Mercruiser water pressure sensor [Update]

Discussion in 'General Maintenance/Repair Questions' started by Devil_Inside, Sep 3, 2019.

  1. Zim

    Zim Active Member

    187
    Jan 16, 2018
    2012 Sea ray sundancer 310
    Twin merc
    From what the guys here st the marina tell me normal h2o pressure is at 2-5psi, at idle, slightly more during load. Good luck, i know the frustration level....
     
  2. Devil_Inside

    Devil_Inside New Member

    17
    Aug 26, 2019
    Proline 251
    2005 Mercruiser 5.7 Horizon MPI w/ Bravo III
    I couldn’t agree with you more. I feel like I am chasing a ghost because I have never had overheating issues, but I still want to investigate all possibilities.

    Like many other things it sounds great on paper, but in reality it doesn’t work all that well. Trust me, the first time you get stuck out on the water due to a faulty sensor you’ll change your mind lol
     
    Zim likes this.
  3. stephenm27

    stephenm27 Active Member

    299
    Nov 3, 2009
    Milton, GA
    2009 310 Sundancer
    350 MAG Mercs/B3 x 2 (Axius Premier)
    Indeed, the standard raw water temp sensor works reliably only once it’s (a) immersed in raw water or (b) the iron housing its threaded into sufficiently heat soaks to the point it conducts heat to the sensor.

    The latter happens with a blockage or impeller failure and by the time the temp sensor indicates this condition, the majority of the block is well past normal operating temp.

    Unfortunately, the PSI sensor is the over-engineered version of the outboard “water stream” that immediately and reliably indicates raw water flow. Great intent, but slightly unreliable.
     
    Zim likes this.
  4. Devil_Inside

    Devil_Inside New Member

    17
    Aug 26, 2019
    Proline 251
    2005 Mercruiser 5.7 Horizon MPI w/ Bravo III
    I am headed to the boat armed with a second, brand new, OEM pressure sensor and a digital pressure gauge certified to be accurate to 0.015psi, which I borrowed from a friend who works at a lab. I also plan on testing the output of my sea water pump as per Mercruiser's procedure. I will post my finding later today or tomorrow. Stay tuned.
     
    Zim likes this.
  5. Bt Doctur

    Bt Doctur Well-Known Member

    Aug 21, 2010
    New Jersey
    Ex SRV 240 Weekender twin
    in between
    Funny how all these years we relied on the engine temp gauge and normal impeller service intervals to keep the motor in good condition. With the advent of FI motors we now have all these useless sensors controlling Guardian Mode and everything else.
    My vote is to eliminate them one way or the other
     
    scoflaw likes this.
  6. Seapig

    Seapig Member

    55
    Dec 25, 2018
    Oakham MA or Winnipesaukee NH
    12 Sundancer 310 w Axius
    Twin 350 Mags w Bravo drives
    Maybe......

    You’re 5 miles offshore with a singe engine boat that’s overheating cause the impeller failed. Choices are to call for a tow $$$$ or try and “make it back” which will mean a new block. Most people ruin their engine and get a tow.

    Knowing days or hours earlier that block pressure was too low offers an opportunity to address it before it gets ugly.

    It’s no different than an oil pressure gauge versus a warning lamp IMO.
     
  7. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    Do your maintenance, change that impeller on a schedule, and you won't be 5 miles offshore with an issue. Seasoned sailors knew this stuff way before they invented water pressure sensors
     
  8. Devil_Inside

    Devil_Inside New Member

    17
    Aug 26, 2019
    Proline 251
    2005 Mercruiser 5.7 Horizon MPI w/ Bravo III
    UPDATE: I was able to confirm that I have LOW pressure and NORMAL flow - whatever that means

    First I decided to test the water pressure with my friend's lab-calibrated digital gauge, and I discovered that I indeed only have about 0.5-0.7psi of pressure at idle.

    I then proceeded to do a flow test and I measured over 12 quarts of water per 15 seconds @1000rpm. I repeated the test multiple times and got similar results. This rules out Bravoitis and any other potential restrictions.

    The boat ran great with no alarms and no Guardian mode issues, but my pressure never went above 6psi even at WOT, and at idle I am always <1psi

    So now what?
     
  9. Seapig

    Seapig Member

    55
    Dec 25, 2018
    Oakham MA or Winnipesaukee NH
    12 Sundancer 310 w Axius
    Twin 350 Mags w Bravo drives
    6 PSI at WOT is quite low IMO. My raw water cooled engine runs in the mid teens at 3500.

    Pressure is created by restricting flow so something downstream of the raw after pump is less restrictive than normal. Being unfamiliar with that FWC system, I can’t guess where to look.

    I think that system is fairly simple: Pump....cooler....heat exchanger......riser dump. I might start by feeling the exhaust hoses after the risers to see if one is cooler than the other.
     
  10. Espos4

    Espos4 Well-Known Member

    Jan 1, 2017
    Long Island NY
    2007 240 Sundeck
    350 MAG Bravo 3 W/DTS
    I have a single 350 MAG with raw water cooling, My pressures have been fairly consistent for the last 4 seasons

    At idle, .8 up to 1.0 psi

    Cruising in the mid 3k rpm range I get 9-12 psi.

    This is with a new raw water pump 3 years ago, new impeller this year, new risers and manifolds last year, splitting the outdrive every winter to clear the water inlet and checking for bravoitis every spring before launch.

    These are just normal maintenance items, none of this was done in an attempt to change the raw water pressure.

    Others have posted higher numbers, I've never seen them on my boat.

    Temps run 158-160 degrees all day long regardless of run time or rpm.

    At low tide, the last 40' of the canal is very shallow. If the wind isn't too bad, I can shut the motor off, raise the outdrive and coast right over the area. A few times I had to keep the prop spinning due to the wind and current. Once I got to the bay and up on plane, I would immediately notice a difference in pressure. Usually only running at 5-6 psi in the 3k rpm range. A little thrust in reverse helps clear the water intake on the outdrive, and the pressure immediately goes back to the 9-12 psi range.

    If you are only getting 6psi in the higher rpm range, You may have a blockage in the supply side, either bravoitis, or debris, like barnacles, in the outdrive water pick up.
    When was the last time the outdrive lower unit was removed to clear the water intake?


    If all of that checks out and temps are good, maybe that's "just the way it is"
     
  11. Devil_Inside

    Devil_Inside New Member

    17
    Aug 26, 2019
    Proline 251
    2005 Mercruiser 5.7 Horizon MPI w/ Bravo III
    I ran the boat over 50 miles yesterday and everything seems to be fine... no change in the pressure readings from what I reported previously, but I haven't had any further alarms either. Temps were fine as well. I am starting to think that as long as the ECM sees *some* pressure it is happy.
     
    scoflaw likes this.

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