Strange overheating problem - it's happened twice now

[wakejunky]

This has happened to me twice now and I initially went down the blown up impeller problem that I posted in another thread and since replacing the impeller and thermostat. I have the boat at least 4 times without problems and then yesterday it overheated after dropping the boat in the water.

Here's the scenario, back boat down into water and start engine, while idling, wait for truck driver to return, pick up driver and then start to head out. Engine reports, engine overheated, shut her down. Wait for it to cool down and in the mean time go for a swim.(it was 102' out) While swimming around notice the engine exhaust flap is basically sucked inward creating a vacuum. I pull the flap away from the exhaust port, which took a little bit of effort, and wait until the engine is cool enough to start. Start it up and everything runs fine. Engine heats to 160' and then thermostat opens up and cools down, run it up to speed and went without incident the rest of the afternoon.
I can't seem to figure out why this has happened. It ran at least four times between the initial problem and doing a raw water flush, impeller and thermostat change.

Can backing the boat down and the exhaust flap being pushed against the exhaust port prevent the boat from breaking the vacuum that is created on the exhaust to not allow water to flow out?

[s_kelley2000]

I have a feeling the exhaust flap is not the cause of the overheat but perhaps happens as a result. I think I remember reading about hose flaps getting sucked in after an overheat in the past but a quick google didn't produce much. I would be checking for an air leak on the suction side before the raw water pump. If you haven't already done so, make sure all hose clamps are tight (use a nut driver/socket, not a screw driver). I'd also make sure that the o-ring is in place for the strainer basket. My theory is that the impeller is having a hard time getting primed due to an air leak. When you notice it start to heat up, if you give it a little gas does it start to cool off? I would give that a quick try before shutting down if it happens again. I haven't followed your other thread you mentioned so forgive me if you already covered any or all of this.