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Merc 7.4L (454) MPI Fuel Pressure

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  • Merc 7.4L (454) MPI Fuel Pressure

    Anyone know what the factory fuel pressure setting is on a 2001 Mercruiser 7.4L MPI at the fuel rail Schrader valve? I'm trying to diagnose a possible faulty (or clogged) injector vs. a possible failing fuel pump. Any help is appreciated.
    2001 Carver Voyager 374 'Stick's Competition'

    Something tells me I should be on the water....

    If I answer your upholstery questions, it's only because I've been doing it 20+ years...


  • #2
    Re: Merc 7.4L (454) MPI Fuel Pressure

    43psi
    just because you found it that way does not mean it is supposed to be that way.

    Comment



    • #3
      Re: Merc 7.4L (454) MPI Fuel Pressure

      Thanks.
      2001 Carver Voyager 374 'Stick's Competition'

      Something tells me I should be on the water....

      If I answer your upholstery questions, it's only because I've been doing it 20+ years...

      Comment



      • #4
        Re: Merc 7.4L (454) MPI Fuel Pressure

        welcome
        just because you found it that way does not mean it is supposed to be that way.

        Comment



        • #5
          Re: Merc 7.4L (454) MPI Fuel Pressure

          Fireman,
          What's your particular issue?
          I had an issue with my Mag MPI (454) where it was down on power.
          I didn't know it for a while as the previous owner had "adjusted" the tach to read higher RPM that actual. Once I put a GPS on it, I knew I had an issue.

          I have a 1998 MPI, which I believe is MEFI-1. The self diagnostics are very limited. I do have the scan tool (now), but the boat showed no diagnostic errors.

          To find it, the boat had to be run on a hose - either the injector clips are hard to get to, so you'll basically need to pull the plug wires one at a time. Each plug wire should result in a drop in RPM.

          Reading the plugs can tell you also...

          It's very odd (in my experience) for an injector to die.. I'm new to the marine stuff. The shop that ended up working on my boat replaced the injector (around $200 for a single injector) - I really don't know if it was dirty or dead, I still need to get back there and ask.

          There's a guy on the web that does great work with injectors, inexpensive, and he'll replace the pintle caps and rubber gaskets for under $20/each: ("www.witchhunter.com")


          I'm curious about this also because I'm still chasing another 6-7mph top end on my boat. It's running 61-62 on GPS, rather than the 68.5 that was tested in original reviews. I might not have any issues at all, but to figure it out for sure, I'm considering putting it on a dyno. The bad news is that the dyno run costs around $300, $400 with air/fuel readings... Pretty expensive...

          email me directly if you'd like: d_ginther@hotmail.com

          Comment



          • #6
            Re: Merc 7.4L (454) MPI Fuel Pressure

            Originally posted by Fireman431 View Post
            Anyone know what the factory fuel pressure setting is on a 2001 Mercruiser 7.4L MPI at the fuel rail Schrader valve? I'm trying to diagnose a possible faulty (or clogged) injector vs. a possible failing fuel pump. Any help is appreciated.
            Can you give more info? Are you still chasing the same problem as in your previous thread about high fuel consumption?

            A failing injector will cause a totally different set of symptoms than a fuel pressure issue, but not a bad idea to check it. As Capt said, 43PSI (with the vacuum line disconnected from the regulator).

            I don't recommend pulling the plug wires as suggested, on a computer controlled engine the ECM will compensate when a plug wire is pulled and the results will not be of much value.

            What really needs to be done is both engines connected to a laptop for a "side by side" comparison of engine data under load, as shown below.

            Comment



            • #7
              Re: Merc 7.4L (454) MPI Fuel Pressure

              His motors are 2001, someone with more MPI knowledge will have to indicate if this version of the MEFI system is capable of self-diagnosing an injector issue. My understanding is that some of these systems can self diagnose by dropping a single cylinder at a time and looking for the corresponding change in RPM.

              He's already indicated that he has a significant performance difference between the motors:
              The port side makes 4500 RPM at WOT but the starboard will only make 4000 RPM
              What would the purpose of detailed diagnostics be if we already know that they're not coming close to performing the same?


              I'd also think that a dead / significantly clogged injector would show when you have a look at the plugs also... It'd at least give you an indication of which cylinder or cylinder(s) are acting up and you could chase from there.

              It was hard for me to hear the boat running on 7 cylinders, but a good marine mechanic with an ear for V8s should be able to hear it if the cylinder isn't firing.


              Dave: What are you using for datalogging?

              Comment



              • #8
                Re: Merc 7.4L (454) MPI Fuel Pressure

                Originally posted by dcg9381 View Post
                It was hard for me to hear the boat running on 7 cylinders, but a good marine mechanic with an ear for V8s should be able to hear it if the cylinder isn't firing.
                You can also tell by pointing a cheapy infared temp gun at each exhaust port while under load . . . Significantly colder one ain't happenin'
                Torque gets you there. Horsepower gets you there in time for dinner.

                Comment



                • #9
                  Re: Merc 7.4L (454) MPI Fuel Pressure

                  Originally posted by dcg9381 View Post
                  His motors are 2001, someone with more MPI knowledge will have to indicate if this version of the MEFI system is capable of self-diagnosing an injector issue.
                  It would actually have to be a 2000 model year, that was the last year they made the 7.4MPI. That would make it a MEFI3 controller which has no misfire detection, and no way to self diagnose an injector problem.

                  Originally posted by dcg9381 View Post
                  What would the purpose of detailed diagnostics be if we already know that they're not coming close to performing the same?
                  Diagnosing any problem is about gaining information, and some information (Ignition Control, Knock Retard, Injector Pulse Width, ect.) is not available any other way. Computer controlled engines have a "mind" of their own, and you've got to know whats it's "thinking".

                  Originally posted by dcg9381 View Post
                  Dave: What are you using for datalogging?
                  Diacom software and a twin engine data link harness.

                  Comment



                  • #10
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                    Re: Merc 7.4L (454) MPI Fuel Pressure

                    Hey! Thanks for all the responses. Good memory on the starboard / port engine difference. Summary:

                    1) Fuel consumption issue turned out to be a faulty fuel flow meter. One was gummed up, showing a lower amount of fuel use than there actually was (isn't that always the case).

                    2) Starboard engine only reaching 4000 RPM when the port was getting to 4500: out of balance prop. I noticed a shudder when running one engine at a time. Both have been reworked on pitch so the engines reach 4250 each.

                    3) Found actual fuel pressure range to be 37 - 43 PSI. Both engines tested and found to be 35 PSI @ idle and 33 PSI @ 2000 RPM (no load). Tech support says this is normal and spring pressure in the pumps override the electrical pump under load and pressure comes up 4-5 PSI. As long as both engines are identical, I have no reason to doubt him.

                    4) Changed the fuel /water seperator (again) and seems to have eliminated the problem. I'm not sure what would have caused it as the previous filter was only a month old, unless it was 'shelf old' and was causing a restriction inside.

                    I took the boat for a 4 hour run today and it performed very well. Planed out well and constant cruise @ 3300 RPM without a hiccup. Of course, this was burning 20 gallons of fuel per hour, but what the heck...gas is cheap, right? Only time will tell if the problem reoccurs. Now to change the impellers, since the previous owner just informed me he hasn't changed them since....ever!
                    2001 Carver Voyager 374 'Stick's Competition'

                    Something tells me I should be on the water....

                    If I answer your upholstery questions, it's only because I've been doing it 20+ years...

                    Comment


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