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How do you do a leak-down test?

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  • How do you do a leak-down test?

    I’m looking for a link or a sticky on how to do a leak-down test on a late 1980s Force 125, and how to use that info to diagnose a compression problem .

  • #2
    You can google that. not sure if there any stickies.

    You will need:

    Leak Test gauge
    Compressor good to 80-100 psi.

    2 cycles are easy, since every revolution is TDC on compression stroke.

    The noise of the air leak will tell you if it is rings/cylinder or head gasket.
    Best regards, Ted . . . . Cape Cod, MA

    Current Boats: Formula 330 Sun Sport, O'Day Mariner Sail #3224, Sunfish
    Past Boats: Catalina 22 Sail #10531, Formula 242 Sun Sport
    Twin Mercruiser 7.4 LX MPI (0F802036, 039), Bravo 3's (0F806198, 199), Mercury 7.5 HP (1969), Johnson 4.5 HP (1980)

    My Boating Web Pages: http://www.tpenfield.com


    • #3
      There is a sticky at the top of this forum. It is the FAQ one. Not sure what leakdown you're talking about though for compression test or if it is even in that sticky. Good luck.


      • #4
        What is your compression? A leak down test is only done after every other test has been exhausted.


        • #5
          What exactly are you looking and what is the problem? A leak down test of what? Compression problem on what?


          • #6
            Late 1980s Force 125. It has been a while since I did the compression test, so I don’t remember the numbers, but it had low compression on one cylinder. I was hoping to find out if the problem is with the head gasket or the rings. Head gasket replacement I could take a stab at. Rings or another matter!


            • #7
              If you have low compression save yourself the hassle and pull the head. No need to do a leak down test because if it is the gasket, you will need to replace it. And if it is scored well you can decide from there.


              • #8
                There you go. As Flyingscott said open and inspect and you just have to take it from there. Take pics of the cylinder and piston on the low compression cylinder on your next post.


                • #9
                  That does make sense! Thanks.


                  • #10
                    When I was a newbie, my boss who knew EVERYTHING about outboards said
                    "don't waste time doing a leakdown test ,a compression test tells you all you need to know.
                    If the comp is low?? remove the head.
                    That ok? remove the port cover and inspect the rings.

                    Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
                    ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

                    In Scenic , Beautiful San Sebastian by the sea Florida

                    88 Bayliner 21CC 2001 Mercury 225 HP EFI



                    • #11
                      I got all 18 bolts out of the head. Only broke two! The head still won’t come off. I’ve smacked it a little with a dead blow hammer and a piece of wood. What’s the safe way to break it loose? Can I pry anywhere?

                      BTW, before I took out the bolts, I warmed it up and ran compression test. Cylinders 1-3 are just over 120. #4 is at 30 psi! And I had a little water come out cylinders three and four, And the number four Spark Plug was a little wet with some rust, so… Yeah there’s some gasket problems.


                      • #12
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                        Here’s a hillbilly question. Suppose I put the bolts back in finger tight, and the spark plugs back in, and try turning the engine over to let the pressure pop the head off. Too risky?