Cadillac Jim Posted March 3, 2016 Report Share Posted March 3, 2016 The later FSMs defer coolant and other fluid recommendations to the owner's manuals. The owner's manual for my current car, which has aluminum block and heads, says in the owner's manual Use a 50/50 mixture of clean, drinkable water and DEX-COOL coolant. This is in the chapter on Vehicle Care, section Cooling System, twice. It ends a warning block on not adding just water or a non-recommended mixture, and again in the text. I would be really surprised if the dealers use distilled water in coolant. This may be OK in areas that do not have high-calcium water, which, as KHE says, can cause problems. If you can say that really hard water is not really drinkable, then this can be a rationale for softened water. I know of many areas in Texas that have really hard water; look for stains in the wash basin as a first clue while traveling. Hard water is common in areas that use deep well water not near a running river. Which is a lot of the country. -- Click Here for CaddyInfo page on "How To" Read Your OBD Codes-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data -- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC Yes, I was Jims_97_ETC before I changed cars. Link to comment Share on other sites More sharing options...
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