Stumped re: ECM/fuel injectors

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by msballin, Jun 8, 2021.

  1. msballin

    msballin Member

    251
    May 30, 2008
    Islip Terrace, Long Island
    2000 340 Sundancer
    Twin 7.4L Horizons w/ Hurth V-Drives
    Good afternoon -

    Subsequent to injectors being replaced w new, port engine presents with rough/vibration from idle to 2200 rpms then she's running like a top after engaging throttle to 34- 3600 rpms. Mechanic states system scan shows ECM is flooding one bank of injectors (3,5,2,8) with fuel as if it perceives engine is running at cruising speed. Swapped ECM to starboard engine, which is running flawlessly, and same symptoms present. Once starboard ECM is connected to port engine, all prior explained symptoms disappear. Deductive reasoning says let's send the ECM out for analysis expecting to either have it recalibrated or replaced with new. After bench test @ OBD Diagnostics in CA, no faults reported and all is fine. Mechanic (very seasoned and trustworthy...been with him for 13 yrs) is stumped and still feels it's ECM.

    Any thoughts or input would be greatly appreciated. Thx in advance

    Mike Ballin
     
  2. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx 502 mag
    2005 Baja 242 islander 496
    502 mpi Bravo 1
    496 mag B1
    2nd that
     
  3. paulswagelock

    paulswagelock Well-Known Member

    Oct 25, 2010
    pa
    2018 SDX 270 OB 300 Verado
    Verado 300
    Did you have a recent sensor fault or failure on that motor, IAC, temperature, etc? Sometimes when they fail, they fry a circuit in the Ecm.
     
    scoflaw likes this.
  4. hughespat57

    hughespat57 Well-Known Member

    981
    Sep 25, 2016
    Rock Hill, SC
    300 Sundancer 1994, trailered tri-axle LoadRite roller
    Mercruiser 5.7 260HP Alpha One Gen II, twin
    Have you re-installed the ECM to see if it is magically fine now that they tested it?
    They may have simply rest everything and it tests fine.

    If it repeats the same symptoms then it's bad
     
  5. msballin

    msballin Member

    251
    May 30, 2008
    Islip Terrace, Long Island
    2000 340 Sundancer
    Twin 7.4L Horizons w/ Hurth V-Drives
    No faults at all.
     
  6. msballin

    msballin Member

    251
    May 30, 2008
    Islip Terrace, Long Island
    2000 340 Sundancer
    Twin 7.4L Horizons w/ Hurth V-Drives
    Awaiting return of unit. However, I do concur and will purchase new. It's a $1700 gamble I'm willing to take.
     
  7. msballin

    msballin Member

    251
    May 30, 2008
    Islip Terrace, Long Island
    2000 340 Sundancer
    Twin 7.4L Horizons w/ Hurth V-Drives
    The frustrating part of this is originally, the port engine was looked at by marina mechanic (no personal mechainics permitted on premises) for a different issue when upon initial look/see he observed a broken #7 injector which was suggested to be replaced. Upon replacement #5 was broken (I've been told if you look at them the wrong way they snap) so that one was replaced. Directly after this, the engine began to show symptoms as described in my initial post.

    My question is, what could possibly cause an ECM failure as a result of potentially reinstalling injectors improperly? Thx for all of your prior responses and in advance for your continued input.
     
  8. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx 502 mag
    2005 Baja 242 islander 496
    502 mpi Bravo 1
    496 mag B1
    A bad ground can fry a ECM. If your injectors are that brittle replace them all.
     
  9. hughespat57

    hughespat57 Well-Known Member

    981
    Sep 25, 2016
    Rock Hill, SC
    300 Sundancer 1994, trailered tri-axle LoadRite roller
    Mercruiser 5.7 260HP Alpha One Gen II, twin
    My point is don't buy the ECM yet, see if it works now when it gets back.

    As to the injectors yes replacing all is expensive, but so is labor. Since this is because they are brittle and physically failing If you are going to remove the fuel rails and usually the intake plenum to get to the injectors you may as well replace all.

    Be sure the wiring harness for the injectors isn't just a brittle and cracking, injectors are controlled by the ground side. If one grounds out that injector will just keep dumping fuel.
     
  10. msballin

    msballin Member

    251
    May 30, 2008
    Islip Terrace, Long Island
    2000 340 Sundancer
    Twin 7.4L Horizons w/ Hurth V-Drives
    Makes sense. As an FYI, all injectors were replaced with new and or ultrasonically cleaned ones while both engines underwent install of new OEM Merc head/valve assemblies, distributor assemblies, and full tune ups. Was not expecting this issue to reappear which was present prior to the rebuilds. It was at this time my mechanic discovered the injector firing issues while scanning for fault codes. Wiring harness appears to be fine as when swapping ECM from other engine, she ran flawlessly.
     

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