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Zero compression in #8 cylinder-gctid393559

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    Zero compression in #8 cylinder-gctid393559

    First off got a 1987 bayliner 2150 capri bowrider 5.0 305 chevy omc cobra outdrive rochester q-jet 4bb. Last year the motor started running terrible so i changed plugs wires distributor cap rotor points and condensor. Then coil,rebuilt carb,had all new shift cables and control box installed. Took it out once this year and still running terrible even worse than last year. So i broke down and went to get a compression tester. These are my results.

    No.1 cyl. 152psi

    No.2 cyl. 147psi

    No.3 cyl. 150psi

    No.4 cyl. 145psi

    No.5 cyl. 150psi

    No.6 cyl. 147psi

    No.7 cyl. 152psi

    No.8 cyl. 000psi

    Yes no.8 has absolutley no compression. Motor runs like a top up to around 2000 rpms then falls on its face and when i try to press it harder it starts trying to die. Worst part is that when i pulled No.8 plug i knew there was real problems ahead because it was melted and mishapen ceramic broken. I had used stethescope to listen to motor and no abnormal noises from engine. So today i pulled valve cover praying with every turn of the wrench a valve would b stuck or a spring busted. But alas valve train seems to b working great. My question is if without pulling the head is there a way of figuring out if its piston ,rings,gasket,or even worse. Oil doesnt smell like fuel at all no creamy color to oil.Valve cover vent had very strong fuel smell not sure if thats normal or not. oil pressure is lower than last year barely in the green on the guage and doesnt rise with rpm. engine will run up to wot when on ears but not under load in water even in neutral. Kinda rambling a little just want to give all info i have . Help would b really appreciated.

    #2
    Any chance you had the compression tool screwed in wrong?

    Comment


      #3
      Unfortunately not did it 3 different times. same result each time no change didnt even try to hop up on the guage

      Comment


        #4
        Borescope
        ..........
        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

        Comment


          #5
          119-120 on the others, tried the old oil in the cylinder trick to see if it was the rings and it didnt improve at all, hopefully thats a sign its in the head or its a big hole in the piston

          Comment


            #6
            Google 'leakdown test'. The cylinder is pressurized on the compression stroke. You can hear where the air is going and determine the likely failure mode. If you hear it out the ex, then you have a broke ex valve. Out the carb, then you have a broke intake valve. Open the oil fill and if you hear it come out there, it's a piston, or heavy scratch in the wall, hissing out the gasket would be a cracked head or gasket.

            http://www.youtube.com/watch?v=WgrfT0LFMhc

            Comment


              #7
              I feel for the original poster. He asked is there any way other then pulling the head. Borescope.
              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

              Comment


                #8
                Remove the Stbd side valve cover, and take a look at the Exh/In rocker arms and push rods for #8 cylinder.

                .
                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

                Comment


                  #9
                  milenko192 wrote:
                  Worst part is that when i pulled No.8 plug i knew there was real problems ahead because it was melted and mishapen ceramic broken.
                  I'd start by stripping down to see how badly the piston is damaged. If you have no compression (zero) and the plug is mangled, something let loose and the compression is going down through the top of the piston.

                  Since you know the valves are intact... you're with near certainty toast. Seeing the piston will let you know if you're looking at a rebuild or new block though.
                  Custom CNC Design And Dash Panels

                  iBoatNW

                  1980 CHB Europa 42 Trawler- "Honey Badger"

                  Comment


                    #10
                    milenko192 wrote:
                    First off got a 1987 bayliner 2150 capri bowrider 5.0 305 chevy omc cobra outdrive rochester q-jet 4bb. Last year the motor started running terrible so i changed plugs wires distributor cap rotor points and condensor. Then coil,rebuilt carb,had all new shift cables and control box installed. Took it out once this year and still running terrible even worse than last year. So i broke down and went to get a compression tester. These are my results.

                    No.1 cyl. 152psi

                    No.2 cyl. 147psi

                    No.3 cyl. 150psi

                    No.4 cyl. 145psi

                    No.5 cyl. 150psi

                    No.6 cyl. 147psi

                    No.7 cyl. 152psi

                    No.8 cyl. 000psi

                    Yes no.8 has absolutley no compression. Motor runs like a top up to around 2000 rpms then falls on its face and when i try to press it harder it starts trying to die. Worst part is that when i pulled No.8 plug i knew there was real problems ahead because it was melted and mishapen ceramic broken. I had used stethescope to listen to motor and no abnormal noises from engine. So today i pulled valve cover praying with every turn of the wrench a valve would b stuck or a spring busted. But alas valve train seems to b working great. My question is if without pulling the head is there a way of figuring out if its piston ,rings,gasket,or even worse. Oil doesnt smell like fuel at all no creamy color to oil.Valve cover vent had very strong fuel smell not sure if thats normal or not. oil pressure is lower than last year barely in the green on the guage and doesnt rise with rpm. engine will run up to wot when on ears but not under load in water even in neutral. Kinda rambling a little just want to give all info i have . Help would b really appreciated.
                    My guess is that if the ceramic is melted, the piston may be damaged. Has anyone set or checked the ignition timing prior to the problem? Griff

                    Comment


                      #11
                      Detonation is a byatch.
                      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

                      Comment


                        #12
                        griff wrote:
                        My guess is that if the ceramic is melted...
                        I doubt the ceramic "melted"

                        It probably just pummeled itself when whatever came loose clobbered it about 3000 times a minute until you shut it down. The other things that got pummeled will be your piston, valves and combustion chamber.
                        Custom CNC Design And Dash Panels

                        iBoatNW

                        1980 CHB Europa 42 Trawler- "Honey Badger"

                        Comment


                          #13
                          Ok thanks for all the replies and some good info found a garage locally that will run a borescope for me. Im pretty certain what ill find. Nice big hole in the top of my piston still got my fingers crossed though. Next thing is that if the piston is trash but my block can be saved. What parts for a rebuild do I need to get for a marine supplier? I realize alot of parts are marine specific on externals just not too sure about internals. Guess ill just pull it and rebuild motor myself if it can be saved.

                          Comment


                            #14
                            i still think a reman long block, would be cheaper. you should be able to find one for $1300 to $1500

                            Comment


                              #15
                              Did you pull the valve cover and look at the #8 cylinder rockers arms?

                              milenko192 wrote:
                              ........... Next thing is that if the piston is trash but my block can be saved. What parts for a rebuild do I need to get for a marine supplier? I realize alot of parts are marine specific on externals just not too sure about internals. Guess ill just pull it and rebuild motor myself if it can be saved.
                              If this 1987 5.0L engine has been raw water cooled it's entire life (even while in Bellville, OH far from salt water), you are only a few hundred dollars away from clean automotive cores.

                              You'll have the option to go with another 5.0L or a 5.7L.

                              You'll also have the option to go with a much better piston selection than the dog gone full dished pistons. Nothing else needs to be altered, and it won't break the bank.

                              Food for thought!

                              .
                              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

                              Comment

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