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  1. #1
    Join Date
    Feb 2007
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    224

    Default 2008 Mercruiser 5.7 TBI rpm loss after 5 min. at cruise spd.

    Hi Gang

    I have a twin engine inboard Pursuit I am working on. The motors are 2008 5.7 fwc TBI with dry joint exhausts and a serpentine belt setup. These are Mercruiser replacement motors. They idle and run smoothly.

    After 5-7 minutes of running both motors at 3200 rpms, the port engine hiccups, bogs and drops down to 2500 rpm. If I return the throttle lever to neutral and accelerate up to 3200 once again the problem re occurs after 5-7 minutes of operation like clockwork.

    I can cruise all day at 2800 rpm on both motors and all is fine.
    The fuel vents are clear and the fuel filters are new. Any ideas?

    Thanks in advance

    Jon Allen

  2. #2
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,267

    Default Re: 2008 Mercruiser 5.7 TBI rpm loss after 5 min. at cruise spd.

    Clogged anti-siphon valve?

    Jeff

  3. #3
    Join Date
    Feb 2007
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    224

    Default Re: 2008 Mercruiser 5.7 TBI rpm loss after 5 min. at cruise spd.

    Hi Jeff

    That's what I was thinking as well.

    However I don't see how it could run for 5-7 minutes at cruise speed if that were the case. I will probably be attatching a fuel pressure/vacuum guage to it to verify.



    Thanks
    Jon

  4. #4
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    6,826

    Default Re: 2008 Mercruiser 5.7 TBI rpm loss after 5 min. at cruise spd.

    could be picking up crud on the dip tube screens as well.....either way, the pressure/vacuum gauges would be prudent....and definitive....

  5. #5
    Join Date
    Feb 2007
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    Posts
    224

    Default Re: 2008 Mercruiser 5.7 TBI rpm loss after 5 min. at cruise spd.

    Hi Jeff and Mark

    Thank you both for your help. I always like to consult the real professionals.

    I will run a fuel pressure check on both the LP and HP pumps as well as measure vacuum just before the tank on the suction side.


    Thanks Again
    Jon

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