Mixing up coolant - tap water or "battery"?

[NiceCupOfTea]

I usually use "battery" water which I assume is the deionised stuff - but does it really matter? Are some cars more sensitive to it than others?

[tr7v8]

I'd always used tap water, but in the US they always use distilled or de-ionised water. I picked this up from the various forums & now always use distilled or de-ionised. Whether it makes a difference or not I'm not sure but given that most engines are alloy these days & the junk in tap water it can't do any harm.

[stevieturbo]

pure water is the best coolant. Although either way, corrosion inhibitor and/or antifreeze is also very important in the cooling system for obvious reasons.

So its ok to sacrifice some cooling performance to make it reliable.

[GavinPearson]

The safest thing to say is that given a choice of 'battery' water or tap I'd choose the battery because it will be low hardness water AND because the process should control a number of other undesirables in the water.

The important thing to understand about coolant is that for the corrosion inhibitor package to work that the pH needs to be controlled, chlorides need to be low, hardness needs to be low or else it affects the inhibitor (it's not just to prevent scaling) and dissolved solids need to be low to stop the water pump seals from failing.

In the USA it is very easy to find distiled water - it's in every supermarket and costs around $1 a US gallon. If you can't find distilled water but have access to a reverse osmosis water filter then that is an acceptable substitute - PROVIDED YOU KNOW THE MACHINE IS WORKING - IE IT DELIVERS THE CORRECT pH, HARDNESS & CHLORIDE. Many machines are not maintained correctly and then deliver clean looking salty water.

It is also extremely important to use the correct coolant - the chemistry of the inhibitors is matched to the materials of the engine. Get it wrong and you could end up with gelling or worse - an engine that has uncontrolled internal corrosion.