I'm a little east of west Texas, but share the same problem...hard water. Alot of auto techs here say to use 50% coolant/50% distilled H2O. I have 2 Japanese vehicles that collectively have 550,000 mi. on them following the afore-mentioned regimen. I DO realize these are not German cars; nonetheless, their cooling systems are quite similar. Still stumped about the distilled water issue and wonderin' why my '83 Toyota truck with just south of 300,000 miles still has the same engine, radiator, etc., all courtesy of Prestone and distilled water. I do premix for what it's worth.

Regards...Mike Murrell