Engine flush is a chemical detergent additive designed to be poured into the engine oil port just before a scheduled oil change. Once added, the engine is idled for 5-10 minutes, allowing the solvents to break down sludge, varnish, and carbon deposits that have accumulated on internal components. It helps free up stuck piston rings and clear blocked oil passages. However, it must be used with caution; in very old or neglected engines, loosening too much sludge at once can clog the oil pickup screen.