If you use a flush kit in line with the heater hoses and keep going till
it's clear, and follow the instructions with the kit, you will get as
close to complete flushing as you can without removing everything and
draining it. I would not worry about the tiny bit that might not get
out, with the thermostat in. Then again, when you change the coolant
you probably should change the thermostat anyways.
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