I am using pre-diluted (50:50) coolant in my car. I have observed that with passage of time and use (around 4 month), coolant quantity in reservoir bottle decreases to middle of MIN and MAX.

I have a question that should I top up the reservoir bottle to MAX level with same coolant or should I top up reservoir bobble with distilled water only?

I heard from someone that for top up purpose only use distilled water instead of coolant because only water evaporates with passage of time not the coolant chemical. so if you keep adding coolant, it will increase the coolant to water ratio and may damage radiator and engine system.

  • Your coolant level shouldn't be dropping like that @Mohsan, if it is you have a slow leak somewhere that needs to be fixed.
    – GdD
    Jun 21, 2018 at 10:09
  • dropping of 100 to 150 ml in 4 months with usage of 2,000 Kms per month. isn't this normal?
    – Mohsan
    Jun 21, 2018 at 10:17
  • 1
    No! An engine's cooling system is a closed loop, you should never lose coolant. If it is dropping then you have a leak, hopefully an external one through a hose or seeping out of the radiator. Otherwise it's a bad head gasket, intake manifold gasket or a cracked head, which is very bad news.
    – GdD
    Jun 21, 2018 at 10:25
  • hmm. car is brand new.
    – Mohsan
    Jun 21, 2018 at 10:43
  • If it's new then it's under warranty and the dealer should fix it @Mohsan.
    – GdD
    Jun 21, 2018 at 10:48

1 Answer 1


The proper answer to this is that you should use a coolant tester to check the concentration of the chemicals in the coolant, and then top up appropriately to keep them balanced.

In practice, however, I'd usually top up with the same coolant/water mix - in general, it's better to have too much coolant in the mix than too little. The amount you're topping up by is quite small - I'm guessing probably only 100ml or so from the mid-point to MAX, depending on the car, so you're not going to make much difference to the concentration.

  • yes need to add between 100 to 150 ml . is it normal that car decreases this quantity of coolant after 4 month usage with AC and running of around 2,000 Kms per month?
    – Mohsan
    Jun 21, 2018 at 9:48
  • Not usually, no. As @GdD says, you ought to get it checked if it's losing that much
    – Nick C
    Jun 21, 2018 at 11:57

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