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?

8 Answers 8


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.

  • This makes the most sense, further research of my own is saying the bi-yearly flush is a bit excessive anyways. Thanks
    – Wulfhart
    Commented May 25, 2012 at 15:18
  • 4
    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. Commented Mar 24, 2014 at 14:46
  • 1
    @Paulster2, thanks, I added the picture at the tail end of the post
    – Bob Cross
    Commented Mar 24, 2014 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. Commented Oct 2, 2016 at 5:49
  • @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
    Commented Oct 2, 2016 at 18:18

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.

  • 3
    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
    Commented May 25, 2012 at 15:11
  • pressurized water boils significantly hotter than unpressurized. upload.wikimedia.org/wikipedia/commons/3/33/…
    – Leliel
    Commented Sep 6, 2016 at 3:08

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.

  • 5
    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
    Commented May 24, 2013 at 13:14

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.


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)  

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.


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.

  • This is a great comment! Not quite an answer to the question.
    – geoO
    Commented Dec 26, 2020 at 17:09

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!


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