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Outboard Water-Pump Impeller Service


This is a fairly simple job, and the average do-it-yourselfer most likely has the necessary tools. Here’s how it’s done.

Ask around at the marina, and chances are that most boat owners have no idea when the water pump on their outboard was last serviced. Some people don’t even know that their engines have pumps. The water pump is a critical part of any outboard. A pump that is blocked or working improperly will cause the engine to overheat, which can lead to permanent damage. If you suspect that the outboard is running hot, it could be time to service the water pump. The manual should say how often this should be done, but often every other year is about right. In most cases, this basically consists of replacing the water-pump impeller. Don’t try to do this with the boat in the water. Have it hauled if it’s large, or do the work when it’s on the trailer in the driveway. That way, you won’t lose anything vital.

The engine we serviced for this story is a 115-hp, 1996 Mercury. Outboards are very similar and, although the pictures might not look exactly like what you have, the sequence should be the same. You should, however, check the service manual for your own outboard because some are different from the one featured here. I took the pictures as Adam Conte at Portside Marine in Danvers, Massachusetts, serviced the pump.

Technical Support

Degree Of Difficulty: ModerateTools and Materials:

  • Water pump kit
  • Gearbox oil
  • Flat-head screwdrivers
  • Oil pan
  • Ring wrench
  • soft mallet
  • Impact wrench
  • Razor blade
  • Emery paper
  • Clean rags
  • Gasket cement/sealant
  • Glycerin or dishwashing liquid
  • Engine spline coupling grease

Time: This project will take approximately 1 to 2 hours

Cost: Will vary based on the engine: Ours cost $50

1. The first step is to drain the oil from the gearbox. Unscrew the drain plug with a large screwdriver, and the oil will start to run out. Place a suitable pan under the engine to catch the old oil. Unscrew the upper oil-level plug, too, which allows air into the gearbox and ensures that all of the oil is evacuated.

2. There are small washers under each screw head that often get stuck in the threads. If they don’t come off with the screw, you may have to pick them out with a small screwdriver or other tool. Let the oil drain as you move on to the next step.

3. Loosen and remove the nuts that hold the lower unit in place. Almost every outboard has four nuts holding it. A socket may not fit, so use a ring wrench to give good purchase on the nuts, which probably will be difficult to loosen.

4. Theoretically, the lower unit should now be free; in practice, it invariably sticks and will need a few taps with a soft mallet. Do not hit the flange cavitation plates at the sides or they’re sure to break. A few taps on the aft end of the gearbox unit, as shown here, is acceptable. With some outboards, disconnecting the shift rod will require different steps. For example, you may need to unbolt the shift rod connection, then re-bolt it after the job.

5. Once a crack opens up, the battle is won, and you can carefully insert a broad screwdriver and carefully pry it apart, being very careful not to damage the castings of the mating surfaces. Lift the unit clear and place it on a suitable bench or jig designed for holding it. Portside Marine services outboards every day, so it had a proper jig. That’s ideal, but you may have to prop the unit upright in the corner of your garage. It works but is just not as convenient, and you’ll be working at floor level.

6. With the bottom clear of the top half of the lower unit, you can get to work on the pump. The first thing to do is slide off the seal, which sits atop the pump housing.

7. Unscrew the bolts that hold the pump housing in place. We needed an impact wrench because this pump hadn’t been serviced for some time, but a ring wrench will work in most cases. Avoid using an open-ended wrench. If you round over the bolt heads, you’ll have a bad day, for sure.

8. Separate the housing and slide it up the shaft. You can see in this picture that the bottom plate is coming off with it. We need to remove this plate, so if it remains stuck in place you might need to pry it up carefully.

9. This pump was in pretty good shape. All of the vanes on the impeller were intact. Note the old impeller on the right, compared to the new one and its straight vanes. The vanes develop a set (or bend) after they’ve been in the pump for several months.

10. Before reinstalling the pump, clean all of the mating surfaces to ensure that there will be no leaks. A sharp razor blade can be used to scrape off the larger bits of gasket and sealant; some fine emery paper will get rid of the remainder. Wipe with clean rags when you’re finished. Everything should be clean and bright.

11. Wipe out the interior of the pump housing, making sure there are no score marks or gouges. If there are, water might leak past the vanes of the impeller and the pump won’t work as efficiently as it should. If there is any doubt about the condition of the housing, replace it.

12. In addition to the impeller, all the required parts for routine service — gaskets, O-rings, seals — are included in the water-pump kit.

13. After cleaning everything, reassemble with the new parts.
 Smear on a little gasket cement. Conte, at Portside Marine, swears by Permatex Form-A-Gasket sealant, but other reputable brands should be fine.

14. Lower the gasket into position, making sure that all of the holes line up. The gasket is asymmetrical, so if something looks wrong you may have it upside down.

15. Install the new bottom plate that comes in the pump kit. We used a little more gasket sealant before dropping this on.

16. Install the smaller gasket, which seals the joint between the top and bottom sections of the pump housing. This gasket has a neoprene bead built in, so no cement should be used.

17. Install the new key, which sits in the flat on the shaft.

18. Slide down the new impeller, making sure that the keyway in the hub lines up with the previously fitted key.

19. A little glycerin or dishwashing liquid makes getting the pump cover on a bit easier and provides lubrication for the second or two before the water gets into the pump and lubricates the vanes. Don’t use oil or silicone, which can attack the composition of the impeller and lead to premature failure.

20. Slide the housing down and ease it over the vanes as you twist the shaft in a clockwise direction with the other hand. This bends the blades and allows the body of the pump to sit fully down onto the base plate gasket.

21. Reinstall the bolts and tighten until they’re just snug.

22. Slide the new seal down over the shaft until it just rests against the pump housing.

23. Push the setting tool, included in the kit, down on top of the seal, which does the double duty of spreading it out and ensuring that it is not compressed too much. With the seal set, remove the compression tool.

24. Smear a little engine spline coupling grease on the top of the drive shaft.

25. Put a little more grease on the gear-shift coupler, which should still be on the gear shift shaft inside the leg. This is a fairly loose push fit, so the coupler may have fallen onto the floor if it isn’t where it should be.

26. Refill the gearbox with the correct oil. Note that the oil is pumped in from the bottom until it comes out of the upper-level hole. Then both screw plugs can be replaced with a new washer under each. Portside Marine services a lot of engines, so it has a big tub of oil. The average DIYer is more likely to use the oil that comes in squeezable quart bottles, but the technique is the same.

27. Reinstall the lower unit. It helps to have someone rotate the engine by hand a little to get the splines to mesh. Replace the nuts and washers that hold the two parts together. There are torque settings for these, but Conte tightens them so they’re just snug. As long as you don’t swing on the wrench, it is difficult to overtighten these. The service manual may list torque wrench settings, however.

28. With everything back together, the job is complete. We ran the engine in a barrel to make sure all was well. You can use muffs on the water pickup, but the pressure of the hose tends to force the water into the engine. Running it in a barrel ensures that the suction from the pump is correct. There should be a healthy spout of water coming out of the engine housing. Record the date and engine hours so next time you’ll know exactly when the pump is due for servicing.

Mark Corke

Contributing Editor, BoatUS Magazine