I'm at the 5 year mark to drain/flush the coolant on my 09 Enclave (okay, 4 months over 5 yrs). I only have 40K miles. I was thinking of draining the coolant in the radiator, filling with coolant and running the engine, draining the radiator again, then filling for the last time. I don't want to remove engine drain plugs for fear of problems with them going back (and I don't know where to find the plugs). How long does it take for the coolant to circulate in the engine, heater, and radiator? Should I drive the car for some time to circulate the coolant? Or would just running the engine for 10-15 minutes be fine to circulate everything (with heater on)?