The coolant in your bike – assuming it’s not got an air or oil-cooled motor – doesn’t just stop the engine getting too hot; it also prevents corrosion in the narrow pathways inside the engine, and lowers the freezing point. If you just put water in there, not only would there be a build-up of rust and limescale, the liquid could freeze, at which point it expands and can crack the engine (it’s why you might find expansion plugs built into some motors, just in case this happens).
Over time, the protective qualities of the fluid degrade, so it needs to be replaced.
If you have to top up your cooling system between services, first question where the liquid has gone. If there are oil deposits in the fluid, it could be a sign of a damaged head gasket. Another indicator of this is a mayonnaise-like substance in the engine oil, though this can often be seen in small quantities due to condensation, especially if the engine is started but not allowed to get up to temperature. If all is well, you should only use de-ionised water, not plain or boiled water.
However, as coolants are now available ready-mixed, and are far more compatible with each other than they used to be, you can top up very easily. If you have an older bike with an unknown coolant in, it should be thoroughly flushed before replacing. Most manufacturers recommend that coolant should be replaced every four years.
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