How To Change Mercury Outboard Water Pump Impeller
On A Mercury Outboard Motor
75, 90, 115, 125 HP
In this article we will discuss how to change the water pump impeller on a Mercury 90HP outboard motor. Its the same water pump as a 75 horsepower, 115 and a 125. Its highly recommended to use a repair manual before attempting service to your outboard engine.
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The first step is we have to remove the lower unit (gear box). There are five 15 millimeter nuts (2 on both sides) and a fifth underneath.
NOTE: Leave one nut on a couple of threads so the gear box doesn't fall off.
When your ready to remove the gearbox, you may have to use a screwdriver to separate the gearbox. Make sure not to damage any sealing surfaces by prying to hard with the screwdriver.
Next take on the weight of the gearbox and remove the last nut. Now slide the gearbox away from the mid unit.
Next position the lower unit in a vice or equivalent to hold the unit firm. Once positioned upright, you will see the impeller housing located at the top. You will need to unbolt the housing to get access to the water pump impeller. There are four bolts securing the impeller housing.
Once you have removed the four bolts, lift the housing up over the drive shaft, the rubber seal may come with it. If you didn't purchase a complete impeller kit, be sure not to loose the keyway.
Once you have removed the impeller housing, take a look inside and you will see the impeller. If the engine hasn't been running in a while, the impeller may be slightly stuck, but should be removable by hand.
At this point, take a look at the blades for any signs of wear or burning caused by friction. Also, you can pull on the blades at an angle to see if its coming away from the bushing. If the impeller blades are heavily curved once out of the housing, it means it must be replaced. An impeller in this condition will not have the capacity to pump water at high speeds.
Underneath where the impeller housing sat will be the plate, it should still be on the lower unit. Pull the plate off and remove it from the unit. Use a gasket scraper to remove the old gasket material and make sure not to damage the sealing surface with the scraper. Then use a fine metal brush to clean the gasket sealing surface in preparation for the new gasket.
If your changing the water pump impeller, you likely purchased the whole kit and this is recommended. The kit should contain the impeller, the plate, a new keyway, top seal and gaskets. You'll notice the new impeller blades are nice and straight as compared to the old one.
When reinstalling the new gaskets and plate, you'll notice you have two very different gaskets. The full gasket goes between the plate and the lower unit while the other gasket sits on the other side of the plate (between the plate and the impeller hosing).
NOTE: You will notice that the plate has a circle directly in the middle of it. This is where the drive shaft goes. However, the plate itself has two very different sides. On one side you'll notice the edges around the hole are rounded while the other side of hole the edges will be sharp or cornered. The side with the rounded edges faces the top (towards the powerhead or impeller housing). The other side of the plate should face in the direction of the lower unit. Its important to get this right as having the plate in the wrong direction can damage the impeller.
Align the gaskets on the plate and reinstall it to the lower unit. You can use some gasket sealing compound on the gaskets.
Now take the impeller housing and apply a very small amount of grease to the outer circle. Applying the grease is simply for the initial start up so to prevent the impeller from burning the blades against the metal housing.
To install the new impeller into the housing, simple take the impeller and turn it counterclockwise as you push the impeller into the hosing. Position the impeller inside the housing so the keyway hole is facing to the side of the impeller housing for installation. If you bought a kit, the kit should have come with a new keyway. If you didnt, you must use the old one.
On the drive shaft (the long shaft sticking out of the lower unit), apply some waterproof marine grease where the impeller sits to prevent seizing the next time you service your impeller. Now reinsert the keyway into the plate located on the lower unit and reinstall the impeller housing. Before you join the housing to the lower unit, make sure to hold the keyway in place and align so it fits.
Take the grease again and apply it to the impeller housing bolt holes to prevent seizing. Now insert the four bolts and tighten them in stages (diagonal stages).
If you purchased the kit, you should have received a top seal that looks like a rubber O-ring. This top seal fits over the drive shaft and slides down all the way to meet the top of the impeller hosing. You can apply a small amount of grease to the inside of the top seal so it slides efficiently down the drive shaft to meet the top of the impeller housing.
To the back of the impeller housing you should see an indentation (where the exhaust comes down) on the lower unit. There may be some old silicone residue. Scrape off the old silicone and apply a bead of RTV or silicone to it. What this does is seals the exhaust gasses from coming up into the water pump.
To the front of the impeller housing you will see the gear shift. Lift the gear shift up and apply some grease to the spline, then reapply the gearshift. Apply some grease to the spline at the end of the driveshaft. Do not get grease on the very tip of the drive shaft, just to the splines.
Now your ready to reinstall the lower unit. Before you install the lower unit, apply some grease to the studs to prevent seizing.
NOTE: Looking into the mid unit you will see the splines for the gearshift. Apply some grease to the splines. When mating the lower unit to the mid unit, make sure those splines meet the gear shift correctly or you will have to remove the lower unit again.
When reinstalling the lower unit, have a nut handy so you can secure the unit to the mid unit. Now reinstall the lower unit making sure everything lines up. If the lower unit does not go all the way up into the mid unit, turn the flywheel until the lower unit slides up. Then mount all the nuts and tighten them in stages.
Lastly, test the motor and make sure its pumping water. Make sure the propeller is truing and the gearshift works.