One maintenance detail a Mercury® outboard owner should never overlook is the scheduled service of the water pump impeller. The rubber impeller turns at high rpm and circulates water through the engine’s cooling system. As the impeller wears, or if it becomes damaged, the flow of water to the powerhead can be diminished. If the impeller fails, the engine will quickly overheat.

Replacing the impeller is a part of normal service typically performed by a marine dealer. However, if you’re a do-it-yourself boat owner, it’s a task that can be accomplished in the home shop, and Quicksilver® offers the premium-quality parts you’ll need to do the job right, packaged in convenient kits.

Depending on the outboard model, your Mercury owner’s manual will recommend changing the impeller annually, at 100 hours or at 300 hours. But time is not the only factor to consider regarding impeller life. If you regularly use the motor in silty water or if you happen to run aground on a sandy bottom, abrasive material drawn through the pump can accelerate impeller wear. In this situation, changing the impeller ahead of schedule will be good insurance against failure. A weak flow of water from the cooling water telltale is a sign the impeller may be worn and no longer pumping efficiently. And if you are reviving an old outboard that has not been used in several seasons, change the impeller before starting the motor. There’s a good chance the impeller has grown stiff and taken a “set” and will either disintegrate on startup or no longer pump efficiently.

Read the Service Manual

Instructions for servicing the water pump are not found in a Mercury engine owner’s manual. A copy of the more-detailed Mercury service manual for your model will have specific step-by-step instructions. Service manuals for many Mercury engines may be ordered online or through a Mercury Authorized Dealer.

The outboard can remain on the boat while the pump is serviced, and this task does not require any special tools. The pump is located on the top of the lower unit. The first step will be to separate the lower unit and its long drive shaft and shift rod from the outboard midsection. The lower unit on a small outboard is fairly easy to handle, but on larger motors the unit’s weight and awkward shape make it handy to have a helper. In a pro shop, the lower unit would be placed on a special – and expensive – gearcase service fixture. You can do this job with the unit on its side on a bench, but it would not be difficult to make a temporary fixture with a little lumber.

Once the lower unit is off, follow the service manual instructions to open the pump housing and change the impeller. Examine the inside of the stainless steel housing. If it’s worn or scored it should be replaced. A Quicksilver Water Pump Repair Kit will include a new housing, the impeller, a new stainless steel wear plate and all of the gaskets, O-rings, water tubes and seals required to complete the installation with fresh components. If your water pump housing does not show wear, a Quicksilver Water Pump Impeller Replacement Kit will have all of the same components except the housing. (Some components vary based on engine model. A Quicksilver retailer can help you select the proper kit.) Some of the seals and O-rings in these kits are not directly related to the pump but can be replaced when the lower unit is off the motor, renewing all these components. Follow the service manual instructions in this regard.

As the water pump housing is lowered over the new impeller, be sure to turn the drive shaft clockwise so the impeller vanes fold over in the correct direction. Before reassembling the pump, apply a light coating of Quicksilver 2-4-C Marine Grease to the wear plate and the inside of the pump housing. This will lubricate the impeller until water begins flowing through the pump.

With the pump reassembled and other new components installed, have a helper on hand to reinstall the lower unit on the motor. They can hold the lower unit while you position the shift rod, for example.

Once you have a fresh impeller installed, you can count on your engine’s cooling system performing at its peak. That means more time to enjoy the water with confidence.