To replace impeller pump parts, close the thru-hull and remove the three to six end cover screws. Most impellers sit on shafts with O-rings, but others are held on with setscrews. Remove the cover and gasket, and get the old impeller out by holding the hub of the impeller using needle-nosed pliers or a channel lock.
Use an impeller puller tool in absence of the two. Never use a screwdriver to lever the impeller out because screwdrivers score the soft bronze on the pump body. This may cause leaks in the pump.
Closely examine the impeller to ensure the tips of the vanes are round by bending each vane to check. If the vanes are flat or cracked, replace them with new ones. In case you lever out the impeller and it falls out in pieces, account for all the bits to avoid any small chunks of rubber from migrating to the heat exchanger or engine and cause damage due to overheating.
Check for proper lubrication in the impeller housing. Reduce the friction in the impeller housing by lubricating it lightly using Vaseline. Easily reassemble the vanes in the impeller by collapsing the vanes using a heavy rubber band or a loop of light line.