Is it critical to change coolant at 100K?

[johnwk]

Originally Posted by RMcG

If I do decide to run distilled water through the system, how long should I let the system run in order to mix & help get the older coolant out of the system? Ten minutes? 20 minutes? Until the engine is hot?

I would drive the car for a few miles, until it's warmed up, with the heat on full blast.

Originally Posted by RMcG

(I know that there is the potential problem of diluting the coolant, since you cannot get all the distilled water rinse out of the system before you add the new coolant.)

If you want, when winter approaches, you can do another quick drain-and-fill, which will reduce the dilution while also renewing a bit of the coolant. There's a very cheap and simple tool for testing the coolant dilution.