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Mercruiser 5.0l Alarming at 4000 RPM

ScooterBob Member Posts: 1
I have been having a problem with my 5.0l alarming at 3800-4000 rpm when accelerating.  It won't always do it.  If I back it down below 3800 it runs fine. I have read some discussions pointing to inlet water or shift sensor.  I tried replacing the impeller and thermostat, and it still does it. Can anyone shed some light on this problem for me?


  • nquirk
    nquirk Member, Moderator Posts: 672 ✭✭✭✭
    First, let me start by saying I don't know I/O very well but have seen similar symptoms on I/Os and outboards.

    The inlet water you've suggested.  The lower end was dropped to change the impeller.  Did this happen before or after you changed the impeller?  Does the alarm happen sometimes but not always.  Check to see if it alarms with the trim all the way down, vs trimmed out.  Do you get a spike in temperature? 

    On the shift sensor, it is possible the Throttle Position Sensor is faulty.  Just happened to my Mercury outboard.  Runs fine at lower speeds, but alarms at higher speeds.  You cannot adjust these (at least on my outboard)
    2010 Hurricane SS 188 OB
    2010 Mercury Optimax 200HP / Mercury SS High Five 19P
    2010 Trailmaster SC trailer