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F115 intermittent sputtering, stalling, rough

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  • F115 intermittent sputtering, stalling, rough

    Hi There Everyone,
    This'll be my first post on Yamaha Outboard Parts

    I've got a Yamaha F115 from the early 2000's. I am the 2nd owner. Its got about 500-700 hours on it. It's been very well maintained by previous owner. The thing looks like new.

    Last year I sailed up and down the coast of Greenland where I live, and on my return trip the engine began acting up. Rough at idle, engine idle RPM seemed to fluctuate quite a bit around 900rpm all the way down to 500rpm, more smoke than usual, seemed to sputter at idle as well. At plane it would act as normal. I got all the way home without further problems.

    Later in the fall, the engine showed further symptoms. Taking a long time to turn over when engine hot. Engine would die when going from "1st notch gear" into higher RPMs (same with reverse gear). More sputtering at idle, but always ran perfectly at higher RPMs.

    For a while after I had returned from my long trip I had an audible engine alarm go off every time i started the engine cold, but I realize now it was due to over filling the engine oil. I drained out excess oil and the alarm ceased.

    I stored my boat inside the winter over, and have just last week begun to troubleshoot and repair the engine.

    Luckily I have an extra engine that is only good for parts, because the powerhead is shot. So i have plenty of parts to swap over with.


    Here is the list of things I've tried so far (in order):

    checked pressure on fuel rail. 30psi at startup and idle. (readings could be wrong because I am using a pressure gauge with a fitting for a car tire)
    checked cylinder pressure. all cylinders at 175psi at WOT.
    top cylinder spark plug shows carbon fowling. Swapped spark plugs to isolate problem to cylinder 1.
    Suspected high pressure fuel pump filter. Cleaned. Wasn't dirty. No change.
    Suspected injector nozzles. Sent them off for cleaning. Came back as new. No change.
    Suspected fuel pressure regulator. Exchanged. No change.
    Suspected VST high pressure fuel pump. Exchanged. Pressure is now approx. 38psi at idle.
    Suspected Ignition coils. Changed both. No change.
    Suspected Battery. Checked voltage. 12.8v off. 14.4v - 14.6v at idle. Exhanged battery. No change.
    Suspected voltage regulator. Exchanged. No change.

    So, with all this said however. It seems the engine has got progressively BETTER now that today the engine started right up. 1-2 seconds and it's on. Also it seems to run more smoothly and stalls fewer times when going from low to high RPMs.

    Tomorrow I will take the boat out on the water for a test run. Because I cannot put the engine in gear while in the shop (the lower drive is sitting in a barrel of water) and have to use the "auxiliary/choke throttle" (or whatever it's called) to rev up in neutral.

    Also, I want to run the engine hot enough to burn off the excess carbon that now is stuck inside cylinders. Perhaps that is the only thing that remains.

    I am hopeful that all my part swapping has fixed the engine, without me realizing it in the shop and that on the water, it will be obvious.

    If the problem persists after tomorrow, I will have no clue where to go from there. Electrical? ECM?

    I've got two friends visiting from overseas, and I desperately want to take them out sailing. Wish me luck tomorrow.


    p.s. I forgot to mention. I don't have access to any diagnostic software, test harnesses or otherwise any F115 wise mechanic in my locality. I DO have the shop manual.
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