Re: 9.9/15 impeller housing tricks?

[Don]

Well, I think we are confusing the "bypass" term we are using. The bypass I am referring to is right in the thermostat. In other words, most of the OMC thermostats don’t seal completely when completely closed. There is a little groove in the thermostat cup/housing that allows a bit of water to bypass through the closed thermostat. And yes, there are actual bypasses cast into the block on other OMC engines, but I do not think there is on this 9.9/15. I will have to look at a head, perhaps there is a little bypass/bleed hole in the head, can’t remember. Later model 9.9/15s could bypass the thermostat by pushing the thermostat off its seat (compressing the pressure relief spring above the thermostat), but the early 9.9/15s did not use the pressure relief system, unless someone has replaced/updated the cylinder head.
You mentioned earlier that there was some sort of gasket under the SS impeller plate when you first looked at the water pump, which makes me question the water pump because no 9.9/15 has used a gasket under the impeller plate that I know of. That is why I recommended replacing the complete water pump, not just the impeller. Don’t forget to check the gearcase/pump plate mating surface, this area is thin and erodes causing an air/exhaust leak into the pump.
Did you have the cylinder head off when you check the thermostat, or did you just pull off the water cover?
Does this engine show signs of overheating? Discolored paint, melted wiring?
I would definitely perform Dan’s drill test, put the gearcase in a bucket of water instead of allowing it to escape into your test tank!
Will brave the cold cellar, and have a look at a head for a bleed/bypass hole.