when I flush a system I remove thermostat while flushing,and turn on heater.I use one gal vinegar,to clean rust and scale.Run in engine 15 minutes then drain and flush till I can't smell it.I fill system at thermostat hole,put stat back in last.aluminum heads I allow head to cool down before using cold water.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran