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Low water pressure, but no overheat

pcmpetepcmpete Member Posts: 110 ✭✭
in Engine Discussions
Mercruiser 377 with bravo 3

engines ran great for approx 1hour.  
Dropped anchor for 2 hours
on my way back to my dock in ‘no wake’ I noticed my water pressure on starboard engine was very low at 2.1psi
i went to idle and continued on.
then psi slowly dropped to 1.2psi, 
so I decided to find the nearest dock and call TowBoatUs. 
I have closed cooling, and the engine  temp only rose to 171f (normally runs 165).
I had new seawater pump and impeller installed approx 1 year ago.
would the impeller be shot already?
I thought maybe the water intake got blocked.  I visually checked it back at my dock.
After about an hour after I got towed back I started the engine to see if any change, but no. Pressure was only 0.9psi at idle.
no alarms or warnings.
what and where should I check first?
thanks guys


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    Dream_InnDream_Inn Member, Moderator Posts: 7,557 mod
    Well, first of all, at idle I only get a PSI of 1-2.  But, you are comparing to the other motor and what you usually get, so I'd start with impeller change first.  That is the easiest thing to change and check.  
    FYI, the voltage is a bit low on both your engines, but especially your stbd (12.5).  I'm assuming the motors are running since you have water pressure.  

    Dream 'Inn III -- 2008 400 Express

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    YYZRCYYZRC Member Posts: 4,907 ✭✭✭✭✭
    Those sensors are notoriously bad. They are located in the power steering cooler. Swap sensors first before you spend any money. 
    2008 350 EC on Georgian Bay
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    earl1z19earl1z19 Member Posts: 333 ✭✭✭
    I second the sensor check especially since it didn't overheat so there is probably more flow than what you are seeing in water PSI
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    LaReaLaRea Member, Moderator Posts: 7,541 mod
    It didn't overheat enough to trigger limp mode, but the temperature did get higher than the Port side.  That doesn't sound like a pressure sensor problem.  It sounds like you're getting some water flow, but not enough.  

    Even a brand-new pump and impeller could get chewed up in a few minutes if you suck up a bunch of sand.  I'd run the engine briefly up to 2500 RPM in neutral, and see what the water pressure does.  Do the port first to see what "normal" looks like, then starboard.

    Pressure is low at idle but normal at high RPM:  worn impeller and/or pump body.  

    Pressure is low at all engine speeds:  intake restriction such as a plastic bag sucked into the intake on the Bravo.  
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    YYZRCYYZRC Member Posts: 4,907 ✭✭✭✭✭
    To my understanding, the boat will only go into guardian mode if the pressure is zero. 
    2008 350 EC on Georgian Bay
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    pcmpetepcmpete Member Posts: 110 ✭✭
    Dream_Inn said:
    Well, first of all, at idle I only get a PSI of 1-2.  But, you are comparing to the other motor and what you usually get, so I'd start with impeller change first.  That is the easiest thing to change and check.  
    FYI, the voltage is a bit low on both your engines, but especially your stbd (12.5).  I'm assuming the motors are running since you have water pressure.  
    Normally both engines readings on the VV are close, I have been chasing why my voltage is low for a while too.   
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    YYZRCYYZRC Member Posts: 4,907 ✭✭✭✭✭
    Check your connections at the DC panel behind the transom door. Mine were loose and resulted in low voltage readings. 
    2008 350 EC on Georgian Bay
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    rasburyrasbury Member Posts: 8,237 ✭✭✭✭✭
    If you do go in after tgat seborrheic, tgat cooler on mine is very soft material and you can destroy it with the turn of a wrench. Want to ask me how I know?
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    tricountytrailtricountytrail Member Posts: 186 ✭✭
    So question when I get PRT EMCT OVRHT alarm. Can I see the value of the sensor on the Port Exhaust coolant temperature or is it just a open close switch set at 212F?
    I know my sea pump housing is scored up and is probably cavitating at cruise speed.
    I just changed out the Port Sea pump but haven't got a chance to take it out yet.
    Attached is the error before pump change, and pic of gage when running as alarm went off. Port engine coolant temp would creep up after 5-10 minutes and alarm PRT EMCT OVRHT would trigger.



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    pcmpetepcmpete Member Posts: 110 ✭✭
    edited September 2022
    Update.  When I cold start both engines the starboard water pressure is still low.  But if i have the starboard only running the pressure is higher.   So now I am leaning towards an electrical issue rather than a water blockage.  See video https://youtu.be/FVa2E_s6SR8
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    YYZRCYYZRC Member Posts: 4,907 ✭✭✭✭✭
    15 mins to swap the sensors.  Worth trying to narrow things down.
    2008 350 EC on Georgian Bay
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    Michael TMichael T Member Posts: 7,227 ✭✭✭✭✭
    .......would be thinking that it was an erratic sensor or ground - both are notorious. Did you feel both hoses. If they are both firm then your engine is getting water. At 1.9 pounds I would expect a much greater engine temp difference that 4 degrees. Very interesting!
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    SpyderwebSpyderweb Member Posts: 879 ✭✭✭
    Agree with @YYZRC, swap sensors.  Even I was able to swap in 15 minutes.  Showed I had a bad sensor.  Even if it doesn’t change, you’ll eliminate one trail to go down.
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