I had my radiator cleaned and flushed yesterday. The mechanic who did the work advised that pure distilled water is better than using mixture of water and coolant. I checked the manual and it just mentioned that you need to pour in coolant but is not specific of the requirement (e.g. ratio, coolant type etc.). So out of curiosity, I checked online and of the many threads I've read, I stumbled on one of our own: Using only coolant or mix of water for radiator?. According to this thread and many more, it is advisable to use 50/50 antifreeze to water ratio.

Now, my question is if this is the case, do I need to flush my radiator again and change all the coolant to make sure that the antifreeze and water ratio is correct? Or is it safe to add coolant little by little?

Btw, I have no problem in freezing since there's no winter season here in my place.

2 Answers 2


There is a tool you can get. It has a reservoir and a bubble on the top. Suck it full of coolant and you can see your ratio and what temperature it is good to freezing and boiling wise.

The primary job of coolant is to keep your cooling system from exploding in the winter. It does help to increase the boiling point, but pure water will typically boil at about 260* F (127* C)in your coolant system. The boiling point is raised 3*F for every pound of pressure, most coolant systems will hold 16psi before the cap starts releasing pressure.

You do always want to keep some coolant in your system as it helps with corrosion and lubrication. I would purchase PURE coolant (not the 50/50 mix), drain out a bit from the radiator's drain cock and top it off with coolant.

  • I would purchase PURE coolant (not the 50/50 mix), drain out a bit from the radiator's drain cock and top it off with coolant. So it is not necessary to maintain very specific ratio. I can add coolant little by little?
    – L42
    Commented Aug 19, 2015 at 9:10
  • Yes, exact ratios are not necessary. I have the tool to gauge what my min/max temp is.
    – rpmerf
    Commented Aug 19, 2015 at 15:16

My aunties Polo had it's radiator replaced and the mechanic who completed this job used pure water. That winter, the water in the radiator froze and because ice expands, when it melted and turned back into water, it leaked out of the now broken radiator.

Antifreeze performs several jobs in the cooling system. It increases the specific heat capacity of the system allowing it to be more effective at high temperatures (i.e. increases the boiling point). It reduces the freezing point thus protecting the radiator from ice. It also includes a corrosion inhibitor which extends the life of components such as the radiator, heater matrix, metal water pipework and metallic components of the water pump.

If it were my car, I'd be inclined to drain the system and refill with a mixture. You can make, by using some additional containers, a 50% mix of antifreeze / water ahead of filling the system and keep any spare sealed in the original container for future use (obviously mark the label to show it's now diluted).

I'd question your mechanics logic and consider using an alternative mechanic in future.

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