How to replace water pump in 9.8hp Mercury outboard motor?

2 comments

1. Havoc Squad on June 2, 2010 at 7:49 am

   Its not too difficult IF you have previous mechanical experience and have a service manual available for this model and year.

   I haven’t done an impeller replacement yet, but I have replaced the seals on the lower unit and gearcase of a 1959 35HP Johnson. It is a similiar process such as William mentions, but his walkthrough is NO subsitute for having a service manual on hand during the repair.

   The two best pieces of advice I can give is:

   #1. Follow service manual instructions 100% in order and to the letter. This is not putting together a bike, it is expensive marine equipment that must be precisely installed.

   #2. Patience. You cannot force things to fit correctly. If it doesn’t align and install correctly, go back and start over again. You probably overlooked something or didn’t install it the method the service manual instructions told you to. I had to attempt my lower unit install twice to get it to install correctly.

   In short, this is far from impossible IF you have some reasonable mechanical experience and the service manual for this model and year. Attempting to do this without a service manual is foolish and could cost you even more expenses because of an improper installation.

2. william M on June 2, 2010 at 7:49 am

   Drain the oil, and decide if you need seals? Shift into reverse. Tilt the motor. Remove front stop nut above and near the front of the lower unit. Remove all the stop nut under the cavitation plate, near the water pickup screen.

   Lower should come off. Sometimes (in salt water conditions) the drive shaft is stuck in the crankshaft splines. The shift rod has splines with one missing tooth for alignment which can suffer the same fate.

   If the water tube followed the lower out, you must find it’s hole and seal and put it back in.(right side center)-use a mirror and sunshine to see it. (push in fit) There is a white thin plastic guide which liked to stay with the water tube. -retrieve it.

   Two stop nuts hold the water pump on the lower.
   Remove the top part of the housing -there is the impeller- If you must sand off the rust on the drive shaft to remove the impeller-now is the time. Don’t hurt the splines. Sometimes the rubber impeller will seize on the shaft. Cut it off the shaft with a moto tool (don’t score the drive shaft).Find the drive plate/pin and cut the impeller there, it’s thinner, and you won’t hurt the drive shaft. Save the impeller drive plate.(like a key way but different).

   Now -did you have water in the oil?- If so get the entire water pump assembly. Otherwise you may get by with just the impeller. There are two diameter drive shafts-so you will need to take your lower unit with you to get the right pump parts.(serial numbers are nice, but these interchange so you might have a different one) Fit the impeller right there at the parts counter. Bring a rag and wipe down the counter in front of the parts guy, so you don’t piss him off.

   Using 2 big screwdrivers, pry the pump base off the lower. Clean the drive shaft and don’t loose the little shims. Grease the lower base of the pump so the "O" ring and seal will go on smoothly. Run your hand over the spline and see of there are burrs or sharp edges that might cut the seal as it slides down the drive shaft. Grease the shaft top to bottom. When your pump base is near the bottom wipe the grease off the shaft, as it will carry crap off the drive shaft and contaminate the bearing.

   Put the bottom wear plate on,and the drive plate for the impeller. Slide the impeller down the shaft and have it catch the drive.- spin it both directions to be sure it is engaged. Check the wear cup in the top of the water pump housing. It should be snug. Grease the impeller blades.

   Spin the drive shaft clockwise as you push the pump body down on the impeller. Look down the water outlet and see the impeller blades move. Snug up the 2 stop nuts as you turn the drive shaft -so you don’t catch any corners of the impeller in the final assembly.

   Put grease in the water outlet for the "O" ring seal to the water tube. Put the thin plastic guide in the outlet hole. Grease the drive shaft splines and shift shaft splines and take a valium -then spend some time putting the lower back on. It was engineered to break your spirit here. So keep at it. Drive shaft splines, water tube and shift shaft must all fit together at the same time.

   Bolt it up, and fill from the bottom with oil and your are done.(unless you have to do it all over again)

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