Engine overheated

[High Bidder]

This August my engine exhaust manifold and riser overheated on the right side of the engine.  Engine temperature was 170 degrees but the manifold became hot enough to melt the flapper and start the rubber coupling on fire.  The manifolds and risers are 3 years old and seem to be open as I can run water through them easily.  The outdrive impeller was new and is in the right direction.  Luckily it ran at an idle and stayed cool enough for me to get the ten miles back to the launch as water was coming in the exhaust and I have two bilge pumps but one seemed to keep me afloat.  Would this be caused by the outdrive pump, while looking good was not suppling enough water to the engine? Or was it a engine problem? This was a used engine I installed and it never seemed to idle correctly. I'm replacing it with a rebuilt long block but still don't know what caused this problem and don't want it to happen again. The only parts I didn't replace is the manifolds and risers. I installed new flappers, water pumps, thermostat and made sure the oil cooler is clear.

[Bozeman Bob]

Is this a closed cooling system ? If it's a Mercruiser and closed system there are two raw water pumps , one on the outdrive and one mounted near the bottom of the engine , usually opposite of the alternator.  A infrared heat gun would be helpful in pinpointing where the temperature starts to climb above the normal range . Odd that it's only on one side , you might swap risers to see if it's the riser . I know you said it's 3 years old and water is seemingly flowing ok . Unless a hose is clogged,pinched ,that's the number one thing that causes only one side to heat up . 

[High Bidder]

Its an OMC open system. The side that overheated is the side where the water flows in the opposite direction in the thermostat housing in relation to the water input hose. I at first thought that the power steering cooler got clogged with seaweed and restricted the flow or the impeller in the outdrive failed but neither seemed to be damaged or clogged.  I guess I'll run water through the manifolds with the muffs on the outdrive and make sure water is flowing through the manifolds adequately before attaching the exhaust outlets and running the new engine.