Plain Water as Coolant: 7 Reasons Why This Is a Very Bad Idea

Of the other 500 (actively running the recommended coolants in their radiators and overflow tanks), it’s usually either because the vehicles are brand new (and the factory coolant is still there) or that the coolant that runs in their cooling system is an ‘overflow’ – literally – from the harsh winters where plain water wasn’t simply an option.

In the (tropical) developing world, these figures are way higher. Out of every 100 vehicles on the road, at least, 95 are running plain water in their radiators and cooling systems. Of the remaining five running on coolants, at least, 2% aren’t using the recommended coolant type…their owners simply throw in anything that has the label of ‘coolant’ in them, not even bothering about a flush (first) or what was in, prior.

The remaining 3% is split between vehicles bought brand new (and still in their newish state), vehicles recently imported into the country or vehicles used by hardcore vehicle enthusiasts like myself.

The overall argument of folks who have mastered the vehicle degrading act of using plain water instead of coolant is that water serves the same purpose or at least, gets the job done.

On face value, this appears like great logic and like the major reason for fuel pump failure, I dare say, sound. But, is it, in reality?

Allow me to dig into the meat of this matter and explain to you in a manner devoid of technicalities or vehicle jargons; allow me to respectfully explain to you like a 5 year old.

First things first though: what’s the primary function of engine coolant in the first place? What does coolant seek to achieve in the block of an internal combustion engine?

What Coolant Does in an Engine

The modern internal combustion engine (ICE) is made up of many moving parts that work together to ensure that locomotion takes place in the most efficient manner possible. This complex placement of various moving parts sees to it that most of these parts (often metals and alloys) come in contact with each other.

To reduce heat and maximize efficiency, engine oil is introduced, and this creates a thin coat of film between the moving parts, reducing heat/wear and generally, maximizing efficiency.

However, engine oil alone isn’t generally enough to keep engine temperatures within reasonable limits. Often (and especially so during spirited driving conditions and elevated ambient realities), temperatures rise in an engine, calling for supplementary cooling. This is where coolant comes into the picture (which together with the cooling fans, do an amazing job of regulating engine temperatures).

However, that’s just the primary purpose of engine coolant. Coolant, flowing in the engine block serves many secondary functions beyond just merely cooling the engine (considered its primary function). Other secondary functions include (but certainly not limited to) engine temperature regulation (by means of the thermostat) and heating (working together with the HVAC system).

Having seen what coolant does in an engine, why not simply use water as coolant, especially considering that water is freely available and inexpensive, at least when compared to coolant?

Should You Use Water as Coolant? Why Plain Water as Coolant Doesn’t Cut It

There are a million and one reasons why using plain water as coolant doesn’t cut it, especially in modern ICEs.

Explained below are the 7 most pressing.

1. Higher Boiling Point (of Coolant)

At around 100ºC (212ºF) water boils. Now, depending on your driving style, vehicle, and ambient temperature, your engine can get truly hot, hitting (and usually, breaking) this limit.

The only saving grace that usually, doesn’t see an overheating episode happen right away is the fact that the cooling system of a vehicle is pressurized, considerably upping the boiling point of any fluid in the system (with the radiator cap acting as final pressure determinant).

With coolant, however, things are radically different with boiling points significantly at the higher thresholds.

For instance, the Toyota Super Long Life Coolant has an impressive boiling point of 108ºC (258.4 ºF) which means, despite your spirited driving, harsh weather conditions, and every other thing that makes an engine hot, you’re likely to NEVER experience an overheating bout if you stick to the use of the recommended coolant as against plain water.

This is in addition to the naturally elevated boiling point in the pressurized cooling system.

For most coolant types, the magic addition that ensures that this happens is Ethylene glycol — a compound that helps lower the freezing point while increasing the boiling point of any liquid it’s added to.

2. Lower Freezing Point (of Coolant)

Of course, everyone who lives in a temperate region where it gets really cold knows this: you can’t try using plain water alone in your radiator, especially in the winter. If you try it, you won’t be needing any convincing, otherwise, before you do the right thing next time (after sorting the damage caused by your carelessness).

Like the boiling point above, the magic here is also handled by the power of Ethylene glycol — lowering the freezing point to an impressive -37 ºC (-34.6 ºF).

Ethylene glycol is thus a ‘double edged sword’: it increases boiling points and equally lowers the freezing points of coolant, making properly formulated coolant indispensable in both the harshest periods of the tropics and temperate regions. This reality ultimately negates the argument that coolant isn’t necessary because of one’s weather (whatever that may mean).

3. Coolant Is ‘Purer’ Than Water

Water is hardly ‘pure’, even the variant we drink (and I mean the best, bottled water that you pay a premium for). Usually, additives go into the treatment and process of killing germs and making it safe to drink.

Traditionally, water is just hydrogen and oxygen , depicted by the classical representation, H₂O except that this is hardly ever the case, especially for water sources that have been exposed, naturally compromised or have gone through several treatment processes to make it safe(r) for human consumption.

Tap water (and other water sources such as rain, boreholes and wells) for instance, generally contain fluoride, Aluminum Sulfate (Alum), lime (to ‘soften’ ‘hard’ water), iron and manganese, trace elements and several organic matter(s),  depending on the actual source.

So, as you can see, H₂O now turns to ‘H₂Z’…

These unwanted elements of water, when introduced to a vehicle’s cooling system, cling unto parts they have no business with, causing a myriad of problems and reducing cooling efficiency. and generally, reduce cooling efficiency.

While slow, these unwanted elements eventually cause several ugly things such as blocking the delicate fins of the radiator, causing overheating and if not noticed on time, ruining entire engines, significantly setting one back, financially.

4. Coolant ‘Residue Value’ Properties

I’ve been cursed to work on vehicles that had compromised cooling systems; systems that simply weren’t in the closed loop fashion any longer.

With their coolants leaking out at will, I knew that it was only a matter of time before things dry up, the engines overheat and the motors fail, catastrophically.

The only challenge was that on all the occasions this happened, I was dealing with systems where plain water was used and no trace or residue was left.

You see, coolant leaves residues when it exits the radiator (or any part of the cooling system, as shown below). Curiously, this residue doesn’t happen when it’s inside the radiator or any part of the cooling system for that matter.

Now, were the vehicles that stressed my life been using coolant, figuring out what was wrong with their cooling systems would have been a walk in the park – and my work would have been done in mere seconds instead of hours/days.

If you’re not looking to stress your mechanic/dealer (and by extension, yourself), it’s always wise to consider coolant, as against plain water, all the time; all seasons round.

5. Coolant Goes Well Beyond Mere Cooling

Traditional coolant goes well beyond the reality of cooling alone: it serves other functions too, like lubrication of seals in the cooling system, particularly the water pump. Remember that the water pump is significantly doused in the coolant, for the most part, in most designs.

This sees that seals remain supple and the water pump runs fine, all the time, being lubricated by the coolant cursing through the engine block.

Unfortunately, water has no such lubricating properties, making the seals in a cooling system of a vehicle not using coolant more likely to fail prematurely, together with the water pump, as a unit.

This doesn’t look like what anyone would want for their vehicles or themselves (for that matter). 

6. Keeping Rust at Bay

Rust is the number one enemy of efficient cooling…and, this is how it affects automotive cooling systems.

The radiator cooling fins are tiny pores where coolant exchanges take place to keep the vehicle’s operating temperature within spec.

What happens is that coolant that has gone through the ‘fire’ of the engine block passes through these fins and mixes with ‘fresh’, cool coolant, regulates itself employing the cooling fan and the process continues. 

With water use, however, rust logically lodges itself in the system and blocks these tiny fins, making it a challenge for the hot fluid from the block to come back to the radiator to be cooled off utilizing the cooling fan.

When this happens, an overheating episode is just around the corner and with it, the real potential of an engine damage, as a result.

7. Coolant Is Cheap & Readily Available

I remember when 0W20 weight engine oil was relatively scarce and I had a friend’s car that called for it. Being the person I am, I insisted on putting the exact weight the manufacturer recommends.

That singular resolve almost saw me run out of a full tank of gas in the quest of looking for this oil weight which, in retrospect, appeared to be something of legend.

In the end, after burning almost an entire tank of gas, I still didn’t record any success and finally resorted to buying the engine oil weight online.

Engine coolant, luckily, isn’t that way. Toyota, for instance, uses the same coolant across its major lineup, including hybrids. The good part? It has been so for decades. For many other manufacturers, at least the major ones, the story is similar and largely consistent.

Availability isn’t all there is to these coolants; they’re also cheap when found in the open market (most auto parts stores stock them). There really is no good excuse for using plain water in place of coolant…

…really.

What if Coolant Isn’t Available at All?

Sometimes, good old coolant isn’t an option — at all. It’s COMPLETELY unavailable (at least at one’s current location)!

If this happens to be your case, then, make use of only distilled water, as substitute for coolant: not rainwater, not tap water, not the borehole but distilled water.

If that is a challenge, collect the condensate from the air conditioner and use it in a supplementary capacity — it’s a viable and safe option.

Still a challenge?

Some bottled water companies offer still water devoid of any additives…this is the next best option.

Still a challenge? Sorry, you shouldn’t be driving a vehicle in the 21st century!

FAQs

Wrap Up

Using plain water as coolant in your radiator or cooling system may look like a cool and inexpensive thing to do.

But, like staving your fuel pump of gas, it brings more grief than savings at the end. Any perceived ‘gains’ are canceled out by the bills that come knocking, sooner rather than later.

If you ever need to cool down your cooling system, use the recommended coolant, only. 

If for any reason, you’re forced to use plain water (for lack of availability), flush and revert back to coolant at the earliest chance you get (and this is valid even if you live in the tropics (the UAE/Death Valley, California) or are have vehicular business at the Eastern Antarctic Plateau.

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