Car mechanics ...topping up engine coolant ?

It’ll be fine, there’s plenty antifreeze in there already.

You can get a cheap tester from Halfords that tells you the antifreeze concentration.

I never knew there was such a thing!

Need to use the car tomorrow morning.  If the water at the top of the resevoir (ie the bit you can see with the gauge line) isn't frozen, is it safe to assume the rest of the liquid in the engine is fine. Obviously if the engine coolant light comes on I'd stop, but want to know that I can't crack my radiator (or worse).

I'll pick up one of those mixture checkers tomorrow but probably not before I need to use the car.

Those testers are an old fashioned thing, but they work.  Never done anything to my car's coolant - 23 years old. Topped up with a dribble of water once every few years

Be careful.

Water isn't a bad thing unless it's overdone, which is what you're checking for^^; the wrong coolant is a problem https://986forum.com/forums/boxster-general-discussions/3859-coolant-use.html

Distilled water??!! I just uses the stuff from my tap!

how old is the Boxster? I'm fettling a 19year old Cayenne - I preemptively changed the water pump and used the opportunity to flush the cooling system and refill with fresh Comma G30. Its not only the anti freezing  aspects but the corrosion protection that's also important.

Its not just an anti freeze its corrosion inihibitor too....

Distilled water??!! I just uses the stuff from my tap!

What does the inside of your kettle look like?

What does the inside of your kettle look like?

Erm.spotless

although tbf it's not 20 years old

The inside of your kettle has seen hundreds of litres of water rather than the same few over and over again. Calcium load of a little top up will be bugger all.

I just buy premixed coolant these days.

19 litres is insane

if the car is used daily probably not any issue, parked for extended periods in freezing temps could be

i would definitely consider adding (the correct) coolant rather than water OR testing it

I'd also try to find where it was loosing before it worsened

Distilled water??!! I just uses the stuff from my tap!

If you're in the frozen north then you've probably got buckets full of white stuff surrounding you (don't use the yellow snow)

Clean rain water will do the same job

If you topped up with a single pint of water, there will still be a significant amount of anti-freeze in there – it would take significantly colder temperatures to what we are seeing right now to even stand a chance of anything freezing.

Coolant doesn't get "used up" though. If the level went low it must have escaped from somewhere, it would be best to figure out where.

19 litres is insane

Rear engine front rad ?  the joys of running a performance car.

The inside of your kettle has seen hundreds of litres of water

Which is why I always empty a cold kettle, otherwise you're progressively concentrating the calcium carbonate

Its not only the anti freezing  aspects but the corrosion protection that’s also important.

+1

It's part of the reason for coolant needing to be changed every 2/3/5/10 years as part of the service schedule.

I did one of my cars and flushed it with a hosepipe connected into the thermostat housing with the bung in the block removed, it took a long time to run clear and so much crap came out (vomit emoji).  Worryingly the liquid in the expansion tank wasn't that bad.

But for a one-off top-up like the OP water does fine, distilled is better as tap water will use up some of the inhibitor as well as diluting it, but again a top up won't hurt.  Like oil, the wrong oil is bad, but not enough oil is far worse!

19 litres is insane

Mid / Rear engine cars need long pipes to the rad at the front, the little intake scoops at the back are usually just feeding the air intake or ancillary things like oil coolers, AC compressors, etc.

It's one of the reasons the Rover K-series got such a bad reputation, the cooling loop was reversed so it would warmup quicker, which meant when the thermostat eventually opened it let in a slug of cold water which made the block/head contract and break the head gasket.  The problems much worse in an Elise or MGF  because there was a much bigger reservoir of cold coolant in the long pipes under the floor.

Check the cross over pipes  theyre a bit of a weekspot

Back when I had the Octavia, I used to keep a bottle of premixed coolant handy to top up the radiator, a once, possibly twice a year event, but it was literally just a top up, the level might have dropped a couple of centimetres or so.
The Ford, on the other hand, I’ve had four years or thereabouts, and never needed to touch it. It’s just had its 40k service, so not something I’m going to worry about for some time.
The washer bottle, on the other hand, takes 5l of water, and at this time of year with crappy roads, it needs to be kept an eye on.

Coolant doesn’t get “used up” though. If the level went low it must have escaped from somewhere, it would be best to figure out where.

This is the key thing, where did the coolant go? You really need to establish that before knowing what to do. Best case scenario is it could just be a hose needs replacing (as long as you replace it before it fails completely and cooks the engine), worst case scenario is it could be a warped or cracked cylinder head or block which would require a very expensive engine rebuild or a complete replacement engine. The sooner you find the source of the leak, the less expensive it will be to fix.

So how would one go about isolating the leak? Tbh it must be pretty minor as the light has come on once, 3 months ago. But it does appear slightly lower now than when I filled it up at that time. Perhaps 1cm lower on the gauge, which is probably less than half a pint

So how would one go about isolating the leak?

A garage will pressure test it and go from there

Ta

Short term - no not a problem.

Longer term - you need to use the right stuff. Correct coolant is important and tap water is definitely not it.

If its dropping, you need to find where. On a Boxster -

- Check your header tank cap - these were revised multiple times over the years, cant remember what the latest one is but if your part number is xxxx.01 then its an old one.

- Header tank cracked/micro cracked and leaking under pressure. Hidden behind the boot linings and carpet.

- Cross over pipes - pipes that tuck under the front subframe, where rubber/plastic pipe and metallic pipes meet - prone to corrosion - check for signs of leaks. I'd put a fiver on them having a pink crust of dried coolant weeps and swollen pipe clips.

- Radiators quietly rotting away on their bottom edge behind the air con condensers in a soup of rotting leaves, hay, mud and rain water and much that has been collected through the grilles.

- Water pump main seal/bearing.