Engine Cooling

Distilled Water - Safe for Engine?

Distilled Water - Safe for Engine?

>>Be careful about using distilled [read de-ionised] water- it can eat
>>through "corrosion resistant" metals like you would not believe.

This appears to have reached the status of Urban Legend. Some comments from the Web:

From the Car Guys:
Dex-CoolTM should be mixed 50/50 with clean, distilled water. When changing your anti-freeze never mix Dex-CoolTM with your current anti-freeze. It is also important that the coolant overflow tank not be contaminated with pure water or other types of anti-freeze. According to General Motors, the use of any additives claiming to improve your cooling system performance are not only a waste of money but could be harmful Dex-CoolTM and to your cooling system components.

From the ASA Shop tech tips.
If you use hard water in these systems, you are asking for mineral buildup problems. Softened water, on the other hand, contains dissolved salts that act as an electrolyte, encouraging galvanic action between the dissimilar metals. That's why distilled water is quickly becoming the water of choice!

From an article by Gregory T. Fieldson B.S., PhD in Chemistry:
Should I use tap water or distilled water?
I recommend distilled water. If you have exceptionally soft water it would be acceptable, as well. Basically, the engine metals are going to corrode to some extent no matter what water you use, and hard water will encourage the resulting metal salts to precipitate.

From the Rover Tech Tips site:
Other contaminants can come from the water that you use with your antifreeze. Most tap water has a trace mineral content. Depending on the source of your water, you may or may not have minerals that will become sediment and block your system. The safest way to refill your system is to use distilled water. This should be a 50/50 mixture with coolant for the best heat transfer.

From the Red Line site:
The anti-scaling ingredients in Red Line WaterWetter allow its use with ordinary tap water. However, using with distilled or deionized water will accomplish some scale removal in the cylinder head area.

From a Saturn GM Tech site:
A 50/50 mixture of this product and distilled water is required.

From Aftermarket World tech site:
Antifreeze is mixed with water (preferably clean distilled water since hard water can introduce unwanted minerals into the system) , usually at a 50/50...

From the General Motors EMD (diesel locomotive) engine handbook:
Distilled water should be used in the cooling system to offset the accumulation of scale and foreign matter, which contribute to overheating the engine.

From MotorTrend online:
In addition, the use of hard tap water in the system leaves mineral deposits that can build up and restrict the flow of coolant; this is why it's recommended you only use distilled or soft water in the system.

And last, and most telling to me -

From Texaco's Product Bulletin on DexCool:
However, to maintain the integrity of a cooling system and to help assure long life, deionized or distilled water is preferred over tap water.

My choice for my 928's radiator is Texaco DexCool and distilled water.
If you want to use distilled water, run the hose from your home air conditioner condensate drain into a clean jug, then filter it with a clean coffee filter. Dehumidifier condensate works well also.

Wally Plumley
928 Specialists

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