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Exhaust flappers

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    Exhaust flappers

    So getting my boat ready for next season, I only bought this boat mid summer so renewing risers and manifolds, all bellows and whole water pump so I know when they will need replacing. The engine is a 2000 mercruiser 5.7 V8, the same basic engine as my last boat but that was a Volvo penta . So to the point, my VP didn't have flappers fitted as standard so why has merc fitted them, do I really need them given the common failure rate of this part.
    I am fully aware of the function of them but in reality should any backwash travel through the outlets, fill the y part and push up to the manifolds against the exhaust pressure.
    Your thoughts please for and against omitting this part
    And wishing you all a happy new year

    The flappers are for when the engine is shut off or at a low idle... some people leave them out, but it depends on where you boat and the sea conditions... in a strong following sea or when drifting, at anchor or docked with waves coming at the stern may require the flappers... and how low the engine is mounted in relation to the waterline all plays a part in the need for the flappers....
    the high failure rate is due to lack of maintenance, as when the flappers get replaced when the exhaust elbows/manifolds get replaced, there are no premature failures, except in the event of an overheat issue....
    they arent restrictive as some.people believe because with the water being induced into the exhaust gasses, it cools and contracts them so they pass thru without unnecessary back pressure...

    I feel its good to have the flappers in place just in case, because even a tablespoon of water in at the wrong time and a piston rod will be bent.when the engine gets cranked...

    your boat, your choice....
    Last edited by Centerline2; 01-03-2019, 10:39 AM.

    Salem, OR

    1989 Bayliner 2556 Convertible
    5.7 OMC Cobra - 15.5x11 prop
    N2K equipped throughout..
    2014 Ram 3500 crew cab, 6.7 Cummins
    2007 M-3705 SLC weekend warrior, 5th wheel
    '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
    Heavy Equipment Repair and Specialty Welding


      Thanks Centreline, I can see the thought about when engine is not running


      • Centerline2
        Centerline2 commented
        Editing a comment
        Some of the earlier VP drives had a flapper at the exhaust exit point near the prop.. im not sure how the newer ones prevent the water backing up, but they must have a provision for it somewhere...

      Exhaust shutters likely fail more from over heat issues than just wearing out. I’d definitely run them.
      Mercruisers new exhaust shutter design is better than the old IMO. However the plastic housing can become slightly deformed causing a bit of a squeeze at the pivot area (sticky flapper issue) I’ve found a ittle creative razor knife work at the pivot area will ensure they stay working as they should.
      Edmonds, WA
      "THE FIX"
      '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
      (.030 over-Vortec top end-part closed cooled)
      The rebuild of my 2556
      Misc. projects thread


        Thanks builderdude,


          Yes you need flappers... what happens if your engine dies while on plane?
          you'll appreciate them when coming off plane.
          also Merc uses more cam shaft overlay than the Volvo.
          Joon, Kathy, Jaden & Tristan
          Uniflite 42 AC, DD 671N
          93 3058 sold
          92 2855 (day boat)
          91 Fourwinns 205 (lake boat)
          Longbranch WA
          Life is Good


            Thank you ruffryder and all of you for your input, flappers it is then


              The "Back Flow Prevention Flappers" are there for good reason! I would NOT omit them!

              There are there not only for while the engine is NOT operating, but also for when the engine is operating during a wave surge from AFT.

              Some of the earlier VP drives had a flapper at the exhaust exit point near the prop.. im not sure how the newer ones prevent the water backing up, but they must have a provision for it somewhere.....
              The AQ series has pretty much always incorporated some form of Back Flow Prevention, either within the exhaust bellows or at the lower gear unit exhaust outlet area.
              Rick E. Gresham, Oregon
              2850 Bounty Sedan Flybridge model
              Twin 280 HP 5.7's w/ Closed Cooling
              Volvo Penta DuoProp Drives
              Kohler 4 CZ Gen Set


                Saw this thread and took a quick picture of my outdrive (VP270) My flapper looks good (never thought I'd say that!) but the two rubber outlets are shot. Should I worry??
                Bruce V. Lake Isabella, Ca
                1986 2850 Contessa
                AQ 260 VP 270
                2005 Chevy 2500 6.6 Diesel


                  Volvo-Penta service bulletin 25-2 5 01

                  Manufacturer discovered that exhaust flapper valve (parts

                  #3653854-1, 3852753-7 & 3855517-3) in exhaust pipe is not effective

                  in preventing water ingestion. The valve has a short life span & has been

                  known to disconnect from its pivot rod & clog exhaust. Manufacturer no

                  longer installs on affected engines. Mechanics are instructed to remove

                  flapper valve & not install new ones, attachment holes in exhaust should

                  be filled w/ silicone or RTV sealer. Volvo also recommends >14" rise

                  in exhaust.

                  Reply to original poster.

                  Should start a new thread with questions about a different boat.
                  Be good, be happy, for tomorrow is promised to no man !

                  1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

                  '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

                  Manalapan N.J