Re: 9.9/15 impeller housing tricks?

[Don]

Those water pumps are fairly simple, yes do inspect the bleed hole in the cup/housing though. Are you using OEM parts? There have been complaints with sierra pump components. The other area to look closely at is the gearcase housing sealing area under the impeller plate. These gearcases are pretty thin under there, the aluminum may have eroded away creating an air/exhaust leak into the pump.
Needless to say, I’m sure you have already checked to make sure to overboard indicator just isn’t plugged up, very common. The crud also gets stuck on the end of the plastic nipple going through the lower pan. I am assuming the engine is actually getting hot, and that you have let it run long enough to confirm this.
What year is this engine? Later 70s models picked up a lousy upper water tube grommet that loves to crush, especially in salt water. There were improvements made in the mid 80s, but the problem still persisted. The early models 74-78 had a different style grommet that did NOT have this problem. Unfortunately, even those early models supercede to the troublesome grommet.
So, once you have checked all the normal stuff, my money says you will be pulling the powerhead.
Sorry
Don