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Don't neglect your outboard's water pump

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.

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The water pump is a very important 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 the outboard is running hot, it could be time to service the water pump. In most cases, this basically consists of replacing the water pump impeller. It’s not a complicated job, and the average do-it-yourselfer probably has the necessary tools. However, don’t try to do it 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 was a 1996 115-hp Mercury. Outboards are very similar, and although the pictures might not look exactly like what you have, the sequence will be the same. I took the pictures as Adam Conte at Portside Marine in Danvers, Mass., serviced the pump. Working thoroughly and methodically, he completed the project in less than an hour, so a competent owner should be able to do the job from start to finish in less than two hours.

A step-by-step do-it-yourself guide


The first step is to drain the oil from the gearbox. Unscrew the drain plug with a large screwdriver, and the oil will run out. Be sure to place a suitable pan under the engine to catch the oil. Unscrew the upper oil-level plug, too, which allows air into the gearbox and ensures that all of the oil is removed. There are small washers under each screw head that often get stuck in the threads. If they do not 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.

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

Once a crack opens, insert a broad screwdriver and carefully pry it apart, being careful not to damage the castings of the mating surfaces. Lift the unit clear and place it on a suitable bench or a jig designed for holding it.

With the unit clear of the top half of the outboard leg, you can get to work on the pump. The first thing to do is slide off the seal, which sits atop the pump housing. Unscrew the bolts that hold the pump housing in place. We needed an impact wrench because this pump had not 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.

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

All in all, the 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 after they’ve been in the pump for several months. Before reinstalling the pump, clean the mating surfaces to ensure there will be no leaks. A 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 are finished. Everything should be clean and bright.

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 impeller vanes, and the pump won’t work as efficiently as it should. If there is any doubt about the condition of the housing, it should be replaced. After cleaning everything, reassembly can start. In addition to the impeller, all of the required parts for routine service — gaskets, O-rings and seals — are included in the water pump kit.

Smear on a little gasket cement. Conte, at Portside Marine, swears by Permatex Form-A-Gasket, but any other brand should be fine.  

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

Install the new bottom plate. We used a little more gasket sealant before dropping this on.

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

Slide down the new impeller, making sure that the keyway in the hub lines up with the previously fitted key (visible beneath the impeller here).

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. Do not use oil or silicone, which can attack the composition of the impeller and lead to premature failure. Slide the housing down and ease it over the vanes as you twist the shaft clockwise with the other hand. This bends the blades and allows the body of the pump to sit fully down onto the base plate gasket.

Reinstall the bolts and tighten them until they are just snug. Slide the new seal down over the shaft until it just rests against the pump housing.

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, then smear a little engine spline coupling grease on the top of the drive shaft.




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 is not where it should be.


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. Reinstall both screw plugs 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.

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 are just snug. As long as you don’t swing on the wrench, it is difficult to overtighten them.

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.