I drained and re-filled 75% of the coolant capacity (Dexcool) from each of our vehicles (99 Silverado - 186,000 km, 97 Lumina - 70,000 km) in March '05 and again in October '05. I did this instead of a flush and fill type maintenence, as we have never had any coolant related problems, got the idea from this forum. Coolant was unbelieveably clean. Question I have is: since I drained this amount (75%) twice in 2005, is it OK for a while? The pH, freeze & boiling point all check out to be OK, checked last week.