Most car workshop use tap water with all sorts or mineral deposit+concentrate coolant which can result rusty water if engine block are cast iron type especially IAT type very reactive

You want best results use OAT type concentrate coolant+distilled water still rusty but not reactive like IAT

If want easy and money not a problem can use those premixed 20%/50% coolant just drain , fill then start engine until thermostat open, stop engine, let it cool and safe to open and drain and repeat the process until the coolant is fully flush

Those premixed reputable brand each bottle 2 liter RM 17 to RM 28 per bottle if radiator and engine together if need 12 liter in total you need 6 to 8 bottle or those come in 4 liter variant

And distilled water the price ain't cheap either for 5 liter if just using for flushing purpose so is viable just all the way using premixed to flush and fill i think is much more easier if money not a problem unless you want use concentrate coolant then using distilled water to flush and fill is a proper way from personal DIY opinion distilled water flushing cheaper but if you drain not enough and some leftovers on engine block which can result uneven coolant from premix coolant with distilled water.

Try buy at official or SC because coolant also got fake punya

If you know what I'm talking about 😂

This post has been edited by zuozi: Dec 8 2025, 09:17 AM

Oh ya forgot to say those premix or concentrate coolant if oat type usually is long life coolant, usually 5 year or 150k/250k km one change can last very long as long you don't go and fill your coolant with tap water can result shorter lifespan become faster rusty what ish look like.

This post has been edited by zuozi: Dec 8 2025, 09:36 AM

Tap water carry many deposit from Rusty pipe and all sorts or organic thing that the reason tap water is not recommend can also become faster Rusty if engine block are cast iron type if any remaining residue u put in premix coolant and leftovers tap water on engine block can result uneven coolant ratio mix

This post has been edited by zuozi: Dec 8 2025, 09:41 AM