The flexible impeller creates suction and flow by bending its blades inside an eccentric pump chamber. It depends on water for lubrication. Running dry can damage it quickly.
Before opening the pump, close the seacock, isolate starting and note cover orientation. Inspect the old impeller for missing, cracked, permanently bent or swollen blades. Account for every vane. Missing pieces often travel to the first downstream restriction, commonly an oil cooler or heat-exchanger inlet, where they reduce flow.
Inspect the cover plate and cam for scoring and the pump shaft for play or leakage from the tell-tale area. A new impeller will not repair a worn pump body, air leak or spinning hub.
Use the lubricant and installation direction specified by the maker. Fit a new gasket or O-ring. Reopen the seacock before starting, then confirm strainer sealing, pump prime and exhaust-water flow. Check again for cover leakage after the engine reaches operating temperature.
Carry the correct impeller plus cover seal and removal tool. An old impeller kept loose in a hot engine bay is poor insurance because elastomers age and deform.