Mercury - Water Pump Impeller Vintage Outboard Cooling System Rubber Elastomer 89982. Overview. Anyone who's run a vintage Mercury outboard knows there's a certain charm to these old 20 horsepower engines. They've got a tone, a feel, and a toughness you just don't find anymore. But they're also fifty-year-old machines, and the cooling system is one place you can't afford to cut corners. When water flow weakens or temperatures start creeping up, the engine tells you it's time for a new impeller.