"First, before you tear down a
"First, before you tear down again, Make sure the pick up holes in the lower unit are not plugged. Find the coolant hose from the outdrive to the engine. Remove it from the engine. Put on "coolant ear muffs" or best, put boat in water or put lower unit in big garbage can full of water and remove coolant hose from engine and aim hose in safe direction like the bilge. Start engine briefly and rev to 1500 ish rpm. Coolant flow should be major..say fill a 5 gallon bucket in a minute. There should NOT be intermetent spurts or air mixed with water, just solid forceful stream.
Assuming it is a mercruiser, it is possible to assemble the water pump gasgets incorectly to cause a major leak from the pump assembly. Some models of pump assembly have two sizes of gasgets with the major pump diameters about a 1/16" difference in size. If you put the larger diameter on the bottom of the assembly, you will loose pump pressure between the base plate and the pump body because that larger gasget will over hang the pump body and not seal.
You should check for leaks before mounting the lower unit. You can do this by pouring water down the exit tube and look for leaks around the pump body and even applying a few pounds of air pressure.
Make sure you install the new impeller in the correct rotation and that you use new parts if any grooves are showing in the steel.
If you purchaced a "KIT" which may contain a new pump body, make sure to install a new "O" ring in the shaft bore and make sure it is not twisted in the bore.
NEVER run the pump dry!!! A few seconds running dry will ruin the pump impeller!
Mercruser water pumps are not very stong. The impeller can weaken just by sitting.
Finally, if you have ever run across a sand bar or mud flat, you can easily fill your system with crud. A standard system where sea water is pumped directly to the engine could fill the engine with sand/mud. Or if you have a closed system, the heat exchanger could be filled with sand. Hence, the need to remove inlet hose from engine to test for coolant flow from water pump.
Hope this helps."
