Impeller Replacement 101

By Tom Burden, Last updated: 11/1/2019

Flexible Impeller Pumps

Assorted flexible impellers

The impeller is a vital component of your engine's cooling system. Keeping a spare impeller on board is a good idea in case of sudden failure.

Inboard and sterndrive engines usually include a flexible impeller type raw water pump. Flexible impeller pumps are common in shower drain sump pumps, some bilge pumps, wakeboard ballast pump systems, oil changing systems and lots of other applications.

Why Impeller Pumps Fail

Flexible impeller pumps work best when run regularly. If your pump sits for months with the impeller in one position with the blades on one side bent, it may “take a set” and may blow a circuit breaker on startup (if it’s electrically driven). The impeller may also stick to the housing and come apart when the pump starts. Impellers are damaged by debris that get sucked in, by chemicals, and especially by running dry. All it takes is a plastic bag blocking the raw water inlet, or a seacock left closed when you start your engine, and your impeller will soon be a shredded mess.

Of course, you should check the wet exhaust on your transom for the proper flow of cooling water each time you start the engine. You should check the impeller’s condition during Spring Commissioning and every 200 hours of operation. Correctly installed impellers may last for several years, but you should stick to a schedule of preventative instead of emergency maintenance. Just the same, an impeller will eventually fail while in operation, and usually at an inconvenient time. You should carry a spare onboard, since the impeller is one of the vital components of your engine’s cooling system.

Selecting the Right Impeller

Drawing of eight impeller drive types

We carry impellers from several manufacturers, including Johnson Pump, Mercury Marine, Sierra and Jabsco. There are several ways to determine the West Marine model number for a replacement Jabsco impeller:

  • If you have a Jabsco pump, the Jabsco impeller part number can be found on the Data Sheet that came with it.
  • You can also remove the impeller from the pump and then identify the Impeller Profile Code Letter by placing the impeller on the silhouettes that appear in the downloadable Jabsco Impeller Replacement Chart below. If your impeller matches one of the silhouettes, you can then measure the depth (See Chart A), identify the drive (See Chart B) and select the material.
  • The correct Jabsco number for a replacement impeller can also be determined according to the number of blades, diameter (Dimension A), impeller depth (Dimension B) and shaft diameter (Dimension C). See the downloadable Jabsco Impeller Catalog below.

Neoprene, Nitrile or Polyurethane

Use neoprene impellers for engine cooling, and for fresh and salt water transfer duties. Neoprene is suitable only for pumps where small amounts of oil or diesel fuel are present.

Use nitrile impellers for bilge pumping and for transfer duties where water is heavily contaminated, for example by oil or diesel. For transferring diesel fuel, use a sliding vane pump like the Jabsco Vane Puppy or Groco Flo-Master. Ballast pumps, used on boats that tow wakeboarders, have unique requirements, needing to reverse direction and withstand systematic abuse. Polyurethane impellers are used in that application.

Replacing Impellers in Raw Water Pumps

Replacement is relatively simple. For raw water pump applications, first close the water intake thru hull valve. Then, remove the pump's end cover screws and take off the cover and gasket. Take the old impeller out by gripping the hub of the impeller using channel lock or needle-nosed pliers. For easiest removal (or where the above tools don’t work) use an Impeller Puller Tool. Don’t use a screwdriver to lever it out, as this may score the soft bronze of the pump body and cause leaks. Some impellers are sealed onto their shafts with o-rings, a few (in some Volvo, Atomic Four and Universal engines) are held on with setscrews.

When the impeller is out, examine it closely. The tips of the vanes should be round, not flat, and there should be no cracked vanes or distortion in their shape. Check each vane by bending it. If the impeller is in pieces, be sure to get all the bits out and be sure they are all accounted for. Little chunks of rubber can migrate into the heat exchanger or the engine and cause overheating damage.

Lightly lubricate the inside of the impeller housing with Vaseline to reduce the friction of the first dry startup. Use a heavy rubber band or loop of light line to collapse the impeller’s vanes, insert it, and pull the loop our with your pliers. See the Jabsco Impeller Catalog for a Troubleshooting Guide with diagrams of impeller wear patterns, which might help you to determine the cause of damage to your impeller.

Your pump may need additional service, particularly the seals, bearings, wear plates or clutch. Nigel Calder’s excellent book, Boat Owner’s Mechanical and Electrical Manual, covers detailed rebuilding. We highly recommend this book as a general reference. If your pump is ready for replacement, most of the “classic” pumps from Jabsco, Sherwood and Johnson Pump are available.