Water Pump Impeller Replacement on Mercury Outboard

Mark Corke has just recently posted about an outboard water pump service. It is reproduced here on this blog, but I encourage you to go over to his blog site to read the whole story. Mark also has excellent blogs on other boating projects and topics of interest to boaters. Well worth a click to read it at the source.

Read it at Mark's site here.

After Mark gives an introduction to the project at his blog site, the project is outlined below.

The engine that we serviced was a 1996 115 hp Mercury hung on the back of a aluminum Starcraft of a similar vintage and the pictures refer to that but all outboards are very similar and although the pictures may 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, MA serviced the pump. Working thoroughly and methodically he did the whole project in less than an hour so a competent owner should be able to do the job from start to finish in under two hours.

 Here's how to do it.

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. Make sure that you place a suitable pan under the motor to catch all the old oil. Unscrew the upper oil level plug too which allows air into the gearbox and ensures that all the oil is evacuated. There are small washers under each screw head which 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 onto the next step.

Loosen and remove the nuts that hold the lower unit in place, almost all outboards have four nuts holding this in place. A socket will not fit so use a ring wrench to give good purchase on the nuts which will almost certainly be stiff to undo.

 The lower unit should now theoretically be free but in practice it almost invariably sticks and will need a few taps with a soft mallet. Do not hit the flange cavitation plates at the sides or they are sure to break, a few taps on the after end of the gearbox unit as shown here are acceptable however.

 Once a crack opens up the battle is won and you can 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. They service outboards everyday at Portside marine so had a proper jig on hand which is ideal but you may have to prop it upright in the corner of the garage, it works but is just not as convenient and you will be working at floor level.

With the unit clear of the top half of the outboard leg we can get to work on the pump proper. The first thing to do is to 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 as 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 will 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. We need to remove this plate also so if it stays stuck in place you may need to pry it up very carefully.

All in all the pump was in pretty good shape, all the vanes on the impeller are intact. Note the old impeller on the right next to the new one on the left. The vanes should be straight, they develop a set to them after they have been in the pump for several months.

 Before reinstalling the pump clean up all 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 old gasket and sealant, then some fine emery paper will get rid of the remainder. Wipe down with some clean rags when you are finished. Everything should be clean and bright.

Clean out the interior of the pump housing checking to make sure that there are no score marks or gouges, if there are water may leak past the vanes of the impeller and the pump will not work as efficiently as it should. If there is any doubt as to the condition of the housing then it should be replaced.

In addition to the impeller all the parts that are required for a routine service; gaskets, O rings and seals are included in the kit.

After cleaning up everything reassembly can start. Smear on a little gasket cement. Adam swears by Permatex Form a Gasket sealant liquid but any other proprietary brand should be fine.

Lower the gasket into position making sure all the holes line up. The gasket is asymmetrical so if something looks wrong you may have it upside down.  Next install the new bottom plate (shown) that comes in the pump kit, we used a little more gasket cement 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 is required or should be used.

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

Then slide down the new impeller making sure that the key-way in the hub lines up with the key previously fitted.

A little glycerin or dish washing liquid makes getting the pump cover on that little bit easier and provides some lubrication for the second or two until the water gets into the pump and lubricates the vanes. Do no 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 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.

Reinstall the bolts and tighten them till they are just snug.

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

A setting tool is included in the kit and this is pushed down on top of the seal and does double duty of spreading it out and ensuring that it is not compressed too much. With the seal thus set the compression tool is then removed.

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

Then a little more on the gear shift coupler which should still be on the gear shift shaft inside the leg, it is a fairly loose push fit so it may have fallen off onto the floor if it is not where it ought to be.

Refill the gearbox with the correct oil. Note that contrary to what you might expect the oil is forced 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 service lots of engine so that have a big tub of oil but the average DIY boater is more like to use the oil that comes in squeezable quart bottles but the technique is exactly the same.

Reinstall the lower unit onto the leg, it helps a great deal to have a helper rotate the engine by hand a little to get the splines to mesh. Then replace the nuts and washers that hold the two parts together, there are torque settings for these but Adam does them up so they are just snug. As long as you don't swing on the wrench it is difficult to over tighten these. 

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 pick up but the pressure of the hose tends to force the water into the engine, running it in a barrel ensures that the suck from the pump is correct. Note that there should be a healthy spout of water coming out from the telltale in the engine housing.