Inspecting and Repairing Your Flexible Impeller Pump

If your boat’s engine or generator is raw-water-cooled, then you are aware that your marine raw water pump is an all-too-common source of trouble. Blockage in the cooling system or a failure of the pump can cause major damage to a raw-water-cooled engine.

Without enough cooling, the engine can overheat and you can damage it permanently. Proper maintenance of the raw water pump now can help you to avoid a mountain of headaches with your boat in the future!

The Source of Trouble in Your Boat’s Engine Room

If you are not getting water flow to cool your engine, then there are several things you want to check. You should always run your engine and water pump with an available water source feeding your cooling system; do NOT run your water pump dry. First, if the pump is belt-driven, you should observe whether the pump pulley is turning when you run the engine. If the pulley is not turning, you may need to tighten or replace the drive belt. Another possibility could be your pump shaft has become frozen in the pump housing. If so, then you would need to disassemble the pump to determine the failure. If the pulley is turning, then most likely there is an issue with the impeller or pump body. Again, you would need to disassemble the pump.

A typical raw water pump is a flexible impeller pump. Inside the pump body is a flexible rubber impeller with several vanes squeezed down one side of the pump as it turns. This pushes out any fluid trapped between the vanes creating the flow needed to pump the fluid throughout the cooling system. The impeller is a common replacement item which you generally need to replace at least every year. You may need to replace it or more often, depending on how much you use your boat. The raw water pump and its impeller can quite often be difficult to access, but the effort is worth it to ensure safe and proper engine performance. Sometimes, you may need to uninstall the entire pump to get access to the impeller.

To access the impeller, first, remove the cover plate. Then you should be able to see the impeller. If you see excessive swelling, noticeable wear and/or cracks, or one or more of the impeller vanes have broken off, replace the impeller pump immediately. You also need to replace the impeller if the vanes are not making contact with the pump body. Impeller pumps work best when you run them on a regular basis. If your boat has sat idle for a long time, your impeller may have lost all moisture and dried up. An impeller lacking moisture is prone to breaking up during its next use.

If vanes were missing, the impeller most likely ran dry, and you will need to find out why and also locate the missing vanes. Debris in the cooling system hoses can limit water flow and cause your engine to overheat. Detach all cooling system hoses and look for any pieces of rubber which might be loose. Check for a collapsed hose, a clogged strainer, a closed seacock, or any other obstruction which would prevent water from getting to the pump.

The Value of an Impeller Puller

An impeller fits snug inside the pump body, and usually requires a tool and some force for you to pry it out. Some people claim they can remove an impeller using two screwdrivers and needle-nose or Channellock pliers. But impeller manufacturers recommend using a specially-designed impeller puller to remove it. An impeller puller is a worthwhile investment. You will avoid unnecessary damage to the pump body or other pump components with a quality impeller puller. Most impellers are a sliding fit on the pump shaft, but a few feature locking screws or retaining rings you will need to remove as well. Very important – before you remove the impeller from the pump completely, note the direction the vanes bend inside the pump housing. Then turn the new impeller so it pushes the vanes down in the same direction.

You must make sure your new impeller represents an exact replacement for the pump model on your engine. Hundreds of impellers exist on the market, all with different diameters, thicknesses, shaft sizes and types, materials and so on. Some variations are slight, but they can make a huge difference. For example, many late model Volvo Penta gas engines use one of two different almost identical impellers. They differ only 1/32-inch in thickness, but this slight difference can cause your engine to overheat! To verify that you buy the correct impeller for your application, get the model and the serial number of your engine. You can then call an experienced salesperson at Marine Parts Source for help in identifying your correct impeller.

Replacing vs. Rebuilding Your Impeller Pump

If you do replace the impeller yourself, make sure to replace the impeller gasket and/or o-rings too! These usually come with a new impeller. Once again, it should be an exact replacement of the gaskets or o-rings you removed. Depending on the condition of the components inside the water pump, you may need to either replace the pump or rebuild it. Replacing the entire pump can be a much quicker repair, but a new raw water pump can cost several hundreds of dollars. Rebuilding the pump might be more cost effective, but it can take longer and requires more technical skill (and patience!). Also, if you’re working on an older pump, many internal components may no longer be available. A pump replacement may be your only option.

The internal components will vary from pump to pump. Inside your pump, there may be a cam, a wear plate, shaft seals (there are several varieties), bearings, a shaft, other retaining rings, gaskets, and seals. Removing some of these components can be tricky. If you do it wrong, you can cause significant damage to the pump and other component parts. We recommend you seek out an experienced marine mechanic for a major pump rebuild.

Regular maintenance of your boat is the key to enjoying to the fullest your time on the water. Taking the time to inspect your cooling system – and especially your engine cooling pump – will save you a lot of headache and money. It will also help to maximize the time you spend having fun on the water. Visit MarinePartsSource.com today!