Also nothing was wrong with impeller or housing when I replaced it. I just purchased it a month ago and was having the same issue. The only problems I have found was a dry rotted grommet at the top of the water tube and a stuck open thermostat.
I’m also having the same problem on my 1990 Johnson 25 2 stroke. I have replaced the entire water pump housing and impeller. Water tube was clear. I took off the power head and replaced the grommet that the water tube slides into. Replaced the power head gasket. And I replaced the thermostat and...