Port Engine loses power, it wont stall, but it wont maintain cruising rpm

Discussion in 'General Maintenance/Repair Questions' started by 340DA, Jul 14, 2020.

  1. 340DA

    340DA Member

    Jul 21, 2008
    LI Sound
    2000 Sundancer Sport Cruiser
    454 Mercruiser w/Bravo III Drives
    I have a 2000 340 Sundancer with 7.4L Mercruiser Bravo 3.

    At the end of last season (Sept 2019) I began to experience an issue with my Port engine (only). Starboard is fine. I would get up to cruising rpm 3300, and cruise at the rpm for a while (15 - 30 mins), then suddenly, the Port engine would lose power and drop down to 1200 - 1800 rpm range. The engine would NOT stall, it would just lose power. Please note that the engine runs easily., smoothly and consistently in the 800 - 2600 rpm range with no issues at all.
    When the engine loses power, I would turn the engine off, then turn it on and the boat would be fine for several minutes, climbing back to 3300 rpm. Then the "issue" returned after several minutes. And Again and Again it occurs

    The Mechanics thought it's fuel starvation issue so they blew out the fuel line in Sept 2019 and this "fix" seemed to work for the remainder of the 2019 season.

    In 2020 season, I took the boat out for a few cruises and it seemed to run fine. Suddenly 2 weeks ago, this issue returned. Exactly the same issue as above. The mechanic blew out the fuel line again using a compressor on July 8. He bay tested the boat that same day and it ran fine for him.
    Today (7/14) is the first chance I had to bay test the boat myself and the issue returned when I tested the boat.
    Note: that once this loss of power issue occurs, the highest rpm I can subsequently get the boat up to is 2600 rpm, but no more than that. If I try to push it past 2600, it loses power

    I am stumped by this issue... Any ideas/suggestions/thoughts???? Is it Fuel???? Electrical??? Mechanical?????

  2. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    For starters Id replace those 20 year old fuel lines. Go thru the fuel system from tank to injectors
    Chris-380 likes this.
  3. Golfman25

    Golfman25 Well-Known Member

    Sep 12, 2009
    Fuel line collapsing?
  4. cfd95

    cfd95 Active Member

    Nov 5, 2013
    Northport Long Island
    2002 310 Sundancer
    350 Horizons
    V Drives
    ignition coil failing when it heats up
    swap coils between motors see if problem follows
    capecodcruiser likes this.
  5. Dabble

    Dabble Member

    May 26, 2020
    Olympia, WA
    1997 330 Sundancer
    2000 230 Bow Rider
    97 - Twin 5.7 Merc V-Drives
    2000 - 5.7 Bravo III
    Definitely sounds like a fuel issue. Fuel filters need replaced?

    When is the last time you did an ignition tune up?
  6. 370Dancer

    370Dancer Well-Known Member

    Oct 2, 2006
    Florida - Alabama
    1998 370 Sundancer
    380hp MAG MPI Gen VI with V drives
    Only need 4 things to run right.
    Fuel Pressure
    Spark (at the right time)
    Controlled air intake (no leaks)

    The hard part is figuring out which one it is, because they can all cause this issue. Sometimes, more than one is contributing. Ask Scoflaw how I know......
    Chris-380 likes this.
  7. capecodcruiser

    capecodcruiser Member

    Aug 1, 2008
    Falmouth, MA
    2001 310, 350 HO's/Vdrives.
    Mercruiser MPI 350 HO's
    Do the ignition coil swap. I had a very similar issue in the last month and ended up having 2 bad coils. Have never had one fail before in anything. Do the "swap" between engines and see what happens. Easy fix

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