1992 4.3 Mercruiser w/ Alpha one Gen 2 Overheating above 3200 RPM

Ok, I'm throwing in the towel here. The boat has run perfect at all speeds until late June. Upon going over 3200 RPM the engine begins to overheat past 190 deg and climbs steadily over the 200 deg range until I throttle back below 3200 RPM.
If I keep pushing the throtte the temp will continue to climb.

I pulled the boat out of the water and here are the following things I did.

1. Removed the exhaust elbows and verified no rust clogs (They looked brand new)
2. Removed the thermostat housing and replaced the thermostat (there was no thermostat in there...odd)
3. Replaced the lower unit water pump (impeller, gaskets, etc...)

After all this work I still have the same problem...

For what it's worth, the temp sender and temp gauge are brand new as of last October and *appear* to be working fine. My IR temp gun measures 140 deg at idle on the temp sender but I haven't checked the temp with the IR gun while it's overheating to verify yet.

My next experiment I think will be to put a clear hose in-line with the raw water pump to look for cavitation at 3200 RPM and above.

So my questions...

1. Should I be looking else where for a problem?
2. If I do find cavitation on the raw water pump line with the clear hose then where do I begin to start fixing that problem?
3. Can anyone give me the no kidding real ohms value range for the temp sender? I'd like to verify the sender and the gauge are correct. It seems every place I read has different ohms values for the temp sender...

Thanks!

Re: 1992 4.3 Mercruiser w/ Alpha one Gen 2 Overheating above 3200 RPM

Have a look at this service bulletin on your exact problem.

View attachment EN_13.pdf

Re: 1992 4.3 Mercruiser w/ Alpha one Gen 2 Overheating above 3200 RPM

Thanks Don, that service bulletin was helpful. It seems to call out the RPMs and problem directly.

So now for the rest of the story...last fall when I bought the boat one of the water pump bolts broke due to corrosion. The previous owner decided to trim the engine up while it was in the water tied up...hence the massive corrosion. I replaced the bolt with a helicoil which seems to work well. When I took the pump apart last time I saw NO evidence the impeller cover was popped or insecure...I suppose that doesn't matter considering a small air leak is all it takes...

It's interesting they ask to install the hose on the manifold rather than the raw water pump. I'm concerned the vinyl tubing I have could melt if connected to the hot water output.

Will try the connection out and see what happens...