The Impellers and Shaft Seals are normal wear items and do not have a "typical" life span. As such, it is common to change them on a regular basis; such as, with every other oil change. The condition of the raw water, i.e.: silt content, contamination, etc., contributes to the deterioration of these components and the frequency at which they should be replaced. The Impellers for both pumps are identical, but the Shaft Seals are different.
An exhaust temperature light going out (alarm) can indicate insufficient flow and therefore a worn raw water Impeller. The appearance of insufficient flow at the exhaust outlet can also suggest a worn raw water Impeller.
A falling coolant level, with no apparent leaks, can indicate a leaking raw water pump Seal (21/10)
. The engine coolant can follow the raw water pump shaft (21/2)
through a worn Seal into the raw water pump chamber. Once in the raw water pump, the coolant will be pumped overboard with the waste raw water.