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I grew up flushing the coolant twice yearly. In the Spring, you drain the antifreeze and fill with straight water for the summer. In the Fall, you drain the water and put in a 50/50 antifreeze/water mixture.

I have heard (from an auto store clerk) that running just water will cause overheating. The clerk also said that antifreeze prevents corrosion and sediment build up and cleans the coolant system.

Despite years of using water in the summer, I have never experienced any problems that were obviously related.

Do I need to start using antifreeze, even in the summer?

DucatiKiller
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Wulfhart
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8 Answers8

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I have heard (from an auto store clerk) that running just water will cause overheating.

Well, that's not true. Water isn't the cause of overheating. Your coolant mixture (of whatever proportion) and radiator work together to get rid of the heat. If it's not hot, you won't overheat. However, when it is hot, the coolant can only absorb heat up to its boiling point.

Here's a super high level summary of a cooling system:

  1. The cool coolant is placed in contact with the metal of the hot engine.
  2. Heat is transferred from the metal of the engine to the liquid coolant, heating it up.
  3. Hot coolant is pumped to the radiator, making room for cooler coolant to move into the engine.
  4. Hot coolant is placed in contact with the metal of the cool radiator, cooling it off.

Liquid cooling requires the best contact possible between the metal and the liquid for most efficient heat transfer. Problems occur as the coolant approaches it's boiling point: steam bubbles start to form, especially at hot metal surfaces. Each one of those bubbles is a less efficient point of heat transfer. That means less heat leaving the engine, meaning a hotter engine, more spots where bubbles will form, repeating until steam starts coming out of the hood.

So, one of your main goals in assembling a useful cooling system is to ensure that the boiling point of the coolant is high in order to prevent high temperature disaster. Water's boiling point is 100 C = 212 F. Straight ethylene glycol's boiling point is at 197.3 C = 387 F. Of course, you shouldn't use straight ethylene glycol in the radiator either for the sake of efficiency.

The clerk also said that antifreeze prevents corrosion and sediment build up and cleans the coolant system.

That depends on the product. Quite a lot of the coolants on today's market will inhibit corrosion and minimize sediments. Some, like Water Wetter, will actually increase the cooling system's ability to carry away heat.

Despite years of using water in the summer, I have never experienced any problems that were obviously related.

Just remember that lack of evidence doesn't necessarily indicate absence of the phenomenon.

Do I need to start using antifreeze, even in the summer?

As always, it's your car. You need to make the call. I can't be bothered to flush out my coolant just to change it from green to clear. When it's dirty, I flush it, not before.

NOTE: I know that a pressurized radiator system changes the physics from this simple "boiling point and no higher" explanation. This is a reasonable first-order approximation for the purposes of discussion.

EDIT: @Paulster2 was kind enough to post a picture of what happens to a water pump when it is run with straight water without the corrosion prevention of coolant + water:

A rusty-ass water pump vs. a brand new one

I submit that the one on the left can no longer be considered a pump.

Bob Cross
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  • This makes the most sense, further research of my own is saying the bi-yearly flush is a bit excessive anyways. Thanks – Wulfhart May 25 '12 at 15:18
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    If anyone is curious about what running straight water can do, check out this photo. It is of a Nissan Maxima water pump. I think it's pretty obvious which pump is new. Imagine trying to run an engine with the circulation (or lack thereof) this pump would have given. – Pᴀᴜʟsᴛᴇʀ2 Mar 24 '14 at 14:46
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    @Paulster2, thanks, I added the picture at the tail end of the post – Bob Cross Mar 24 '14 at 16:58
  • "the coolant can only absorb heat up to its boiling point" -- while this answer is generally a great one, the part I just quoted isn't completely correct, if I remember my physics. Once the coolant reaches its boiling point-- at a given pressure-- you can't raise its temperature. But it can still absorb heat. In fact, it MUST do so, in order to make the phase change from liquid to gas. This is called the 'latent heat of evaporation'. I suspect the real reason steam is less efficient at cooling your engine, is that it vapor locks the water pump. But I'm not 100% sure of that. – Cognitive Hazard Oct 02 '16 at 05:49
  • @RyanV.Bissell, are you trying to argue that coolant that has boiled is useful for heat transfer? That's the purpose of the cooling system. A liquid-based cooling system needs to maximize contact area between the heat transfer medium, the hot parts and then, later, the radiating parts. – Bob Cross Oct 02 '16 at 18:18
  • No, steam is certainly not as useful. I was talking about coolant that was on the cusp of boiling-- 'up to its boiling point', in your words. Once water reaches that point, it still needs significant energy input to make the phase transition, and so can still cool. This is probably semantics though, you apparently meant 'can only absorb heat' until it has turned to steam. (Which also isn't strictly true, as 213℉ steam can cool molten aluminum just fine... but that's clearly out of scope of this discussion.) – Cognitive Hazard Oct 05 '16 at 17:16
  • TL;DR I was probably being too pendantic. Sorry. – Cognitive Hazard Oct 05 '16 at 17:25
  • The recommended glycol/water mix has a higher boiling point than straight water. Cooling systems are pressurized to raise the boiling point of the coolant. However, the boiling point of straight water at the designed system pressure might be too low for the designed operating temperature, in other words, it can boil over. Also, tap water is a bad idea; there are minerals in tap water which can promote corrosion, so you need to use distilled, but corrosion will occur even with straight distilled water. Best is to always use recommended mix. – Anthony X Mar 12 '17 at 16:56
  • @BobCross Good explanation. I believe scientific term for what you explained is called "Leidenfrost effect". Very often "Leidenfrost effect" is demonstrated by briefly putting hot hand in liquid nitrogen where the hand does not get immediate frostbite due to reduced heat transfer. In that scenario you want "Leidenfrost effect" to happen because it protects your hand. However, you don't want to see "Leidenfrost effect" to happen in your cooling system, because it would reduce heat transfer the same way and engine would not be cooled by the cooling system anymore. – user2138912 Jun 06 '17 at 02:36
  • Some water pump designs actually use the properties of the glycol to lubricate the bearing. Run one of those on straight water and you'll wear it out in a couple of months. The most recent pump I purchased though uses sealed bearings, so I suspect such designs are being phased out. – Perkins Aug 09 '18 at 22:10
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The main ingredient in automotive antifreeze, Ethylene glycol, has a higher boiling point when mixed with water than water alone. Wherever you live, I bet it's not Arizona or Texas. Steam voids suck, you don't want any in your cooling system. Antifreeze also contains corrosion inhibitors. I bet you don't have hard water, either.

Mark Johnson
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    I live in New Mexico (the state between Arizona and Texas) and we have very hard nasty water. But that is what water softeners are for. With all your "betting", how much money did I win? :p – Wulfhart May 25 '12 at 15:11
  • pressurized water boils significantly hotter than unpressurized. https://upload.wikimedia.org/wikipedia/commons/3/33/Phase_diagram_of_water_simplified.svg – Leliel Sep 06 '16 at 03:08
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If you were to do a Google search for water vs. antifreeze, winter or summer, you will find no support for running water for anything other than pressure tests. Absolutely do not use regular water for an extended period. It will cause more problems. Antifreeze helps keep corrosion away and does aid in cooling in warm weather, better than straight water.

Nick C
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william
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    Many race tracks require you to drain all coolant and replace with just water, as spilled coolant in the event of a crash or mechanical failure can make the track slippery. This is common on strips for drag racing, and for tracks with motorcycles. I've always used Water Wetter in my motorcycle, though many just use plain water. – Tim B May 24 '13 at 13:14
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Water is corrosive. You will shorten your engine's life if you don't add antifreeze. Antifreeze, on top of everything else it does, also acts as a rust inhibitor. Yes, you can run straight water, but your coolant boiling point will be lower AND you'll corrode the inside of your engine and destroy your water pump.

Captain Kenpachi
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When it comes to coolant. It is not recommended to run with either 100% water or ethylene glycol. Manufacturers (Ford in this case) recommend the that the coolant concentration not go below 40% or exceed 60%. Doing so will do damage parts and they won't function properly.

50/50 mixture provides protection from -37C(-34F) to +129C(265F). Well within safe operating limits

Compare to properties of the components separately.

100% H2O-water: Freeze point 0C(32F) Boiling Point: 100C(212F) Note: As air pressure increases or decreases the boiling point also changes. For example, changing altitudes.

100% Antifreeze: 

         Melting Point: -12.9C(8.8F)

         Flash Point: 111C(232F)-closed cap

         Boiling Point: 197.3C(387.1F)

         Auto ignition point: 410C (770F)
Old_Fossil
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I do not buy a bit about overheating except, mayhaps, severe service or a lot of stop and go traffic (meself, if I have to top off with water in the summer, I do so), but I would worry about corrosion and sediment. Distilled water would take care of the latter, but for the former I would use some sort of protective product, even if for the peace of mind alone. Either summer coolant or some anti-corrosive additive.

theUg
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Water ain't water. Depending on where you live water contains dissolved salts that deposit scale and reduce life of components such as sacrificial items like thermostat housings. Having said that, here in western Australia temperature often exceed 120F in the shade. I've always used rainwater collected in a kids paddle pool, not via a metal roof or down pipe. Another old bushys trick is to soak 200 cheap teabags in hot water. Cool, then strain through a very fine sieve and add to radiator. Look inside an old enamel or alloy teapot. Never corrode do they! Just replaced my radiator after 38 years. Says it all doesn't it.

Pᴀᴜʟsᴛᴇʀ2
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mike
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Very simple answer despite what everyone else is saying. If you live in a hot state say like Texas or Arizona, I would use no less or no more than 70% water 30% coolant in the summer and 50/50 in the winter! If it's a colder state like Colorado or Montana I would reverse it for the winter as in 70% coolant and 30% water and 50/50 in the summer! It's your choice but personally I would just do 50/50 year around unless your state see's 110 summers and -5 winters, if it does then follow the above example!

Jay
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