It almost seems like open-heart surgery. Changing the impeller on your 115-horse Mercury, a small part in the middle of your outboard, means you must remove the lower half of your outboard and dive, willy-nilly, into the "gray area" between the engine block and the propeller. Fortunately, it's not open-heart surgery--there's very little in the area that you can damage and you can save money and shop-time at your dealer by doing the job yourself.
Set the motor up in a vertical position. If it's currently stored on a "lay up rack" for the winter, ensure the rack is high enough that you can remove the lower unit. If the motor is still on the transom of your boat, disconnect the cables from the battery, to prevent accidental starting. Place one of the oil drain pans under the lower unit.
Use the adjustable wrench to remove the four bolts that hold the lower unit of the outboard. The lower unit is the bottom section of the outboard that includes the gear housing, the propeller shaft and the propeller. On newer Mercury motors, all four bolts are readily accessible; on older Mercury outboards, removing three of the bolts exposes the fourth, which you can then remove.
Prepare to support the lower unit's weight after you remove the bolts. Remove the screws that connect the wiring harness to the lower unit with a screwdriver, and pull the lower unit away from the upper part of the motor. The driveshaft and transmission control shaft will pull away with the lower unit--the crankcase oil will begin to leak as you separate the driveshaft from the upper unit, hence the need for the drain pan.
Set the lower unit horizontally into a motor vice above the second oil drain pan and use the socket wrench to remove the four 9/16-inch bolts that connect the water pump to the transmission. Slip the body of the water pump housing upward--the face of the pump will remain in place--and off the top of the driveshaft. Remove the impeller from the water pump housing; pry it out with the tip of the screwdriver if necessary.
Examine the water pump housing for cracks or chips and, if you see any bits of plastic loose in the pump housing, remove them. Remove the three gaskets--one rubber gasket, a metal gasket and a second rubber gasket. Replace them with the new gaskets from the water pump replacement kit, wiping each gasket with motor oil before setting it in place.
Find the new impeller in the water pump replacement kit and slide it onto the driveshaft, down to the face of the water pump. Slide the water pump housing and its new gaskets down the driveshaft, over the impeller, to join the face of the water pump. Grasp the driveshaft and twist the driveshaft clockwise to set the impeller fins. Replace the four 9/16-inch bolts that secure the water pump to the transmission.
Move the lower unit back into place under the upper unit--you will need assistance, because of the weight. Ease the driveshaft and transmission housing back into place in the upper unit and reconnect the wiring harness. Bolt the upper and lower unit housings together, tightening the bolts to 55 pound-feet (75 Nm) with a torque wrench.
Change the oil and oil filters in the motor. Replace the gear lubricants, per the instructions on pages 80 and 81 of the Mercury owner's manual. Connect the flushing attachment to a garden hose and the engine, and turn the water on full force.
Connect the battery cables to the battery and start the motor. Watch the water purge hole on the lower unit--a steady spray of water coming out indicates the pump is working correctly.
Use only the Original Equipment Manufacturer shop manual.
Dispose of the lubricants that drain from the motor in accordance with local laws.