Which of these statements is true or false:

  1. Using 100% anti-freeze/coolant in your engine radiator will NOT decrease the cooling ability of your cooling system.
  2. Using regular tap water, especially water that is considered hard is perfectly fine to mix with engine coolant, and will not over time cause any deposits to form in the cooling system.
  3. Distilled and deionized water are not at all the same and neither should be used in an engine cooling system, use regular tap water.
  4. Straight water in a cooling system will actually cool better than a 50/50 or 100% antifreeze mixture.

OK, questions 1, 2, and three are totally FALSE, question 4 is a trick question, it is TRUE but, it is not good to use only water in your cars cooling system. Question 3 is also a trick question, distilled and de-ionized water are basically the same product and the best choice for mixing with coolant. Distilled water has been boiled and the steam recaptured, leaving most of the mineral deposits behind. D-ionized water has gone through a process to remove the ions suspended in it, ions in other words are dissolved minerals in the water.

Here are some interesting factoids:

If you drain your cooling system completely and refill it with 100% antifreeze, it will actually run hotter than it would with a 50/50 mix or straight water. I recently proved this by picking up the wrong jug (not a 50/50 mix) and filling my system with almost pure coolant. The net result was the engine ran a LOT hotter than it did prior to the service. When I drained out 50% of the cooling system capacity and refilled with water, the system then ran cool as it should. I made no other changes.

Concentrated antifreeze in a cooling system should never be used without adding water. At least 40% of the mixture should be water. If antifreeze is mixed with distilled water at the ratio of one part antifreeze to one part water, the freeze protection will be down to 34°F and boil- up to 265°F that is if using 15 PSI pressure cap.

The question comes up: Do I use regular tap water when mixing my coolant and water together? No, you should use distilled water or better yet, de-ionized water. The reason for this is that tap water has minerals that can form deposits inside the radiator and the cooling system passages of your engine. And over a long period of time they can lead to over-heating.

For best all-around and most convenient protection, use a 50:50 mixture of antifreeze and distilled or deionized water. Convenient because it is widely available as a pre-mix at the store. Other ratios actually provide a better cooling mixture however, you would have to measure and dispense your on custom mix.  The chart below shows typical performance based on the mixture of coolant and water.



Coolant should not be mixed with hard tap water. Hard tap water has excessive calcium and magnesium deposits that can cause scaling, which will result in inadequate heat transfer. Use only soft or de-ionized water that is not treated with salts or chlorides. (OEMs publish limits in parts per million (ppm) for hardness, chlorides, sulfates and total dissolved solids for the water used to dilute antifreeze.)

For most applications, distilled and deionized water are the same thing. Both are just water with ions removed. Distilled water has been deionised by distillation as apposed to another technique such as ion exchange chromotography. The end product is the same and either can be used. Most labs will not have distilled water anyway even if some people still call it distilled rather than deionised.

You can buy distilled water at the pharmacy CHEAP! At CVS it is $.99 a gallon. The most I have seen it sell for is $1.98 Amazon sells distilled water for $19/gallon...it's a total rip-off. Go to the pharmacy or Walmart. Look for distilled water that has NOT been flavor enhanced. Flavor enhanced means they put back in salts and other minerals to make it taste 'better'. Desani water, a coca cola product, has more than a dozen additives for example to 'help the flavor.'

The safest and best solution is to purchase a 50/50 mix of coolant and water, premix. All coolant manufacturers use distilled/deionized water to reduce the coolant concentration. Else, you can save a few bucks, buy distilled water at the CVS pharmacy and mix it 50/50 or better yet 40/60 (water/coolant).

Here is our take on the indiscriminate mixing of coolant types:

In brief, it's not typically a good idea and a system designed to run 'green' should be filled with green. And the same can in GENERAL terms be said for the other colors.

Here are some things to consider if you are into details and a little supposition (perhaps common sense and/or experience) on my part:

Some folks just take the yawn approach to what goes on between consenting chemicals in the steamy privacy of a car's cooling system. It's just not me because of a recent check that was written to the machine shop for aluminum welding services to replace material in some places in a cylinder head that did it. Funny how much aluminum went AWOL in the 30-some years since that engine left the factory, enough to leave gaskets hanging in midair.

Funny? I laughed all the way to Coolant College and did a little Googling.

Modern antifreeze, is 96-percent ethylene glycol, which provides the freeze protection, and four-percent additives...typically. When you dilute that blend 50-50 with water, as the makers intend, you push down the freeze point to minus 34 degrees Fahrenheit. In normal circumstances, you also gain corrosion resistance . . . for a while. The freeze protection is permanent, but the additives are consumed in battle, so to speak.

About half the additive is made of buffers to control acid buildup; the other half is corrosion inhibitors to protect metals.

Perhaps the battle is already going badly in your car. A sticking thermostat can be an early indicator. The next stage: As detritus migrates through the system, it settles in the most confined spaces. If your heater blows cold, uh-oh.

I was hoping that technology, as it marches relentlessly toward obsoleting everything I own, might also have created new antifreeze formulas that would bring forbearance and frustration to the chemicals frolicking under my aging radiator caps.

Of course, no doctor writes the prescription before he considers the patient. The "old" antifreeze technology started in the '60s, improved in the '70s, and was superseded in cars of the '90s by two new technologies. It turns out that an antifreeze transplant into older cars will work fine with one of the new types; the other will probably kill the patient.

The old technology, a.k.a. "conventional," a.k.a. "inorganic," is green in color. Most of what you see on the shelves at Wal-Mart and AutoZone is conventional, including the yellow bottles of Prestone and the white bottles of Zerex.

One of the new types is "organic acid technology," or OAT. It's orange. General Motors pioneered this chemistry starting with 1996 models in the U.S. and using the name Dex-Cool. Ford changed a few models to OAT, then backed away from it. VW, Audi, and Porsche are OAT users, too, but most others have resisted.

Instead of OAT, most new cars now use a "hybrid" antifreeze that's formulated with both OAT and the silicate inhibitors from green (Japanese hybrids have different inhibitors). It comes in too many colors to pretend this type is color-coded. Interestingly, the materials improve for the white plastic overflow bottles of new cars, and they become less yellowing over time, automakers are becoming more venturesome in choosing coolant colors.

The promise of OAT is long-life corrosion protection, on the order of six years/ 100,000 miles for the initial fill instead of the two years/50,000 miles that was typical with the old green stuff. The GM Dex-Cool formula works fine in systems designed for it. But it eats old-style radiators with lead solder, and the inhibitors work too slowly to protect against the sort of corrosion that happens so fast it actually erodes metal-for example, the cavitation likely in the imperfectly designed water pumps of older cars.

"Cars born with green coolant shouldn't be changed to orange," It's also a bad idea to mix the two, although the result doesn't immediately turn into witches' brew.

I theorize that coolant technology is driven by the makers of new cars to solve new-car problems (same with motor oil.) By the time a car gets old enough to be interesting to a collector, the latest antifreeze blends have moved on to protecting newer alloys and gasket materials. Fortunately, the after-market lives by catering to older cars.

As for those aging turbo Dodge cars of the 80's and early 90's we're keeping around as playmates, no matter what brand green antifreeze we choose, and no matter how often we replace it, the best medicine is to play often. Coolant down in narrow crevices can become isolated, then overwhelmed by corrosion. Once it starts, the best you can hope for is a stalemate. You can't undo corrosion. To keep protection active in all the crannies, the system needs to be heated and circulated every 30 days. This is by recommendation of the engineers at Zerex.

Obvious question: What about the water we mix in? As mentioned in my original post, coolants are designed to work with "reasonable" levels of hardness and chlorides in tap water. But magnesium and calcium, the hardness ions, unquestionably contribute to scale and deposits, which hurt cooling efficiency. And chlorides are corrosive. Distilled water gets rid of all the worries. (It was $.98 cents a gallon at my local Wal-Mart yesterday. Or you can buy "predilute" coolant already mixed and ready to go.

In my vision of purgatory, I'll be sentenced to changing antifreeze in all my cars, day after day, and some archangel with white gloves and a test tube will be checking the color of my flush water for contaminates. I have to keep flushing until he can't tell the drain-out from the distilled he carries in another tube as the control.

Here in this life, I've always changed my coolant. I'm one of those guys who agonize over details. So the job takes a full afternoon for each car. I drain everything that comes out through the cock, then top up with clear water, warm the engine, and run the heater to circulate fully, then drain again. Repeat twice.

What to do with the drainings? I called the local pollution controllers. Antifreeze? Their book had no mention of it. After thinking a bit, however, they told me to put it out back in buckets and let it evaporate. Rocks evaporate at about the same speed.

Old coolant "hanging up" in the system is a real concern. But we also know that nobody gets it all out.

If you open a drain cock or drop a bottom hose, you might get 50 to 60 percent out. The best machines, the new ones going into Valvoline quick-oil-change shops, get 80 to 85 percent. This is a manageable level of contamination, as long as the new antifreeze doesn't fight with the old.

Don't mix and drive!