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Thread: New motor problems

  1. #1

    Default New motor problems

    Got my new 350 Mercruiser put in my Sea Sport, it replaced a 305 Volvo, these are both Chevy blocks. Every time for the first hour heading out it cruises along nicely then will miss or drop off about 400 rpms then pick back up like nothing every happened. Feels like hitting a log or something, just instantly drops off for a quick second, then resumes right along. Will do this about a dozen times in an hour and won't do it at all a day later coming back.

    THINGS THAT WERE ATTEMPTED THAT MADE NO DIFFERENCE;
    -Timing has been re-checked. (made no difference)
    -Gas tank emptied completely (yet never did this ever before, why now with a new engine???) (made no difference)
    -Water/fuel filters changed (made no difference)

    Next test is; Changed the coil and put in new batteries.

    Why would it do this in the first hour of a run but not after fishing all day???

  2. #2

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    plugs are cheap...could be an arcing wire as well. Points or electronic ignition. If points I would think the condensor is failing. Kinda wierd that it doesn`t persist past the first hour...???


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  3. #3

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    Sounds possibly like a bad throttle position sensor (potentiometer) if your engine has one. If you move the throttle to a different position does the problem persist? Vibration damage at a set throttle position can cause the sensor to begin acting up.

  4. #4
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    Recheck every electrical connection.

  5. #5

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    As 338 said check the electrical ,retighten any ground as 95 % of the time if it is electrical power it will be the ground. Good Luck

  6. #6

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    Something else to add; The voltage gage would sometimes, not at the same time necessarly as when it cuts out, would be sitting at 9 or 10 then when you rev it up sometimes it would dance back up. Also, before the new engine the volt gage always sat around 12, with this new motor it sits around 13 to 14. Seems weird.

  7. #7

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    Tried alternating the throttle position, didn't seem to matter. I was told one theory; The coil acts up when it's cold that first hour, then gets warm and sort of "heals" it's self??? I have a new coil in it to try. This is crazy without being able to readily run it nearby, got to go to the ocean and do the whole gig to test it.

  8. #8
    Member spoiled one's Avatar
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    Quote Originally Posted by Myers View Post
    Something else to add; The voltage gage would sometimes, not at the same time necessarly as when it cuts out, would be sitting at 9 or 10 then when you rev it up sometimes it would dance back up. Also, before the new engine the volt gage always sat around 12, with this new motor it sits around 13 to 14. Seems weird.
    While it is running, it should be charging at 14+/_, so if it is at 9 or 10 volts it seems to me that it is an electrical problem (ground?). I am no expert, but know enough to get myself into trouble. I did stay in a Holiday Inn Express one night, not last night if it makes a difference.
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  9. #9
    Member Cap'n Ron's Avatar
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    Not sure how old your outdrive unit is or whether it is also a Mercruiser...but older Mercruiser outdrives had a "kill switch" that momentarily cut electrical power to the ignition when you shifted from forward thru neutral to reverse. It kept the outdrive from locking up. If you have one of these switches, they sometimes go bad and cause the symptom you are experiencing. It is a toggle switch activated mechanically by the shift mechanism. But, if it is malfunctioning, it can cut ignition intermittently without any shifting activating it.

  10. #10
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    Quote Originally Posted by Myers View Post
    Tried alternating the throttle position, didn't seem to matter. I was told one theory; The coil acts up when it's cold that first hour, then gets warm and sort of "heals" it's self??? I have a new coil in it to try. This is crazy without being able to readily run it nearby, got to go to the ocean and do the whole gig to test it.
    Rarely does a coil act up when cold and improve as it heats up, almost always it fails after heating up. Seeing a variance in the charging output as you mentioned is either an electrical issue as I suggested or you are noticing something normal, ie; low voltage after first start up and idle RPM, then seeing output increase after the alternator RPM increases enough to excite the alternator and normal charging voltage attained. 13-14 volts is a good normal charging voltage, also the voltage will change depending on demand and RPM.

    Rechecking all electrical connections is easily done in the driveway, check them at the batteries, starter, alternator, ground points, coil, distributor, etc.

    If this is an EFI engine you have much more to check, injectors, throttle body/tps, map and maf sensors and so on.

    Be thorough enough to look for bent pins in the connectors, wires broken at the terminals and under the insulation, corrosion and loose fasteners.

  11. #11
    Member Frostbitten's Avatar
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    Could it be an issue with the ECM? I had an identical issue right after I repowered with a Mercruiser 350 a couple years ago. When I would get the boat home after each trip out, I always disconnected my batteries. The mechanic connected the motor to his computer a couple times looking for codes indicating the problem, but never found any codes indicating anything was wrong. I bought the connector to plug the boat into shore power at the house, and I stopped disconnecting my batteries...never experienced the problem again. Coincidence? Maybe, all I know is it runs like a top now.

  12. #12
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    Disconnecting the ECM from power resets all the parameters to a base value creating less than nominal performance until the ECM relearns an optimum 3D map of what the engine requires and how the engine is used (demand and habits). Sometimes it is very evident how the performance of an engine is affected by a power disconnect, sometimes it is hardly noticable.

    In general it is not a good idea to disconnect the ECM from power.

    I am guessing that the OPs engine is Carbureted, if it is EFI then it had better have a new ECM to match the new engine or an appropriate reflash(if possible) of the existing ECM to match .

  13. #13
    Member tlingitwarrior's Avatar
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    Quote Originally Posted by Big Bend View Post
    As 338 said check the electrical ,retighten any ground as 95 % of the time if it is electrical power it will
    be the ground. Good Luck
    In my experience its closer to 98%.
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  14. #14

    Default Found something...

    I took the advice to check electrical connections; I touched the three wires connected to the back of the alternator, two are (don't know correct terms) "circles" or "eyelits" with the nut holding them on and the third is a square-ish male/female type connector. The male/female "slip-on" connector (purple wire with REGULATOR printed on it) fell right off when I touched it, the only thing holding it in place was the rubber boot! I crimped it a bit to tighten it and much, much better.

    Now, I haven't started or ran it yet, but what do you guys think now??? IN MY MIND, THAT'S GOT TO BE IT! What does that wire do???

  15. #15
    Moderator bkmail's Avatar
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    Sounds like you rewired the end of the ignition switch that throws the starter on.
    BK

  16. #16
    Member tyrex13's Avatar
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    Carb or EFI?

  17. #17

    Default Carb

    I didn't change the motor myself, shop did it. Not sure what you mean by rewired ignition, etc???

    Would that lose connection make for an occasional stutter? I'll have to run it now and find out.

  18. #18
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    The loose wire on the 'alternator' is probably not an ignition feed (I don't have this type of motor and have never worked on one so take my comment with a grain or three of salt). The ignition feed would come off of a starter relay. The alternator wire you tightened very well could be your issue that is now fixed. It would/could cause complete or intermittent failure of the charging system.

  19. #19
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    I don't know that I would 'read too much' into the voltage reading. If you really want to know what it is doing, then check it back at the alternator or the battery, since they are connected by a big cable. The gauge on your dash is most likely tied to the bus, and when things come on, like a fan, heater or GPS, they may draw the voltage down there a bit but this may not be a real indicator of what the battery voltage is. As for your main problem, it sounds electrical to me. I would try to push back on the installer to fix it as part of their installation.
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  20. #20

    Unhappy Still messed up!

    I was all excited thinking the loose wire on the alternator (RESISTOR NOT REGULATOR -my bad) was the problem, so I went for a test run. All was great until about twenty to thirty minutes in and it did it again! I read in my manual that a cracked distributor cap can cause "hard starting and miss fires"? I took it back to the shop again to let them have at it some more, they're going to change all electrical, plugs, wires, distributor, etc, etc, and go from there. Also, they are going to run it in a barrel full of water instead of just idling off the garden hose to get a load on it. I'm not real sure this will do much, especially if it's just a wire bouncing around that causes the problem.

    It's been a monster disappointment, new motor and nothing but test trips this summer looking for a wild gremlin! (Why did I replace my motor again???)

    Many thanks for everyone's input, all your information has been greatly appreciated. I hope to report information soon about what could possibly be wrong, the curiosity is killing me!

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