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  1. #11
    Join Date
    Oct 2006
    Location
    Oakland, NJ
    Boat
    Sea Ray
    Details
    1998 330 DA E80 GPS, Radar, & heading sensor
    Engine(s)
    7.4 Mag. Horizons 380hp
    Posts
    1,103

    Re: 7.4 Horizon (380 hp) idling rough

    " Originally Posted by ShoreFlyer
    I was curious what was the fix for the rough idling in your thread from May 2010."


    Turned out to be low fuel pressure, we were getting about 20psi when we should have gotten about 40psi (static preasure). I don't know if it was the regulater or the fuel pump as I replaced both allready, it was a PIA job due the tight area we had to work in on my boat so once we were down there I didn't wan't to do it again so we replaced both. This fixed the problem. The interesting part of this was that at higher RPM's the motor ran fine, all the way up to WOT...
    Any questions feel free to ask!
    Larry

    1998 330 Sundancer "Miss Melina"
    2 X 7.4 Mag. Horizons 380hp FWC
    Ryamarine E80 GPS, Radar, & heading sensor
    Home Port: Norwalk Cove Marina, Norwalk CT

  2. #12
    Join Date
    Aug 2007
    Location
    Grand Lake o' the Cherokees
    Boat
    Sea Ray
    Details
    2005 320 Sundancer, Raymarine C80 GPS & Radar
    Engine(s)
    Twin 350 Merc V Drives
    Posts
    194
    Happy to hear all worked out. I had a rough idle a couple seasons ago that turned out being missing and/or blown IAC filters. A .97 cent part in my case made all the difference. Crazy stuff, boats that is. :-)

    2005 320 Sundancer, Twin 350 Merc V Drives
    Raymarine C80 GPS & Radar
    Grand Lake o' the Cherokees
    ...Prev 2003 280 Sundancer, Sgl Merc 496 Mag Horz.
    ...Prev 1998 215 Express Cruiser, Sgl Merc 5.0L EFI

  3. #13
    Join Date
    May 2012
    Location
    Toms River, NJ
    Boat
    Sea Ray
    Details
    1995 280 SunRunner Raymarine C70 Chartplotter
    Engine(s)
    7.4L MPI Horizon Mercruiser Bravo III
    Posts
    13

    Re: 7.4 Horizon (380 hp) idling rough

    Fuel pump test indicated fuel pressure was good at 39 PSI.

    New problem developed....I loses power after reaching 4100 RPM.....drops back to 3000 RPM if I bring the power all the way back then WOT I can get 4000 RPM back then drops to 3000 RPM again.

    Would a bad MAP or Throttle sensor cause the idling and the power loss problem...Runs find in between.

    Still waiting on a computer from my buddy.

  4. #14
    Join Date
    Oct 2006
    Location
    Oakland, NJ
    Boat
    Sea Ray
    Details
    1998 330 DA E80 GPS, Radar, & heading sensor
    Engine(s)
    7.4 Mag. Horizons 380hp
    Posts
    1,103

    Re: 7.4 Horizon (380 hp) idling rough

    Quote Originally Posted by ShoreFlyer View Post
    Fuel pump test indicated fuel pressure was good at 39 PSI.

    New problem developed....I loses power after reaching 4100 RPM.....drops back to 3000 RPM if I bring the power all the way back then WOT I can get 4000 RPM back then drops to 3000 RPM again.

    Would a bad MAP or Throttle sensor cause the idling and the power loss problem...Runs find in between.

    Still waiting on a computer from my buddy.
    Is the 39 psi reading just at idle? Or does this maintain throughout RPM range? If you havn't done so allready, keep the fuel pressure gauge attached while running and when the RPM's drop take notice of fuel preasure readings.
    Larry

    1998 330 Sundancer "Miss Melina"
    2 X 7.4 Mag. Horizons 380hp FWC
    Ryamarine E80 GPS, Radar, & heading sensor
    Home Port: Norwalk Cove Marina, Norwalk CT

  5. #15
    Join Date
    May 2012
    Location
    Toms River, NJ
    Boat
    Sea Ray
    Details
    1995 280 SunRunner Raymarine C70 Chartplotter
    Engine(s)
    7.4L MPI Horizon Mercruiser Bravo III
    Posts
    13

    Re: 7.4 Horizon (380 hp) idling rough

    It was about 35 PSI at Idle and maintained steady fuel of 39 PSI throughout the RPM Range and even when it was doing its rough idling thing. I am thinking it is more of an "air" thing. Going to check the vacuum lines again and start unplugging sensors to see what the result is. Mercruiser manual says the ECU will set a value to a sensor if loses indications from it. If I get different performance from one sensor vs another that might isolate the problem.

    I am reluctant to hand it over to a mechanic because they seem like they just changer parts out these days till they hit the right one.

  6. #16
    Join Date
    May 2012
    Location
    Toms River, NJ
    Boat
    Sea Ray
    Details
    1995 280 SunRunner Raymarine C70 Chartplotter
    Engine(s)
    7.4L MPI Horizon Mercruiser Bravo III
    Posts
    13

    Re: 7.4 Horizon (380 hp) idling rough

    Removed connections from various sensors one at a time today. Engine ran rough and stalled in neutral without a load when I removed each of them. The IAT one would not allow me to start the engine unless I pumped the throttle...but I think that was more because the engine was already hot.

  7. #17
    Join Date
    May 2012
    Location
    Toms River, NJ
    Boat
    Sea Ray
    Details
    1995 280 SunRunner Raymarine C70 Chartplotter
    Engine(s)
    7.4L MPI Horizon Mercruiser Bravo III
    Posts
    13

    Re: 7.4 Horizon (380 hp) idling rough

    My boat is now fixed.

    Timing was off by about 10 degrees. It has to be set using a laptop AND timing light.

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