Which coolant for my car?

How short is it? I’d be giving it a glug of water to bring it up a small amount. 

It has a top-up warning on the dash (not a red do-not-drive warning).

Everything I've read online says you shouldn't use water unless it's an emergency. Would a top-up with water be as good as coolant?

I cant speak for Merc, but last time I bought some coolant, ford stuff from a ford dealer was the same price as the halfords stuff, give-or-take for arguments sake...

...So I just bought the official Ford stuff from the Ford dealer.

How short is it? I’d be giving it a glug of water to bring it up a small amount.

Not to be confused with antifreeze/screen wash, I'd only add plain water in an emergency, or to top it up a TINY ammount 😉

@eckinspain, give the service department at your local Merc garage a ring and ask them how much 'x' litres of coolant is... then you'll know more.

It has a top-up warning on the dash

Also, if possible, check the coolant refil tank under the bonnet, you should be able to see a min and max line, and shine a torch behind it if needed to try to see the physical level of liquid.

Thanks @mattyfez

Absolutely fine using Halfords lookup tool.

May be slightly cheaper going to a motor factors, they’re usually very friendly but so are Halfords.

Top up with water if we’re talking ~100ml. Look at the expansion tank with a torch as above.

But do not continually top with water when proper coolant is needed. Don’t ask me how I know.

The car manual will state what type of coolant to use.  If you don’t know where your copy is just search online for the manual. 

You can also try a few other motor factor websites and check they all recommend the same type of coolant. <br /><br />

Also check how low the coolant is on the header tank. Don’t go buying 5 litres if you just need to do a ~250ml top up. 

But do not continually top with water when proper coolant is needed. Don’t ask me how I know.

Very much this.

Also, if the engine is hot, the expansion tank (the bit where you refill it) will be under pressure if the engine is hot, so DO NOT remove the refil cap until the engine is cool, unless you want to pay for skin grafts as well as coolant 😀

And if you're adding water, ideally use deionised water - Tesco sell it for irons.

Definitely look at the tank - the difference between the 2 lines is probably less than 500ml.

There are some universal coolants but if you are going to top it up it’s quite important to make sure the new stuff is the same colour as what’s already in there.

Absolutely fine using Halfords lookup tool.

I've found it to be quite error ridden which is only a waste of time when its something like windscreen wipers (their results for my car have been consistently wrong for nearly a decade) more of a concern if its something you're going to pour into the engine.

It couldn't give me a consistent result for oil for my van - the online look-up doesn't match the results that the staff's own in-store system provides which in turn is at odds with the customer touchscreens in the shop.

ford stuff from a ford dealer was the same price as the halfords stuff, give-or-take for arguments sake…

…So I just bought the official Ford stuff from the Ford dealer.

give the service department at your local Merc garage a ring and ask them how much ‘x’ litres of coolant is… then you’ll know more.

It's probably bad form to quote myself, but I will, anyway, as I have no shame.

I'd generally never recomend buying stuff from a main dealer, but purely from a price comparison point of view, the cost of a phone call. Some stuff is actually cheaper, not many things, but some.

Prestone coolant claims to be compatible with "All makes, all models" although I was bit dubious so just got some my local motor factor (think mine was G13 - the purple stuff).

Prestone coolant claims to be compatible with “All makes, all models”

This is probably right if you're doing a full change,  but I'd not mix different grades. Rockhopper's point about matching the colour is correct to my knowledge,  and I had to research this once to refill the V18 blackstart diesel's at work. 

My only contribution would be to ask if you know why it's lost coolant.

Modern cars use a sealed system for most fluids but you can lose a little coolant and still be "normal", however more than (number off the top of my head) 50ml regularly and I'd be wondering why

Otherwise, manufacturer's handbook and/or main dealer are your easiest sources of anti-freeze info

Unless it’s been a long time since you had it serviced I’d not expect it to lose coolant unless there is a leak.

In any case I’d recommend going to the MB parts desk. Some things are surprisingly cost competitive. I seem to remember oil and windscreen wipers being fairly priced.

Just a quick point. Some modern cars have very specific coolant these days. It to be a choice between two. These days i would check with Merc as sticking a "compatible" coolant in could cause issues.

Just a quick point. Some modern cars have very specific coolant these days.

Indeed to meet the desire for ever longer service intervals there are some very specific coolant set ups (that can cause their own problems further down the line - beyond the warrantee period of course) and changing the coolant type means changing the servicing schedule

Most* coolants are wither Organic Acid (OAT, the red stuff, needs flushing every 5-7 years), or older (like pre 90's cars) use Silicate (the blue stuff, needs flushing every 2 years). Prestone is yellow and works in everything* and lasts forever.

*generalizing, YMMV, check the manual, etc.

Topping up with water is fine. Most shops sell "ready to use" coolant, this is generally 50/50 mix. If you live in Siberia, need more than 5L, or previously topped up with water then get the neat stuff that needs diluting. Tap water is fine, the additives are in there to prevent scaling, corrosion etc.