I just had the same problem with my Westerbeke
W58 which is a perkins
4-154 that Westerbeke
uses. I had an overheating problem, changed new salt water
bump, no difference. Cleaned heat exchanger, no difference. Tested thermostat in boiling water, worked good. Checked all hoses, all good.
Fresh water pump belt was nice and snug but pump pully had a little play in it but there was no water leaking from the pump weep hole.
Finally removed fresh water pump and the shaft that holds on the metal impeller had shared completely off so pump did not pump any water.
These pumps are well built with three sets of bearings so even with the shaft broken the belt was still snug so the only way to check it is to remove the pump.
These pumps are very difficult to find, Westerbeke no longer sells the pumps but they sell a kit that include the shaft bearings and seals
but you have to use back tour casting body, impeller and pully flange.
To make things even more difficult there are two types of perkins 4-154 and although the water pump for both look exactly the same they will not interchange because the taper on the outside of the impellers are different. Everything else is the same. Mine is the later perkins 4-154 series 200 and I can tell because the engine have glow plugs, the waterpump has only one weephole and there is no casting number on the pump.
Today I fould a supplier who just started carrying the pumps for the Perkins 4-154 200 series (later version) after months of trying to find a replacement, they are JTServices in Minneapolis and the cost is $120.00