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Mercruiser 350MPI idle dies after run?

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  • JT FIN
    replied
    Hello again Lineman1 & others.

    Today we have solved that 350 MAG MPI with ecm555 , 2003/2012 missfiring problem.

    I have recorded before whole ECM data using Rinda diagnostics devices.
    It was seen from curves that when the spark advance is between 26 and 32 degrees the problem starts, horrible sound and backfiring.
    Also vacuum and rpm varies all time.

    Now thinking that the distributor can not deliver the sparks to right cylinder, some kind of cross firing?

    We put the cylinder 1 against TDC line and noticed that the rotator was not pointing enough accurate for cyl 1, shown service manual 31 page 4B-13.
    So we adjusted one teeth forwards but it was also not on the adjustment area, is was on the other side of the allowed area than before?

    Maybe the distributor gear was assembled 180 degrees wrong (4B-11) or camshaft timing is wrong, which I dont believe.

    New Vortec motor was changed 2012, maybe this needs different kind of distributor, now it was the original Mercruiser type?


    In any case I decided to modifye the Distributor Hold Down Clamp and grind a "long hole" for bolt.

    No I got the distributor rotator adjusted to point cyl 1 and between the shown lines in Service Manual 31.

    With these MPI engines you can not turn the distributor, the clamp prevents that, fixed assemly (mine now not any more...).


    So when the ECM puts more spark advance the spark goes to closer pin, not the right one ---> missfiring.


    Lineman1, Rinda diagnostics was now very important to get this solved, especially the recorded curves.

    Get some hours to learn...at least from me. Do not hesitate to ask features You like to use.



    Thaks for all, have to go boating, +22 degrees outside (water only +18...)


    br JT FIN

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  • JT FIN
    replied
    Yes, also the 350 MPI is absolutely too small in our 2858, the base weight is near 4000kg = 8800 pounds.
    If you have six people on there are problems to get vessel on the slide.

    I think also for us 383 would be much better...the bigger the better even the gas costs here 1,60€/litre , near 2USD / litre ~ 7,5 USD/ US gal.
    On the other hand bigger engine does not always mean bigger consumption.

    JT FIN

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  • Lineman1
    replied
    No sir my 5.0 is long sold. If I ever complete the engine swap I will have a 383 mpi.

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  • JT FIN
    replied
    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

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  • Lineman1
    replied
    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

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  • Lineman1
    replied
    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?

    Leave a comment:


  • JT FIN
    replied
    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

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  • Lineman1
    replied
    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?

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  • JT FIN
    replied
    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

    Leave a comment:


  • C-ya
    commented on 's reply
    Glad to read that you got it sorted! Thanks for the follow-up report.

    Happy boating!

  • JT FIN
    replied
    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:

    NOW IT IS WORKING! IT WAS THE IAC VALVE!

    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 noise...be careful!

    Nice boating summer for all and thanks again,

    Br JT FIN







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  • Ameen
    replied
    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

    Leave a comment:


  • JT FIN
    replied
    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










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  • builderdude
    replied
    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.

    Leave a comment:


  • JT FIN
    replied
    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 battery...to high amperages is loading the voltage too low for EMC.

    br JT FIN

    Leave a comment:

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