In this video on our annual service of a 40 hp mercury Four stroke outboard motor I go through all the steps involved in replacing the water pump impeller. Including removing the Lower unit, inspecting and replacing the impeller. I also give a demonstration and description of how an outboard motor water pump actually pumps water. I show you how to install the lower unit and how to identify what gear the lower unit is in. I show you how to get the splines to line up so the lower unit is easier to install. They can be very difficult sometimes.
0:00 How to remove the propeller and Lower Unit
5:45 How to Inspect & Replace the Water Pump Impeller
10:07 Description of Water Pump Operation
17:02 Installing the New Impeller
27:16 Description of the Shift Shaft Operation
29:00 How to Install the Lower Unit.