These impeller vanes have eroded, probably due to long coolant change intervals. Not only do the steel blades CORrode from old coolant not preventing it; they also Erode due to microbubbles (cavitation) created due to the lowered boiling point.

http://www.fourdoorbronco.com/board/showthread.php?t=5230

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Owner caption January 2005 · Photo 185 of 980