Mercury - Water Pump Impeller OptiMax Cooling System Elastomer Vanes 89984T4. Overview. If you run a Mercury OptiMax 2.5-liter outboard, you already know these engines like to work. They're crisp, responsive, and built to cover water fast but only when they're getting a solid stream of cooling water. When the telltale weakens or the engine starts running warmer than usual, that's your signal the impeller is wearing down. The Quicksilver 89984T4 Water Pump Impeller restores that confidence. It's