Re: impellar causes...
The impeller is the working, rotating, part of the water pump.
The function of the water pump is to pump water through the engine for cooling. If the impeller, or the pump itself for that matter, is worn or damaged then insufficient, or even no, water will be pumped through the engine and overheating will be the result.
The engine temperature is controlled by the thermostat which regulates the volume of water passing through the engine.
While you have the gearcase unit removed to access the water pump connect a hose to the water tube and make sure you can get a good water flow through the engine. During the course of its 36 year life the water passages in the engine may have become obstructed with scale and corrosion and if that is the case you may not get sufficient water flow to cool the engine even with a new impeller. Action will then be required to clear the water passages.
