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Mercruiser water pressure sensor

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  • #16
    Agree with AC Chris here. Just get another sensor. Problem solved. Lasted years in mine and hundreds of hours since.

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    • #17
      OK, so the plot thickens - I replaced the sensor with an OEM Mercruiser sensor, and my pressure readings are still kinda low. According to VesselView, I am only getting 0.4psi to 0.7psi at idle, about 2psi @1000rpm, 4-5psi around 1500rpm, etc. No alarms or Guardian mode this time around, but I only ran the boat for a short period of time, at the slip.
      I went to Home Depot and I bought a cheap 0 to 15psi gauge, and a couple of fittings, and I installed it in place of the sensor. My readings went up ~1psi, so at idle I was getting a reading of 1.4psi to 1.7psi, about 3psi @1000rpm, 6psi @1500rpm and 8psi @2000rpm.
      I don't know how accurate the cheap $10 gauge is, nor do I know how accurate the new pressure sensor is, but even the higher values shown by the gauge are kind of low aren't they?

      This motor is new to me and thus far I've only ran it for about 2 hours total (everything from idle to WOT), during which time the temperatures stayed normal according to both the analog gauge and VesselView. This engine replaced a raw water cooled 1995 Mercruiser 5.7 TBI/EFI, which I ran for 6 years and over 800hrs, and it too never had any overheating issues so I don't think the problem is "Bravoitis". I am embarrassed to admit that I never checked what's behind the water outlet while I had the motor out, but at the time I hadn't even heard about "Bravoitis".
      The impeller and wear plate are brand new, and so is the water distribution housing, and water hose (transom assembly to raw water pump). While I had the raw water pump apart I didn't notice any excessive scoring inside, but then again I probably wasn't paying as close attention as I should have as I wasn't aware of that known issue either....

      I am really not sure what to make of this, but I think I should do a flow test as a next step. I recall reading about a target value of ~7.5qts water for 15 seconds of runtime as a baseline - does that sound about right?

      Comment


      • #18
        Pressure is merely a resistance to flow. If you have no resistance (like the engine, manifolds and elbows are clear of obstructions) then you could have a high flow, 60 litres/minute, and still show little on a pressure gauge. You could also have the opposite, high resistance and low flow. That would show as a high pressure. Without knowing the flow for any given pressure, the numbers are almost meaningless.

        The flow is more important then pressure.

        Chris........

        Extract from the Merc service manual..

        Click image for larger version

Name:	pump output test.PNG
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        Last edited by achris; September 7th, 2019, 10:31 PM.
        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
        The world takes on a whole new perspective when viewed from 100’ below.
        1972 Bertram ‘Bahia Mar’ 20
        2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
        (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


        Comment


        • #19
          If your in fresh water Bravoits may not be a concern, but would be easy enough to check, if you can get at it. Remove the hose fitting on transom assembly

          Has the exhaust elbow gasket been replaced, and if so which ones?
          94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
          95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
          07 Seadoo GTX
          Merc Adults Only VP Adults Only
          G-Dad always said "First Liar doesn't stand a chance"

          Comment


          • #20
            Originally posted by achris View Post
            Pressure is merely a resistance to flow. If you have no resistance (like the engine, manifolds and elbows are clear of obstructions) then you could have a high flow, 60 litres/minute, and still show little on a pressure gauge. You could also have the opposite, high resistance and low flow. That would show as a high pressure. Without knowing the flow for any given pressure, the numbers are almost meaningless.

            The flow is more important then pressure.

            Chris........
            Chris, your comment is once again spot-on. Thank you for sharing the proper procedure for measuring the flow - I will perform the test as soon as possible, and I will report my results.

            Comment


            • #21
              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.

              Comment


              • #22
                Standing by with baited breath.
                xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                The world takes on a whole new perspective when viewed from 100’ below.
                1972 Bertram ‘Bahia Mar’ 20
                2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
                (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


                Comment


                • #23
                  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?

                  Comment


                  • #24
                    pressure never went above 6psi even at WOT
                    Have never thought much of the idle flow test, but would be good if it was overheating at idle

                    You either have:
                    Drive inlet ports are clogged
                    Worn impeller and/or pump housing.
                    Air leak from drive thru to pump in one of the connections
                    Bravoits

                    Don't see it being after the pump because your not building any pressure

                    Use a piece of 1 inch clear tubing and place it at the thermostat housing and inlet hose. Then take it for a run in the water and look for bubbles. If no bubbles then no air leak and then look between pump and outdrive
                    94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
                    95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
                    07 Seadoo GTX
                    Merc Adults Only VP Adults Only
                    G-Dad always said "First Liar doesn't stand a chance"

                    Comment


                    • #25
                      Originally posted by AllDodge View Post

                      Have never thought much of the idle flow test, but would be good if it was overheating at idle

                      You either have:
                      Drive inlet ports are clogged
                      Worn impeller and/or pump housing.
                      Air leak from drive thru to pump in one of the connections
                      Bravoits

                      Don't see it being after the pump because your not building any pressure

                      Use a piece of 1 inch clear tubing and place it at the thermostat housing and inlet hose. Then take it for a run in the water and look for bubbles. If no bubbles then no air leak and then look between pump and outdrive
                      I suppose anything is possible however I don't see how I could have a clogged inlet, Bravoitis, etc and still manage to get a flow rate of over 12 gallons / minute at 1000rpm. I am also not overheating - it does't matter if I am idling around or if I am WOT - temp doesn't change.

                      Since this is a fresh-water cooled motor I would have to find a different place to put a piece of clear hose...it is worth a shot I suppose.

                      Comment


                      • #26
                        I’ve never checked my pressure at cruise speed because I’m using a scan tool for the values but I can confirm at idle I’m at .7 psi also so perhaps that’s normal. I’m not getting any low pressure alarms are you?

                        edit-that’s with a new impeller this year
                        2004 Trophy Pro 2359
                        Mercruiser 5.0 MPI OM656133
                        Bravo 3 OM598515

                        Comment


                        • #27
                          Originally posted by metsfan3197843 View Post
                          I’ve never checked my pressure at cruise speed because I’m using a scan tool for the values but I can confirm at idle I’m at .7 psi also so perhaps that’s normal. I’m not getting any low pressure alarms are you?

                          edit-that’s with a new impeller this year
                          I haven't gotten any alarms since I replaced the pressure sensor, but Mercruiser is telling me that the pressure at idle is supposed to be greater than 2psi and anything less than that could trigger an alarm (though I havent been able to find any confirmation)


                          P.S. My impeller is also brand new and so is my WP wear plate.

                          Comment


                          • #28
                            The original post was because the motor went into guardian mode, so there was a low pressure issue either by sensor or actual. The sensor was changed and have same readings, so guessing the sensor was not the issue.

                            Fresh water = closed cooling and gets mixed up quite often with folks boating in fresh water. Just something we will always deal with, because folks have not seen a HX filled with "fresh water", always AF

                            Tubing can be connected to inlet of the HX

                            Use a Tee fitting so you can have both sensor and mechanical gauge installed.
                            94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
                            95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
                            07 Seadoo GTX
                            Merc Adults Only VP Adults Only
                            G-Dad always said "First Liar doesn't stand a chance"

                            Comment


                            • #29
                              Sign up today
                              Originally posted by AllDodge View Post
                              The original post was because the motor went into guardian mode, so there was a low pressure issue either by sensor or actual. The sensor was changed and have same readings, so guessing the sensor was not the issue. - That's not entirely correct - the thread was started because the motor went into guardian mode due to a pressure sensor which was reading 0.0psi. After I replaced the sensor I am no longer going into alarm, however the pressure readings remain "low". I have not been able to find any published values those values are supposed to be - according to Mercruiser customer support, the magic number is &gt;2psi at idle, but in reality folks seems to be getting anything from 0.7 to 5 psi at idle.
                              The only reason I keep re-hashing this is because I want to prevent my engine from going into Guardian mode because of the same issue again. Frankly, I couldn't care less what the pressure value is - all I care is if my engine is operating at the correct temperature.


                              Fresh water = closed cooling and gets mixed up quite often with folks boating in fresh water. Just something we will always deal with, because folks have not seen a HX filled with "fresh water", always AF - Point taken - closed cooling it is

                              Tubing can be connected to inlet of the HX - I need to look at the routing of the hoses but that sounds like a good spot

                              Use a Tee fitting so you can have both sensor and mechanical gauge installed. - I could do that, but I feel like I already proved that the replacement pressure sensor is reading accurately. I had some doubts initially, when I compared it's values to an inexpensive mechanical gauge, but it turns out it was the mechanical gauge that was reading inaccurately. Unfortunately we are splitting hairs anyway, since we are dealing with such low pressure values (for ex. 0.7psi vs 1.5psi)
                              Answers in bold

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