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Is it OK to top up coolant with tap water? Needed a bit today for the first time - I hope just a consequence of the Canberra weather.
It was put in by the person who rebuilt the engine and I only know that it is green.
If not preferable, what should I buy?

Buy some coolant. Get green to match as the other colours may not be compatible. Coolants are like oils, super technical stuff.

generally speaking if it was an oem spec build you should use oem spec coolant (assume you need anti freeze in canberra). so probably best to top it up with that.

if you need to top it up in a pinch you should use demineralised water (bunnings sells if cheap for $4 for 5L in the cleaning section), not tap water. but if you can get demineralised water you can get coolant.

depending on how much water you put in or need to put in when you are forced to put in tap water you would probably change the coolant at your earliest convenience.

Oops!
Thanks guys.

I'm fluffing all over the internet looking for the right coolant. Anybody got a link please?

Info I have in my SE's specs sheet is ethylene-glycol based coolant, just pick a brand. DO NOT USE alcohol or methanol based coolants. That's from my handbook. Pretty certain any of the models would be the same. As Bruce mentioned check the color and match, don't mix.

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Deminerialized water will do if it is just a small top up.

Tap water has so many minerals in it that can cause rust.

FWIW, this is what I've been using for years in my NA6... available in 5 litre and 1 litre packs...
https://www.nulon.com.au/products/cooling-systems/green-premium-long-life-pre-mix-coolant

Nulon also sells a coolant that they market as being safe to mix with any other coolant. Life span defaults to what is already in the radiator system.

Yup, nulon website has a search function.
But I wouldn't be too fussed as anything green should suit. Just do a big proper change down the track when due.

You can over do these things - a top up with water is not going to be a problem. De mineralised water- ok but thats not going to be a prob either - plain old water at least in Melb is no concern for a top up. Bear in mind that Coolant is nasty stuff- store it carefully and never put it where pets / children can get to it- toxic.

SuperMazdaKart wrote:Nulon also sells a coolant that they market as being safe to mix with any other coolant. Life span defaults to what is already in the radiator system.

I've got some of that stuff, used it to top up my Rodeo, works well.

I've generally just used the cheap premixed stuff from Supercheap or elsewhere when needed, never had any problems. I never use tap water though, had to flush out too much sludge over the years from previous owners who did. Old days I always collected the humidity water that drains off the airconditioner and used that, just as good as demineralised if done so that keeps clean.

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How much are we talking? Did you "top it up" to the tune of litres or more like a cup? A little tap water won't make a lick of difference.

Sorry, helpful answers and a couple of questions to which I've not responded. (Getting ready to go to the DRC.)

The top-up was roughly the amount to take the expansion tank from Low to Full, maybe a tad more.

Someone told me that because coolant has water in it, a little extra water shouldn't be too bad.

I don't think I drove the car after that - because I found a hole in the top of the expansion tank!

I've got a new one to fit. So, if it's a reasonable risk to leave the current coolant as it is, is it possible to instal the tank without doing a complete swop of the coolant?

Or, given that I have to instal a new tank, and given that there is a little water in the system, would it be sensible to drain the lot? I know it's simple, but I haven't done it before and any mechanical aptitude doesn't seem to avoid me having problems, so I'd prefer, if possible and not crazy, just to replace the tank without draining the system.

Topping up the expansion tank with water is not a problem, but in future top up with the normal ratio as per owners manual.
The coolant mixture ratio in the car's cooling system is preferably 1/3rd coolant : 2/3rds demineralised water, but can be up to 50:50.

You can remove, clean & refill the expansion tank without draining the system, but if the whole system hasn't been drained/flushed & refilled with new coolant mixture for a few years, then it would be wise to do so.

Expansion tanks have either a small breather hole in the top cap, or a breather/overflow pipe coming out the top.
Cheers