First let me qualify this by saying I am no longer a profesional mechanic. Unless you have something drastically wrong with your tap water, use it, I can just about guarrantee MB does not use deionized water. Use a coolant that is campatible with aluminum.

FYI. In the old days of open cell batteries, deionized water was used for batteries so you can determine their charge by using a hydrometer which reads specific gravity. In an emergency, tap water could be used without harming the cells, but it would through off any future hydrometer readings.

I also just got sucked into paying $15.00 for 0.75 liters of DOT 4 brake fluid at MB, for which you will see a more detailed post shortly. Discount parts stores sell the same stuff for $4.50. MB engineers spend their time designing mechanical systems, not designing fluids (although they obviously are very proud of their brake fluid packaging). Save your money to buy OEM parts (brakes, hoses, etc) rather than over the counter fluids.

I will gladly retract these comments if one of the pro's corrects them, but I will be surprised if that happens.

Regards,

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Deezel
87 300TDT
150,000 miles