I am not familiar with the myvi, but go take to a shop and do a pressure test. They will pressure the cooling system to 1.5 bar and see if there are any leaks.

Many leaks happen only when the car gets up to running temperature and under pressure.

If there are no leaks, you can start by changing the cheapest item, the radiator cap which would would be a few ringgit. A 9 year old car would probably have hardened hoses, and if you have not changed them, perhaps it is time you do so. A set of hoses shouldn't cost more than 100 for a set. Once you take off the hoses, you may see if the steel hose necks are not corroded...replace the neck if corroded, or in some instances if salvagable, a mechanic can slap on more RTV silicone to seal it better.


but having to refill water every ten days sounds very serious to me. The leaks should be quite easily spotted.

Water pump leaking would be rare, given if you had only run 140k, but not an impossibility. You can only spot any leaks from the water pump after you remove the accessorize drives and the timing cover.

Regardless, a normal outside mechanic would probably be better experienced to catch these sort of leaks since they would be more experienced dealing with older cars than the ones at the dealer.

Get it properly diagnosed before replacing anything.