Unlike some of the larger Johnson outboard motors with bulky lower units, the 15-horsepower model is small enough to work with that you will not need an extra set of hands to disassemble it. The impeller is a small plastic piece in the water pump that is responsible for drawing water up into the engine to prevent it from overheating. The water pump impeller requires regular replaced.
Disconnect the negative battery cable, if it has an electric ignition. Tilt the motor so it is upright and remove the engine cover.
Remove the six bolts that secure the lower unit to the engine. There are three on each side of the lower unit that secure the lower unit from below. Shift into forward, and remove the shifting rod screw with a screwdriver. Slide the lower unit down and store it upright while you replace the impeller.
Remove the four bolts that hold the water pump head in place. Keep track of which bolts go where, as the two in front are longer than the two in the rear. Lift the pump head off the driveshaft to reveal the black plastic impeller. Note the direction in which the fins are bent -- the new ones must bend in the same direction.
Pull the impeller out from the pump housing using a pair of needle nose pliers, and pull out any shards that may have broken off. Remove the O-ring and the key insert on the driveshaft.
Place the new key insert into the driveshaft slot where the old one was. Lubricate the impeller and slide it in place, bending the fins in the same direction in which the old ones bent. Place a new O-ring on the pump housing and replace the pump head. Replace the bolts in the same pattern as they were removed.
Slide the lower unit back in place, making sure the shaft is properly aligned, and tighten the six bolts back in place. Screw the shift rod back in place and put the engine cover back in place.