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Bad Piston- Symptoms

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  • Bad Piston- Symptoms

    2002 5.7 350 Mag MPI Mercruiser- reported by marina #7 cylinder low compression (30psi), won't start, spark plugs broke off during removal. Motor has history of water ingestion/head replacement, initially due to incorrect boat builder installation (riser height). My uneducated guess is the same thing has occurred again.

    Can anybody give me ALL of the symptoms of a bad/burnt piston, so that I can go ahead and prepare myself for the bad news I expect to receive when the head get's pulled..??..thanks

    Also, can I expect the codes from the ecm to tell me anything..??

    By the way, the boat came in to the marina under it's own power. The only thing unusual about the delivery was on start-up the engine appeared starved for fuel, cycled the throttle a few times and it started up and ran smooth and cool over to the yard.


  • #2
    Re: Bad Piston- Symptoms

    If anything has low compression the very next step is to do a leakdown test to find out where the compression is going. Tell the marina (or kindly ask) if they did a leakdown test.
    just because you found it that way does not mean it is supposed to be that way.

    Comment



    • #3
      Re: Bad Piston- Symptoms

      Originally posted by captJason View Post
      If anything has low compression the very next step is to do a leakdown test to find out where the compression is going. Tell the marina (or kindly ask) if they did a leakdown test.

      CaptJason,

      Because it has a history, I've gotten Mercruiser involved...paid an authorized tech to travel to the boat....he verified compression bad on #7 cylinder, and reported that 1-he didn't have access to compressed air (????), and 2-he didn't have the right adapter for the ecm module, and 3-regardless of what the problem is, the head has to come off to figure it out, and 4-they want $1000 bucks to remove the head.....

      Now, do you have any idea what the symptoms are of a burnt piston..??..

      Anybody..??..

      Comment



      • #4
        Re: Bad Piston- Symptoms

        Well i don't know what to tell you then. Sounds like your looking for an answer that you want to hear.

        Originally posted by beneben
        1-he didn't have access to compressed air (????),
        I have a 8hp 80gal at my shop, but i travel with this...

        http://www.sears.com/shc/s/p_10153_12605_00915310000P

        I got it on sale for 90 bucks.... and it's perfectly capable of doing a leakdown... shame shame on your mechanic. Maybe you can pay him for a free hour of his time so he can go out and buy this.

        Originally posted by benben
        2-he didn't have the right adapter for the ecm module,
        who cares... the ecm doesn't know wether the cylinder has 30psi or 130psi of compression. Again... you need to know where the compression is going.

        Originally posted by benben
        3-regardless of what the problem is, the head has to come off to figure it out,
        no it doesn't. if you do a leakdown and you the gauge shows squat and you hear a rush of air coming out of the oil dipstick tube, why go through the trouble and expense of pulling a head when the whole motor needs to come out????

        Originally posted by benben
        4-they want $1000 bucks to remove the head.....
        that is a bit pricey just to pull a head.... but then again, the boats not in front of me.

        you need to find a new mechanic my friend. You could have a fried motor or you could have valves that are just a bit to tight... or anything in between. but the only way your going to know..... IS WITH A LEAKDOWN TEST!!!! sorry for the caps... but there's not much you can do until you do a leakdown... other than WAG'in and throwing money/parts at it.
        just because you found it that way does not mean it is supposed to be that way.

        Comment



        • #5
          Re: Bad Piston- Symptoms

          Originally posted by captJason View Post
          Well i don't know what to tell you then. Sounds like your looking for an answer that you want to hear.



          I have a 8hp 80gal at my shop, but i travel with this...

          http://www.sears.com/shc/s/p_10153_12605_00915310000P

          I got it on sale for 90 bucks.... and it's perfectly capable of doing a leakdown... shame shame on your mechanic. Maybe you can pay him for a free hour of his time so he can go out and buy this.



          who cares... the ecm doesn't know wether the cylinder has 30psi or 130psi of compression. Again... you need to know where the compression is going.



          no it doesn't. if you do a leakdown and you the gauge shows squat and you hear a rush of air coming out of the oil dipstick tube, why go through the trouble and expense of pulling a head when the whole motor needs to come out????



          that is a bit pricey just to pull a head.... but then again, the boats not in front of me.

          you need to find a new mechanic my friend. You could have a fried motor or you could have valves that are just a bit to tight... or anything in between. but the only way your going to know..... IS WITH A LEAKDOWN TEST!!!! sorry for the caps... but there's not much you can do until you do a leakdown... other than WAG'in and throwing money/parts at it.
          Capt,

          Like I said in the original post, I'm trying to prepare myself for the bad news I expect to hear. I'm not looking for someone to tell me "what I want to hear"...Except for that comment, I appreciate your response and agree that I need another mechanic....The reason this mechanic is involved is that he's an "authorized" Merc mechanic, and I suspect water ingestion back up the exhaust, which is what caused the original failure of both the port and starboard engines, and I suspect that the same thing caused the second failure of the starboard engine, and that's what leads me to suspect the same for the third failure (this one) of the starboard engine, and want to find the cause of the problem more so than what is broke, so that it won't break again.

          In speaking with a local auto mechanic, he's confirmed what you've said, that a leak-down test is the next thing to do, and that it is simple. He also said that the problem starting the engine is a separate problem that has nothing to do with the valve/piston/cylinder issue.

          He also said that the symptom of a holed out piston would be no different than a bad spark plug......You agree with that..??..

          Comment



          • #6
            Re: Bad Piston- Symptoms

            I just replaced a 2003 5.7 Mercruiser (carbed). No compression on #4 and #8. It had a tuliped intake valve in #4 and a tuliped intake valve and melted piston in #8. In that case, a leakdown would have indicated a valve problem on both cylinders, but would not have indicated the melted piston.

            I wouldn't bother with a leakdown, and I wouldn't pull the engine until I confirmed piston damage. In my opinion it make no sense not to pull the heads at this point, before you pull the engine, because the heads need to come off as part of the failure analysis anyway.

            As far as connecting a scan tool, it's true that it would not tell you anything about the compression, but I always review the fault history and operating history for possible clues as to the cause of the failure.

            Comment



            • #7
              Re: Bad Piston- Symptoms

              Originally posted by newport dave View Post
              I just replaced a 2003 5.7 Mercruiser (carbed). No compression on #4 and #8. It had a tuliped intake valve in #4 and a tuliped intake valve and melted piston in #8. In that case, a leakdown would have indicated a valve problem on both cylinders, but would not have indicated the melted piston.

              I wouldn't bother with a leakdown, and I wouldn't pull the engine until I confirmed piston damage. In my opinion it make no sense not to pull the heads at this point, before you pull the engine, because the heads need to come off as part of the failure analysis anyway.

              As far as connecting a scan tool, it's true that it would not tell you anything about the compression, but I always review the fault history and operating history for possible clues as to the cause of the failure.
              newportdave, thanks for the post....the most important thing at this point is trying to determine what has caused the failures....if all failures lead to a single cause, then repairing the damage would only buy some time so, I concur that scanning the ecm would make sense...thanks again...

              Comment



              • #8
                Re: Bad Piston- Symptoms

                Originally posted by Bubba1235
                Sometimes the old methods are the best. You need to perform a dry and a wet compression test. A dry test is what you have seen before. Hook up the guage, crank the engine, read the guage. Now with 30 PSI you can be assured something is leaking.

                A wet test is when you remove the spark plug and shoot a bit of heavy oil into the cylinder and repeat the test. If there is ring or cylinder damage the oil will seal it (temporarily) and you should get a much higher reading on the guage. If the readings are the same or almost the same you have head damage (valves), if the reading is a lot higher the damage is to the rings, cyl., piston.

                So no, you do NOT have to pull the head to deterime if the damage is in the heads or the cyl.
                Bubba1235, thanks for your reply as well....I understand the wet compression test and how it would indicate ring failure...but will that also hold true for a holed out piston..??..If so, things might not be as bad as feared, since the second tech performed compression test wet and dry, and saw no change in the readings.....

                Comment



                • #9
                  Re: Bad Piston- Symptoms

                  Originally posted by Bubba1235
                  So no, you do NOT have to pull the head to deterime if the damage is in the heads or the cyl.
                  True, but in either case the head still has to come off to fix it, no? I think newport dave's suggestion was to just cut to the chase...
                  Full Gallop
                  1998 Larson 176 Flyer
                  VP 4.3GSLK
                  VP PJX PowerJet

                  Comment



                  • #10
                    Re: Bad Piston- Symptoms

                    Originally posted by Bubba1235
                    Honestly, if you have a holed piston I would be suprised to see any compression (you said 30 PSI) from engine cranking. (starter RPMs) Unless it was a very small hole.

                    Either way, a wet test costs next to nothing and can tell you a lot about what is going on inside the engine.
                    Thanks, I'll get wet test done next, before I spend money on anything else...

                    Comment



                    • #11
                      Re: Bad Piston- Symptoms

                      Addressing the original question, a damaged piston may be detectable before head removal by using a borescope to inspect the inside of the combustion chamber. A good tech should have one. Also you may see very tiny bits of aluminum splattered on the spark plug electrode. Below is a pic of what that looks like.
                      Attached Files

                      Comment



                      • #12
                        Re: Bad Piston- Symptoms

                        I think if you had a holed piston you would not even see 30 psi. My guess is a tulipped valve. Doing a leakdown test will verify this, it will also tell you if you have a holed piston.

                        Have a gret day,

                        Rob.

                        Comment



                        • #13
                          Re: Bad Piston- Symptoms

                          now that you are all worked up over all the bad things it could be. have you pulled the valve cover and just checked to see if you have a valve sticking. take a hammer and tap the intake and exhaust vavle and see if they clap shut, or is one of them sticking and not closed all the way, a peice of carbon stuck on the valve seat can show on low pressor also, why must every one always think of tearing something down until your sure what it is, how would you feel if you tore it down and fouind all it was was a peice of carbon and you spent a grand for a two sec fix and 5 bucks or less.
                          sigpicif you dont know it, learn it, if you cant learn it, leave it alone!!!

                          Comment



                          • #14
                            Re: Bad Piston- Symptoms

                            Originally posted by buggiedad 67
                            have you pulled the valve cover and just checked to see if you have a valve sticking
                            my points exactly. Seriously... how long does it take to leakdown 1 cylinder??? maybe 5-10 minutes after you have the plug out.
                            just because you found it that way does not mean it is supposed to be that way.

                            Comment



                            • #15
                              Sign up today
                              Re: Bad Piston- Symptoms

                              Originally posted by buggiedad_67 View Post
                              now that you are all worked up over all the bad things it could be. have you pulled the valve cover and just checked to see if you have a valve sticking. take a hammer and tap the intake and exhaust vavle and see if they clap shut, or is one of them sticking and not closed all the way, a peice of carbon stuck on the valve seat can show on low pressor also, why must every one always think of tearing something down until your sure what it is, how would you feel if you tore it down and fouind all it was was a peice of carbon and you spent a grand for a two sec fix and 5 bucks or less.
                              buggiedad, yes, we have done that...there are no sticky valves......good point though.....thank you.....

                              Comment


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