I need a coolant flush soon but I was wondering how thorough the auto shops do it? I know auto shops want the cars in the bays for the least time possible and a thorough coolant flush can take a couple hours or more (waiting for drainage/temps to be right in engine, etc). Do they just flush it once and put new coolant in or do they wait for it to drain fully and flush it again or? What is your experience? Also do they use tap water generally?