350 Merc Mag MPI Surging Above ~4k RPM

aves137aves137 Member Posts: 26 ✭
in Engine Discussions
Hey all. New to the community. My parents have an '03 342 Fiesta Vee with the Merc 350 Mag MPIs that they are trying to sell. The boat has some issues that I am trying to help them sort through to hopefully secure a sale. We had a potential buyer take her out on a sea trial today and noticed that one of the engines was exhibiting a surging when she was really being pushed. The tachs in the boat suck, but it looked like anything above ~3,700-4k RPMs was when the surging was exhibited. No issues on idle or below that ~4k RPM mark. The fuel in the fuel system is probably 6-10 months old, but there was Stabil poured in around that time as well. I did check for water in the fuel as well and did not see any. I have not put a fuel pressure gauge on the rail yet to see where it's at as it started raining here, but I'm suspecting either fuel pump or fuel pressure regulator? I'm a classic car guy more than a boat guy though so I figured I would come here and ask the experts. Really appreciate the help and am very grateful that a community like this exists!
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  • Black_DiamondBlack_Diamond Member Posts: 5,439 ✭✭✭✭✭
    Might be a few things: water/fuel filter, fuel filter, cap/rotor. 

    Past owner of a 2003 342FV
    PC BYC, Holland, MI
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  • Michael TMichael T Member Posts: 7,227 ✭✭✭✭✭
    The fuel that was 6-10 months old was that ethanol infused fuel. If it was it can certainly cause surges at higher rpms due to significantly reduced octane. However does sound, a B D said, a restriction to fuel/air flow. What shape are the fuel supply lines in? They can collapse if spongy from sitting with ethanol fule in them/
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  • aero3113aero3113 Member Posts: 9,045 ✭✭✭✭✭
    Did he notice the surging only on the tach or did he hear/feel it? You are right, there have been issues with the tachs.
    2008 330EC
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  • aves137aves137 Member Posts: 26 ✭
    Michael T said:
    The fuel that was 6-10 months old was that ethanol infused fuel. If it was it can certainly cause surges at higher rpms due to significantly reduced octane. However does sound, a B D said, a restriction to fuel/air flow. What shape are the fuel supply lines in? They can collapse if spongy from sitting with ethanol fule in them/
    No ethanol in the fuel from what my dad tells me. Usually filled up with 89 octane no ethanol fuel, which I'm told is what in the tank (about a half tank). The fuel supply lines look to be in decent shape from the outside, though that obviously doesn't mean that there are not issues there. Think it might be worth throwing in a bottle or two of octane booster to see if it smooths it out?
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  • aves137aves137 Member Posts: 26 ✭
    Black_Diamond said:
    Might be a few things: water/fuel filter, fuel filter, cap/rotor. 
    Non-ethanol fuel being used, though I would not be surprised if the rubber seal around the fuel filler inlets had allowed some rain water down in there as the seals are dried out.

    Fuel filters were replaced ~2-3 years ago, though the boat has seen maybe 10 hours of engine run time during that period.

    Cap/rotor I believe are fairly new, though I may just throw a new set on each motor for peace of mind. Only one engine is doing the surging, so I'm thinking I may swap the cap and rotor between the two motors and then take her out for a run. If the issues switches sides, that should help isolate the issue without just throwing money at new parts.
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  • aves137aves137 Member Posts: 26 ✭
    aero3113 said:
    Did he notice the surging only on the tach or did he hear/feel it? You are right, there have been issues with the tachs.
    I was on the boat with them at the time and you could hear it and feel it. You could watch the sync gauge bounce from side to side when the surging would occur. Take her down below ~4k RPM and she smoothed right out without issue.
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  • Handymans342Handymans342 Member Posts: 10,375 ✭✭✭✭✭
    Man you gassers have so many problems I feel sorry for you and I wish I could help. LOL
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  • aves137aves137 Member Posts: 26 ✭
    Handymans342 said:
    Man you gassers have so many problems I feel sorry for you and I wish I could help. LOL

    The two happiest days of a boater's life - the day they buy the boat, and the day they sell the boat, haha.
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  • Handymans342Handymans342 Member Posts: 10,375 ✭✭✭✭✭
    Yup
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  • Black_DiamondBlack_Diamond Member Posts: 5,439 ✭✭✭✭✭
    All good until you get a runaway on that diesel!

    Past owner of a 2003 342FV
    PC BYC, Holland, MI
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  • Handymans342Handymans342 Member Posts: 10,375 ✭✭✭✭✭
    Never happen
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  • 212rowboat212rowboat Member Posts: 2,593 ✭✭✭✭✭
    those filter start deteriorating the moment you put them on... it's both use and time, which start ticking as soon as gasoline hits them... and, it's the same with diesel filters, they just deteriorate slower- or asphalt up... 
    click here for faults (BEEP) list
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