Can't Figure out Overheating issue

[Begester]

5.7L carbureted Mercruiser alpha 1 gen 1 in my 1989 SeaRay. I replaced my impeller, water pump, gaskets, etc. Also replaced thermostat and temp sensor. Boat is overheating on the muffs but I'm getting good water flow from the exhaust. Any suggestions?

One item I did notice is that while water is always coming out of the prop, there is not water coming out of the exhaust until the engine hits about 150 degrees. I don't believe it was that way previously.

There is no milky oil. Closed cooling system.

Any ideas? I'm banging my head on the wall here! The overheating seems to be defying conventional wisdom here.

 

[Begester]

Forgot to mention. Closed cooling.

 

[TunaFish389]

Have you checked a different gauge? Done a compression check?

 

[Begester]

It's hotter than normal down there. Before I had the boat out of the water, it was running fine, no power loss at all.

 

[Bondo]

Forgot to mention. Closed cooling.

Ayuh,...... Can you see coolant flow through the heat exchanger,..??

If raw water is comin' out the exhaust, raw water is flowin' through the heat exchanger,......

Have ya checked it with an IR temp gun,..??

 

[Begester]

Here's what I've done over the past few days, to no avail:

Removed the heat exchanger and had it boiled out by a radiator shop. They said it was pretty bad...has been 10-15 years since its been done. Also replaced thermostat (143 degree) and one of the two temp sensors.

I've also checked the raw water lines from the heat exchanger to the manifolds and on the antifreeze side of the heat exchanger.

Temp still continues to rise.

Temp Sensor readings:
With Thermostat housing reading 160:
Port Manifold - 160
Port Riser - 134
Port Elbow - 167
Starboard Manifold - 171
Starboard Riser - 153
Starboard Elbow - 180
Heat Exchanger - 140 on the top side (antifreeze), 145 on the raw water side
Water inlet on inside of transom: 91

As the temp gauge continued to climb I took the readings again:
With Thermostat housing reading 171:
Port Manifold - 176
Port Riser - 150
Port Elbow - 167
Starboard Manifold - 193
Starboard Riser - 174
Starboard Elbow - 185
Heat Exchanger - 143 on the top side (antifreeze), 163 on the raw water side
Water inlet on inside of transom: 93

From these readings, the antifreeze side of the heat exchanger is staying relatively consistent, as are the elbows.

Any ideas? This is driving me crazy and my boat has always run religiously at 145-155.