Coolant level change S70

[phly50]

Hi all, I have just returned from a trip in our '98 S70, undertaken to give the car a long ride at speed (100klm/hr) with cruise control on so that speed maintained up & down long windy roads to see how it performed. It ran and handled beautifully with 2 adults/2 kids and returned a very good ~8.5 litres per 100 klms in fuel consumption using 95 octane fuel. (mobil 1 synthetic engine oil and Dextron III ATF flush done ~ 2000 klm prior to the trip).

My question is this. I have had to add ~ 400 mls water (distilled) to the coolant tank during the trip. Outside temperatures ~ 30 Centigrade and air conditioner on all the time. Temperature gauge always at normal 3 o'clock position. Is this coolant loss normal? Should I now add a further 400 mls of volvo coolant (when able to) to maintain the correct concentration of coolant? My thoughts on this is that could the loss be likened to loss of water in a battery and that with a battery you don't add acid to top up only distilled water?

Cheers, Phil

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'98 N/A S70 136,000 Klm