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Water Leak Coming Out From Behind Spark Plug! (Mercury 20 HP)

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  • Water Leak Coming Out From Behind Spark Plug! (Mercury 20 HP)



    I just purchased a 1970's Mercury 200 Series (20 Horsepower) outboard two stroke. I was doing routine maintenance on my newly acquired engine when I noticed that water is leaking out from underneath the upper spark plug when the engine is running. The water comes out faster with higher RPM, as would be expected. I've researched a bit and found that this can be a difficult fix due to the tendency for the cover bolts to break. Is it the waterjacket gasket? What is a cheap fix? WILL THE WATER RUIN MY ENGINE IF I RUN IT LIKE THIS? Thanks!

  • #2
    Re: Water Leak Coming Out From Behind Spark Plug! (Mercury 20 HP)

    Oh, and BTW, the engine runs great, starts first or second pull every time. No missfires. Tell tale pee hole has good water flow. I believe the engine has not been used for about 5 years before I got it but I am pretty sure the previous owner did proper maintenance and winterizing.


    • #3
      Re: Water Leak Coming Out From Behind Spark Plug! (Mercury 20 HP)

      just the waterjacket leaking. Motor has gotten hot at one time in the past.
      Replace the water pump if you haven't done that yet.
      If it doesn't leak too bad, just leave it alone, it won't hurt anything.
      If it leaks a lot, just pull the cover off and replace the gasket.
      The problem with the bolts breaking is that people think it's a head bolt and over tighen them. It's just a waterjacket cover not a head gasket.
      If it still leaks after replacing the gasket, the cover is warped, may need a new one.


      • #4
        Re: Water Leak Coming Out From Behind Spark Plug! (Mercury 20 HP)

        It shouldn't ruin the engine, the way you describe it its just a leak around the waterjacket seal. There is no way that water can get in the engine unless there is another problem. You should be able to remove the bolts, pop the cover with a screwdriver, and put on a new seal. Problem solved. The bolts shouldn't break, but I would spray some penetrating oil on the bolts just to be sure.

        OR, just let it leak. The worst it could do is ruin the electronics due to way to much humidity under the cowl but its unlikely since its an outboard and designed with water in mind.
        1978 Invader 17' Restoration in progress

        Admiral Ranger


        • #5
          Re: Water Leak Coming Out From Behind Spark Plug! (Mercury 20 HP)

          Thanks for the responses...I don't think I need to replace the water pump because there is very good flow through the pee hole. Plus the guy I got it from told me he had the lower unit rebuilt and this would include the impeller, correct? I'm guessing his impeller wore out, the engine got hot, so he had the LU done, and then this gasket began leaking shortly after. If this doesn't sound right let me know.

          I'm going to go get some high temp RTV sealant and seal the area around the spark plug. I'll let you all know how I make out.


          • #6
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            Re: Water Leak Coming Out From Behind Spark Plug! (Mercury 20 HP)

            I got some regular gasket sealant since I don't really need high-temp stuff for this application. It turns out the previous owner lied about the lower unit being rebuilt (figures). I drained the old oil, of which there was about 4 tablespoons of super milky, light brown oil. It looked like an oily frappacino or something. Anyway, I decided to remove the lower unit to inspect for damage considering the condition of the gear lube. Everything looked great, including the seals, so I reassembled everything and flushed with fresh gear lube. Currently waiting on the gasket sealent to cure. I will let it sit 'til tomorrow morning before starting it. If the spark plug still leaks water I will just take engine skirt off when I'm out fishing this weekend. This way any water will evaporate into the atmosphere rather than remain trapped around my critical electral components. Thanks again for the help!