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!

All good info, but again, I need to get the motor back together before I can go further. I am going to run it without the thermostat first (which hopefully will show it running cold), then put a new thermostat in and retest. If all goes well, it should be OK at that point.

Dan I looked at the water flow diagram a few times before my initial post and they are very helpful. This is my first batch of 9.9/15’s I’m working on so there is a learning curve going on here, as I’m sure there will be more to come as I move onto other motors once I’m done with this style. Being able to pull the whole motor apart obviously is the best learning lesson.

Donny, I am in agreement with you between the two different bypasses; there is a thermostat bypass (on the thermostat, a small vent basically for water), and then some motors have a water bypass altogether (and from what I said before, it appears the 9.9/15’s do not have this). My point before is that the predecessors (9.5’s) had a water bypass, but if the thermostat was faulty the engine would still overheat with a stuck closed thermostat.

You’d see water pumping from the water outlet, but the motor would still overheat. It appears with these 9.9/15’s you’ll see the telltale but that doesn’t mean water circulation around the cylinders. It’s sort of a handy feature to help trace the issue – because at least you know water is flowing through the gearcase/water pump. If the stream becomes weak, that tells you ahead of time you have a gearcase originated water pump issue, or perhaps to your point, a grommet that is folded over or a leak somewhere previous to the tell tale.

DONNY!!!!!

The telltale was clogged up initially, but there was slight water flow. I unclogged it and now water flows freely throughout the motor. I haven’t been able to start it up because I’m waiting for the thermostat gasket and replacement thermostat to arrive this week. Then we can >hopefully< put this one to rest!!!!

Will report back 🙂