The Importance of Changing a Water Pump on an Outboard
Outboard engines, inboard gas engines, and diesel engines all get hot and almost all of them require raw water to help cool the engine. Even inboard engines with cooling internal cooling systems require raw water to help cool said coolant. Raw water-cooling systems rely on a water pump to draw water from the body of water the boat is sitting in. This raw water pump is usually powered by an impeller which is powered by the drive shaft of the engine.
Why the Impeller is Important
The impeller is critical to keeping an engine cool. The impeller is a rubber star shaped device that pulls water in and helps circulate water throughout the rest of the cooling system. I once knew a Marine Technician that told me the best way to test if an impeller is still good is to take it out of its housing and throw it into the water to see if it still floats. Impellers never float. He was simply explaining that if you go through the hassle of taking apart the water pump housing you should just replace the impeller and other parts of the water pump while you are in there.
When to Change your Impeller
Once the impeller reaches a certain age or level of use it will start to deteriorate and break apart. Once the impeller starts to fall apart it stops being as effective. Bits of the impeller will start to clog the cooling system and the engine will overheat. This can cause catastrophic problems with the engine, like damage to the pistons or the powerhead itself. If your impeller has broken apart, you need to get every piece out of the cooling system. Keep the pieces and try and piece the impeller back together to verify that you have all of it. You can flush the cooling system to get all of it out with a hose. Never run the engine out of the water. Impellers can break apart in less than 20 seconds without water around it, then you have a guaranteed project of getting the pieces out of the system.
Making Sure Your Cooling Passages are Clear
It is critically important to get the pieces of an impeller out of the cooling system of a water-cooled engine. If you are lucky, the pieces of the impeller are just caught up in the thermostat and thermostat housing. This is easily checked by opening the housing and checking the contents. The thermostat gasket will likely need to be replaced if you pen the housing, and you should always consult your service manual. Hopefully you can simply piece the impeller back together to check for the missing pieces. If any pieces of the impeller are still in the cooling system you will likely still have a recurring overheat condition. Pieces of the impeller can also block the tell tale on the engine. You can use an IR thermometer to track through the cooling system to try and identify where the blockage is happening.
Prevent Rather Than Repair
The best way to prevent having to search for pieces of a damaged impeller is to maintain regular service intervals on your engine. Make sure you change the impeller per the manufacturer specifications, which are usually every one or two years or every 200 hours of use. Check the service or owner’s manual for the specifications. It has already been mentions, but make sure the engine raw water intake is under water. Rubber impellers will fall apart without water around them. Make sure your engine cooling system is safe by operating the engine in water and use specified flushing muffs to make sure the impeller is getting sufficient mater while flushing the engine after use.