I pulled the motor completely apart, but not the manifold of course (not necessary…well, actually, now I have pulled the manifold to replace it because the manual starter screw orifice was stripped out and beyond salvage…another whole batch of cursing). The tell tale was gushing water with the garden hose connected to the water inlet tube, and there was lots of water passing through the thermostat orifice. I had to clean out the telltale because the "L" off the water jacket was nearly 100% obstructed with dried up sand/debris. The cylinder water jackets had nothing in them (I pulled the cylinder head…despite this being a salt water motor, only had 1 stuck screw…of course, it snapped and required a drill out and retap).

When I originally started the motor (first run), water came from the telltale briefly, then turned to a steady drip. There was a very minor amount of water coming from the exhaust of the motor (just below the pan). So there was probably a little water getting through that thermostat bleed hole.

Upon further education on my own behalf, these 9.9/15hp motors do not have a water bypass as did their predecessors, the 9.5hp’s. So if the thermostat is stuck, that will basically create a ‘damn’ of sorts to the water flow. The telltale will still have water flow but it won’t be able to circulate past the cylinders, because once it passes the thermostat it goes up and around the thermostat cover (which is the outer cylinder head cover), then right out of the motor. Thus, stuck thermostat = overheat condition.

So once I have the gaskets and replacement thermostat, I’ll be able to button her back up and confirm if this was the issue or not (I feel pretty confident that it is). I had put a new impeller in before I went more invasive and it didn’t solve the problem, which is why I started this thread originally, to see if there were any passages in the impeller housing that I wasn’t aware of that should be examined for clogs.

Recall a couple of weeks back I ran into the same issue on an older 20hp motor; Frank was kind enough to point out there is a tiny pinhole in that housing which if clogged, essentially shuts off water supply to the motor. That was such a dramatic change (basically like turning a faucet on and off, that’s how much of a difference it made), that I wanted to be sure this wasn’t the same issue. I don’t think the 9.9/15 impeller housings have this type of configuration. If I’m wrong, I hope somebody who’s following this thread tells me for future reference!