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Mercruiser 350MPI idle dies after run?

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    Mercruiser 350MPI idle dies after run?

    Hello all!

    We have here in Finland on lake Hoytiainen 2003 2858 equipped with Mercruiser MPI 350MAG Smartcraft engine and Bravo3 drive.

    The problem is dying idle after normal travel run with warm motor. Also the idle is a bit too high, 620..630 rpm.

    Engine has frozen during transportation from Germany and a brand new base motor has been changed in 2012 before we got it .
    The same fault has been already with the old frozen block than now

    We are trying to find the reason and have changed some parts as IAC valve, throttle position sensor, distributor, plugs etc?
    Fuel pressure is also OK all time.

    Does anybody have some tips for this?

    br Juha

    JT FIN

    Have you tried opening the fuel cap to see if the breather is plugged?
    2014 Double Eagle 176EXL. But we are not done with Bayliner yet…this is just to get us to and from our cottage in the Gulf Islands.


      does it die as the rpm ramps down from a run, or does it just randomly die as its idling?

      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 for the comments.

        No, I have not checked the breathing tube etc. itself, only I have measured fuel pressure through the measurement fitting for that before nozzles and its OK also with full

        Sorry I did not explain the fault carefully.
        For example when I have driven some tens of minutes with 20...25 Knots speed and slowing down the speed to zero such rapidly, the engine does not start good idling
        but dies in some seconds.

        What should it be?

        I'm going to check at first the leakages in intake manifold because of the motor has frozen.

        Then I check with Rinda analyzer some motor and sensor values...even I do not know yet where to get the right ones, does anybody know?

        Btw it is very nice that we can get contact with real users here in BOC forums!
        Here in Finland there are only a few persons handling these motors compared to US.

        br JT FIN


          Hello again, more fault explanations ....

          If you slow down very slowly by degrees from 20...25 Kn to zero it is possible that the idle does not die!
          Also after that it dies easily for example you have to use trims or bow thruster...some ectrical defiency?

          The the bow thruster power should be taken in any case before next summer from own battery.
          I have heard that dying idle can happen if power is taken from the drive high amperages is loading the voltage too low for EMC.

          br JT FIN


            You are stating it will die when a load is applied to the DC electrical system?
            Im no expert on the mpi engine but I do believe if DC voltage is lower than “normal” it can cause some strange issues. Have you tested the voltage being produced by the alternator at cruising and idle rpm? How old are the batteries?
            Others with more knowledge of the mpi can confirm or deny my theory.
            Edmonds, WA
            "THE FIX" '93 2556
            Carbureted 383 Vortec-Bravo II
            The Rebuild Of My 2556
            My Misc. Projects


              Yes, it is a good point to measure voltages when falling down, thanks. Maybe I have some Rinda saved voltage data, Rinda anyzer can make graphs as You maybe know.

              Now in wintertime it is difficult to demonstrate the dying idle, but we can run the engine inside and use Rinda to analyze sensors etc. (even we have -6C temp and 20cm snow...).

              Note that the idle dies very often after run without doing anything but it's sure after using power trim etc.

              Batteries are such new Vetus 105A x 2. The possibility of a electrical fault is existing.

              We are thinking that it can be the IAC valve, that is the old one in use?

              br JT FIN


                Looool, it's so tricky, go to the above the throttle underneath the Idl, you will find tiny small filter to suck the air through Idle to the throttle, replace the filter with new original one, believe or not this is has major effect to your engine power and Idl operation and life


                  Hello Ameen and others and very big thanks for the good comments!

                  After the test arrangements cooling, exhaust, Rinda set up etc we have driven the engine with the old original IAC valve.
                  The fault came also without any load after engine at normal running temp . It nearly stopped and got after that higher about 800rpms revs.

                  Last summer we bought "cheap" IAC valve through MARINEPART, it was also not working, runs al time high rpm. It was ordered by using engine numbers etc but looked very different
                  compared with the original one, there was seen spring etc. extra parts inside...was totally wrog model. So we used the old original one with faults...

                  Now we have bought original IAC through the local Mercruiser dealer and the part was like the original old one. After the new test drives last weekend we have to say:


                  So I'm recommending to use the original IAC valve if You have idle problems!

                  Other faulty MARINEPARTS parts was also the rear engine foots, was totally too soft compared with original we have now.
                  The engine went into wrong "position" after taking torque out and got vibrations and careful!

                  Nice boating summer for all and thanks again,

                  Br JT FIN


                  • C-ya
                    C-ya commented
                    Editing a comment
                    Glad to read that you got it sorted! Thanks for the follow-up report.

                    Happy boating!

                  Hello Bayliner people and others too...

                  This is very fine channel to get helpfull information when problems are existing, thanks for all !

                  We have only one small fault going on nearly every time when we are getting 2858 on the slide.
                  The motor is 350 MPI Magnum 300 hp, year 2003.

                  Near 2800 rpm there happens something like misfiring, not sure that's it, but you can feel it like twitching ?
                  Also can happen when dropping off the slide, near same rpms.

                  Changed new parts: plugs, plug wirings, whole distributor, IAC valve, fuel filters, whole spool with eletronics.

                  What it could be? I can not find the fault even using Rinda testing software...but not done yet the test for nozzles.

                  What happens if some nozzle is leaking or jammed etc?

                  Here in Finland nice summer starting and today 2858 on lake Hoytiainen!

                  br JT FIN


                    No fault codes?
                    Can you hold the rpm right at where the problem happens for a few seconds or more to see if the ecm will set a fault code?
                    1992 2452
                    5.0 alpha1 gen2


                      Lineman1, yes I can hold that rpm , good idea!
                      We can keep PC with Rinda connected during test drive.

                      There was some codes but they was pointing transducers which this boat does have not at all.

                      br JT FIN


                        I would not hold it long if its missing and spiting. The ecm should code in just one or two rpms. Sometimes they take there time about it.
                        How do you like your Rinda software? Im thinking about purchasing it. Can you data log?
                        Hook it up run the engine and store some run time to look at later?
                        1992 2452
                        5.0 alpha1 gen2


                          Just a thought have you changed the filter that Ameen talked about in post#8? My understanding is that little piece of foam can cause all kind of strange things to happen
                          1992 2452
                          5.0 alpha1 gen2


                            Ok,not a lot spiting...

                            The Rinda Pc version takes some time to solve how it works, at least for me, but you can "record" up to 4 sensor values and look them direcly and afterwards.
                            For example rpm, timing, voltage, MAP (manifold pressure), all in same time window. And graphs are seen or data logging.

                            Unfortunately I am not very qualified yet with Rinda but going to test nozzles with that soon.

                            Yes I have replaced the foam when we were hunting the faulty IAC valve.
                            This tiny foam kills a lot of noise and effects at least to idle.

                            Btw is Yours 5.0 also MPI?

                            br JT FIN