Reply To: 1961 Johnson 40 (RDSL-23) stops pumping water

[Don]

Give us more on the history of this engine…Is it new to you? If not, did it ever pump correctly? Sounds like you have thought and looked at most everything. Sounds like the gearcase is buried deep in the water also. How did the aluminum impeller housing look? Was the little vent hole clear? You speak of running it with the thermostat in and out, made no difference. How are you determining that it stops pumping water? Is the plastic thermostat housing and check valve/spring in place?
As far as sealing the SS impeller plate, I don’t think that would make too much difference when running the gearcase real deep in the test tank at lower speeds. Good sealing is required when running on a boat at higher speeds when the water level drops and the water pump gets surrounded by exhaust. BUT, you need to check the condition of the gearcase casting underneath the SS impeller plate, perhaps it has eroded away allowing exhaust to be pulled in under the pump. Any evidence of broken water pump screws in the past? Perhaps there was a botched thread repair or something like that creating a sealing problem….