Drain plug removal can often be a major pain. I suggest doing the flush as per MBZ: Remove thermostat (you will probably need to use both old and new seals to keep a water tight housing), get engine temp to normal and run at medium RPM for 5 minutes, let engine cool to about 120f and drain at radiator, repeat process, replace thermostat w/new seal, fill system. Just use plain water unless you know there is rust or crud build-up.

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