If the gunk in the engine oil gets too bad, the parts of the engine can get gummed up. An engine flush removes the gunk, which keeps the parts running smoothly. That in turn can make the engine more efficient, bringing the power and fuel efficiency closer to what the car had when it left the showroom floor.
In very old cars, however, the engine flush might clean these piston rings, valves, and other parts a little too well. Sometimes, the gunk acts as spackle in non-metallic parts like rubber seals that have cracked with age. Cleaning out the gunk exposes those cracks, and the weakness of those parts becomes apparent.
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