Troubleshooting the Water Pump
So how can you tell if the outboard water pump has issues? If water should stop flowing from the telltale hole on your outboard, or if the stream becomes weak, carefully check the outlet tubing for obstructions. Mud daubers and other insects love to call these places home, especially during periods of extended storage.
Not all outboards will emit waterflow from the telltale hole at idle speed, even when operating normally. Once RPM increases a bit, however, you should see it. If you don’t, keep a close watch on your temperature gauge and listen for a warning horn. Additionally, Yamaha outboards have an RPM reduction mode (as do most brands), which will limit the engine RPM if an overheat condition is detected.
Time without use can lead to the impeller “taking a set,” or becoming permanently deformed, due to its off-center positioning inside the cup. This condition makes water flow much weaker.
Additionally, periods of non-use can cause the rubber to become more brittle, perhaps even breaking pieces off and sending them into the cooling system. For these reasons, it’s best to replace your water pump impeller or the entire water pump assembly when servicing these items. It’s also important to never rotate your outboard’s crankshaft or driveshaft in a counterclockwise direction.
Tip: The rubber impeller is located inside a stainless-steel cup, and it uses the water for lubrication. If this water is not present, the friction of the rubber on stainless steel will very rapidly overheat and destroy the rubber impeller. This is why it’s imperative NOT to operate, or even turn over, your outboard without there being a proper supply of water to the outboard beforehand.