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Thread: 25HP Mercury 4stroke

  1. #1
    Join Date
    Nov 2016

    Question 25HP Mercury 4stroke

    Just a question on the water pump on the 25HP Mercury 4stroke outboard.
    I have it on my 16' 2002 pontoon, I am looking at upgrading it to a 60HP but need to do some saving as its going to be $10,000'ish so it seems
    I have never changed it since I have owned the pontoon, 2years... it would be safe to say the org. owner didn't do much. The boat only runs in fresh water and is only used in the Ontario, Canada boating season.
    I have been told that the water pump is something that should be changed (Impeller) every so often as its something that when it does fail its to late. Last year I pick up a Impeller replacement kit and have yet to change it out. Anyone have any idea how long a job like this takes, and what I am in for trying to tackle this myself??
    I am handy and in the Trades with a lot of tools but I am no mechanic !!

  2. #2
    Join Date
    Feb 2014
    Boat at Lake of the Ozarks Missouri


    Hey Huggyd:

    Well if you are handy and have the tools; ratchet & sockets, flat blade putty knife, the proper grease,(not lower unit lube) - it's not too bad of a job.

    Get hold of a good repair manual and/or google a YOUtube video for your exact engine. Be sure your rebuild kit is for your exact engine, (by serial #/model#)
    and you are in business.

    Generally, unless it is time to change the lower unit lube, you won't need to drain & refill the gear case as it comes off intact when working on the water pump / impeller area.

    Remove the 6 or more nuts from under the cavitation plate just above the gear case, check the manual to see if your particular engine needs to be in reverse gear to remove the lower unit.
    Some engines require this in order to line up the internal shift fork with the sliding collar so the L/unit can come down. Also be aware of this when going back together.

    The drive shaft will come down with the L/unit and you will see the metal pump cover plate over the impeller housing. Your rebuild kit will most likely have the
    necessary gasket & o'rings for the reassembly along with the impeller rotor. Be aware of the directional rotation of the old impeller as the new one needs to
    be placed in the same direction when reassembling.

    When you have replaced the impeller, any o'rings, gasket(s) etc. reinstall the metal housing; then take the tube of Mercury grease you bought at a dealership,
    (or online) and place a glob of grease on the top of the drive shaft before inserting the completed unit back onto the motor.

    You will be trying to get the splines on the top of the shaft to line up in the bottom of the crankshaft up inside the engine housing. You may find it
    useful to rotate the propeller by hand to get this to line up the splines. Work slowly & carefully. It will go in.

    Start bolting the assembly back to the housing with the nuts & washers you removed earlier. Tighten all in a criss-cross pattern to the
    torque specs in the manual. Put the shifter back to the neutral position while hand rotating the prop. Obviously a two person job.

    Before you start the engine, (with ear muffs & the flowing water hose); try forward, neutral & reverse shifts by hand to ensure
    you got everything back together correctly. Again a two person job. Turn on the water medium force to the ear muffs, start the engine
    and be sure the pee hole is flowing water - after a short time for warm up & circulation.

    Good luck - you just saved $$$$ if all went well. Not rocket science but doable.
    If you needed to change the lower unit lube, now would be a good time before launching the boat.

    Hey Moser - did I miss anything ??
    Captain Tony
    2005 Summit by Triton - 220T Platinum 3Log
    2005 115HP Mercury Optimax - 228 Marine Master Trailer
    St. Louis & Lake of the Ozarks, MO.

  3. #3
    Join Date
    Jun 2004


    Just make sure you leave both the lower unit and shift lever in the same gear you start in. I prefer to use reverse as it makes it easier to turn the prop when realigning the lower unit when doing it alone. Forward will work fine too. Especially on that small of engine. Lot less weight to deal with when reinstalling. Grease the splines, but only put a small film on top of the driveshaft, if any. Also be aware that the rebuild kit may contain parts you will not use on your particular model, as that kit may fit different engines too. It's really a pretty quick job and one you should learn to do as part of general maintenance. Some throw away rubber (Nitrile) gloves are a good thing to wear when working on it. Lower unit fluid has a distinctive nasty odor to it and the more you can keep off your hands, the better. Harbor Freight is where I get mine. A factory Manual is your friend when it comes to jobs like this. And like tony said, YouTube is a great resource to refer to and a friend to help out would make it easier when reinstalling. But that lower unit is pretty light and manageable by yourself. When you remove and install the lower unit, it helps to have the engine tilted up some so it doesn't want to slide out on it's own and also makes it easier to push back on. Time wise...A few hours your first time at the most. The next time you do it will be cut in half..

    St. Louis/Lake of the Ozarks

  4. #4
    Join Date
    Jul 2006
    Raystown Country, PA


    Pretty concise Tony.

    I think this motor is designed to come apart in neutral.

    If I can leave the shifter linkage untouched while the lower is off, I do. I prefer to reach up and turn the powerhead a little to the right to get the splines realigned. It is usually just a matter of a couple degrees.

    Watch the water tube as you pull it down, noting the position in the pump housing and make sure it goes back together with the tube in the same spot, the tube goes into the pump housing not off to the side, it is not hard to miss. The water tube carries the water out of the pump up into the block.

    Make sure to look for the famous hidden bolt up under the trim tab, most engines have one there too. Note the position of the trim tab and remove the bolt holding it on, there is usually a bolt in there going up into the lower unit. Once all the bolts are out the lower should come off with relative ease. You should not have to beat or pry this apart, a couple good wiggles and it should come down off.

    I put the impeller on the shaft, position the key and get that all where it belongs and slide the impeller down onto the key and pump plate, put a good dollop of grease on the impeller to make it slide against the housing, slide the housing down over it and turn the shaft to the right, clockwise as you push the housing down over the impeller. Everything should slide together nicely and the impeller is in position, key in place, ready to go. Take notice to the key orientation with the flats on the shaft and the keyslot in the impeller, it is possible to get some of them back on upside down. I take pictures as stuff comes apart so I get it back together right.

    Like Rick said, if you want to do some of this maintenace yourself the factory manual is the best $40 investment you can make. If you have time, get one before you tackle the impeller. It will tell you just how to do the job on your motor, what position to put the shifter (it varies on different models and doing it wrong can create a big headache) and a variety of things pertaining to your motor. I get them used on ebay, usually $20 or so.

    If it is working you can visit boatinfo.no for access to free digital Merc and other manuals. It is a Norwegian sight, sometimes it works sometimes it doesn't if it's working it has every Mercury manual.

    Take your time, a helper is a good thing, and enjoy.

    Go to boats.net and look at the parts blowup for your lower unit, you can get a good idea of how many fasteners and their location, and how it all comes apart and goes back together.
    2006 Forester 19 Fish (new deck and carpet, Pontoonstuff interior, 2019)
    1996 Mercury 50 ELPT4S
    1983 Sea Nymph FM171 Striper (complete rebuild from hull up, 2014)
    1985 Johnson 70 J70ELCO

    Raystown Country, PA

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