Johnson outboard motors have internal water pumps that induct water through the lower unit side vents and send it through the engine passages. The motor driveshaft that extends down to the propeller drives a small rubber impeller inside of a sealed housing. The vane-like blades on the impeller rotate with the engine rpm, sucking water through one passage and sending it through the other, within the housing. Over time the impeller loses its elasticity because of heat and constant wear. Replacing the impeller on a Johnson outboard involves removing the lower unit and disassembling the water pump housing.
Trailer the boat and stabilise the trailer in a secure position, either with the tow vehicle or setting chocks behind and forward of the trailer wheels. Use a socket to disconnect the negative battery cable. Shut off the fuel valve to the carburettor. Refer to your motor repair manual for the location of the lower unit case bolts. There may four bolts for a smaller Johnson motor, and up to six or eight for the larger models.
Use a socket and wrench to remove each lower unit case bolt (a few might be nuts attached to studs). Look for any upward-facing bolt on the underside of the cavitation plate. Gently pry away the lower-unit case from the upper unit with a screwdriver, but only about 1/2- to 3/4-inch clearance. Shine a flashlight inside the case seam and notice the small coupler bracket holding the shift linkage together. Insert a screwdriver inside and loosen the hex bolt on the linkage arm.
Pull the lower unit down from the upper unit case, disconnecting the top of the driveshaft spline and the heat tube. Leave the copper heat tube attached to the upper grommet. Place the lower unit upright in an engine stand and secure the clamps to immobilise it. Pull the O-ring off the top of the driveshaft. Use a socket to remove the bolts that hold the water pump housing in place. Pry the pump housing loose and lift it straight up over the shaft.
Flip over the pump housing and look at the old impeller blades. The blades will face a certain direction --- remember this orientation. Recover the impeller key on the driveshaft and set it aside. Pry the old rubber impeller out of the pump housing with a screwdriver and discard it. Clean the inside of the pump housing bore with carburettor cleaner and an abrasive sponge pad. Wipe it dry with a rag. Look for deep grooves or cracks. Apply some marine grease to the inside of the housing.
Pull the thin pump plate off the lower unit case and clean it with a sponge pad and carburettor cleaner. Set it back down over the lower unit case. Lubricate the blades of a new impeller and slide it down over the driveshaft. Align the driveshaft key inside the impeller keyway. Clean the gasket surface on the pump housing with carburettor cleaner and a sponge pad. Wipe it dry with a rag and install the new kit gasket over the base plate.
Push the water pump housing down over the driveshaft and twist it against the tops of the impeller blades in the correct orientation that you removed it. Turn the propeller with one hand and shove the housing down with the other hand, to seat the pump housing. Insert the water pump housing bolts and tighten them with a socket. Replace the kit O-ring on top of the driveshaft groove. Wipe the heat tube grommet hole with a rag and lubricate it with grease.
Push the engine stand under the upper case. Lower it with the adjustment controls, if you need the clearance. Gently pump the lower unit up into the upper case. Align the driveshaft with the spline socket in the top of the engine, while you also guide the heat tube into the lower grommet hole. Make sure the driveshaft seats in the splines and the heat tube just enters the grommet. Stop with 3/4 inch between the lower and upper unit.
Place a screwdriver between the case seams and tighten the bracket bolt that holds the shift linkage together. Finish raising the lower unit flush with the upper unit case. Install all the lower unit case bolts by hand. Tighten all the case bolts with a socket, rotating the sequence from one side of the engine to the other in a crisscross pattern. Release the clamps on the engine stand and remove it.
For a very heavy lower unit on a larger engine, obtain assistance from a friend to help you lift the case into and out of the engine stand.