Chrysler 79 75hp outboard overheating

"recently purchased 79 75hp, a

"recently purchased 79 75hp, and overheats but impellar fine, (have checked and replaced), dealer says could be corrosion where exhaust and cooling water meet and exhaust stops water from going thro system. The inlet on the leg is not blocked, and if we put a hose thro it it flows fine. Water flows at idle but not at operation speed.
Any thoughts or suggestions?
thanks"

"If it pumps water at idle but

"If it pumps water at idle but not when it's "revved" up that usually indicates a seal problem, and most often on the water pump end of things.

Water will take the easy way out and the increased pressure may be "opening up the seal breach" and letting the water escape.

A second issue could be old bits of impeller or other crud which causes a blockage in the water-tube or powerhead with the increased waterflow at higher pressure.

In any event, the lower unit should be removed and the pump housing inspected for gasket and seal problems.

While it's off, a hose can be connected to the water tube to check for flow up the tube and through the powerhead. Alternately, air can be blown through the system from the top down to try and dislodge any debris.

The problem could be more complex, but that is the easiest place to start your troubleshooting and most often the location of the problem..."