain bolt at the bottom of the pump impellor cover – the one near the bottom that is normally away from the sealing edge bolts. But to be sure check your owners manual for the location of the drain bolt. With a catch tank in place (be careful, as it comes out fast) and cloths to avoid splashes getting on the floor, remove the drain bolt. Usually, not much will come out. But, once you have the bolt out and have removed the radiator cap, the flow will begin. By taking the cap off, if releases the pressure, letting the fluid flow out freely. The old coolant should be carefully disposed, too, at a chemical recycling tank.
Ideally, replace the drain bolt and fill the radiator and cooling system with demineralised or distilled water to flush the cooling system. With the water in and the drain bolt in place, you could run the engine for a few minutes, which will help pull out the old coolant. Don’t let the engine run too long, ‘cause you’ll need to wait for it to cool before removing the water without risking damage.
Once done, remove the drain bolt and radiator cap (like you just did) and add that water to the coolant that is to be disposed.
Replace the drain bolt, torque it up to manufacturer’s recommended setting (remember, not too tight or too loose) and slowly fill the radiator (to minimize the amount of air bubbles introduced) all the way up. It is a good idea to run the motorcycle for a few minutes, as it will circulate the new coolant and work air out, before switching off and waiting for the motorcycle to cool.
Before you go riding, check the cooling liquid level, as chances are it will have dropped as air bubbles were displaced. Now is also a good time to check for any leaks or perished plumbing, taking a good look at the system while you’re up close with it.