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5ltr V8 strange engine issues - help-gctid382653

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  • 5ltr V8 strange engine issues - help-gctid382653

    Hi All,

    I have a strage issue on my Bayliner Arriva which has been re-engined to a Volvo Penta AQ211A.

    The engine will runs well on the slip, the auto choke works and she idle's well at 900rpm, rising to temp and holding perfectly.

    Everything works perfectly until after running at around 2000rpm+ for more than 20 mins or so. Upon arrival at my destination I will knock back the throttle to idle and the engine drops below its idle point and eventually dies as if the idle point has magically changed.

    If i try to re-start it, it appears as if the idle point has moved to 450rpm so it barely catches before dying again.

    This has happened on both my last x2 trips out recently.

    I have recently had a full service including plugs, cap rotor, leads, filters, tank flush and fresh fuel and a tune up etc etc.

    Has anyone ever experienced anything like this before? Any thoughts would be appreciated.


  • #2
    one or more cylinders is low on compression maybe. Perform a compression test warm motor all the plugs out and the throttle fully down.

    In gear on a warm motor on the water the idle should be in my opinion between 600-700 rpm the lower the better.
    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


    • #3
      Basics- As you already completed the maintenance items, I'd look at the carb. Sounds like there's an air leak somewhere- perhaps around the base gasket

      Cold engine- all clearances are tight. As the engine warms, and metal expands, the mixture changes (gets leaner) and your running problems begin....


      • #4
        The AQ211A was fitted with a standard ignition system and a very simple 2 brl carburetor.

        Since it runs well for 20 minutes or so, perhaps the ignition coil is beginning to fail once it heats up.

        I read your info re; the tune up, but no mention of ignition coil replacement.

        IMO, of all the parts that we can replace without proper diagnosis, something such as replacing an Igntion coil never hurts.

        They do fail.

        Rick E. (aka RicardoMarine) Gresham, Oregon
        2850 Bounty Sedan Flybridge model 31' LOA
        Twin 280 HP 5.7's w/ Closed Cooling systems
        Volvo Penta Duo Prop Drives
        Kohler 4 CZ Gen Set

        Please, no PMs. Ask your questions on the forum.
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        • #5
          Hi All,

          Right I think I might know what the problem is, whilst I have an AQ211A block and internals, the boat has retained most of the original Mercruiser 5ltr parts, including risers, manifold, outdrive and thunderbolt ignition etc. The mechanic that serviced it was somewhat surprised but ended up stating that it all appears to work well (he had fun getting the right parts for the job with two engines to have to source parts for). The only difference being that the AQ211A has a higher HP output than the Merc (and the leg has been re-geared accordingly).

          Long story short, it appears that whoever completed the mish-mash conversion fitted an ignition coil that states "requires an external ballast resistor". This is not a standard thunderbolt ignition coil. I assume the thunderbolt ignition system doesn't have an external resistor as standard and I can't find one in the wiring which confirms my suspicions.

          On this basis, I am assuming that the (incorrect) coil is overheating after 20 mins and when the throttle is knocked back, it isn't providing enough power to the plugs to run the engine.

          Before I worked out that the boat had thunderbolt ignition, I purchased a Lucas Gold High Power Sports 3Ohm ignition coil with internal resistor.

          The blurb on this coil states that it will work with standard points systems and electronic ignition conversion kits etc Does that mean that I can use it with the thunderbolt ignition system?

          Basically I want to know that I don't have to splash out another lump of cash having spent out on a coil already.

          I have also noted that the ignition control unit is mounted directly to a riser and is missing its heat resistant "pads". I will rectify that at the same time as replacing the coil.

          If anyone can help that would be amazing.



          • #6
            The thunderbolt ign has been common problem for many on BOC.

            Can be the coil or the module, and you can replace only to have it crap out again and they are expensive.

            For long term happiness I'd replace the whole dizzy with a different unit. I think you can get nice billet aftermarket units new for less than $300, forgot who makes them.


            • #7
              You MUST use the proper mercruiser coil for a thunderbolt ignition. The module is designed around that coil. I even tried a NASCAR grade coil on my previous boat and it did not work right.

              When the coils go bad, they do exibit their first problems when hot.

              Note: the engine you have should idle at 650 in gear.
              Captharv 2001 2452
              "When the draft of your boat exceeds the depth of water, you are aground"