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Idling RPM

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  • Idling RPM

    Hello- I am a bit new to boating. I have a 2950 ProLine 1996 with twin Mercruiser 5.0 MPI engines with Bravo III outdrives.

    my question is the port engine idles at about 1,500- 1,300rpms while the starboard is at about 1,000-1,000rpms.

    Should I be concerned about either engine at this point? No clunking, knocking, smooth sounding engines currently.

    any advice etc will be much appreciated!

  • #2
    Welcome,

    I'm no expert on fuel issues, but I think that is a little high. Should be around 650 rpm in gear. I'm sure some of the experts will chime in shortly,

    Ken

    1995 Sea Ray Signature Series 200
    5.7 Mercruiser(OF363726)
    Alpha I, Gen II

    Comment


    • #3
      Yes that’s what I have been told also. I am having my mechanic change out the idle air control with a new one.thabks for the reply.

      Comment


      • #4
        Welcome to iBOATS
        .
        1979 is correct. The idle should be 650-700.
        Start by making sure that both throttle bodies are returning all the way to idle.
        "Growing old is mandatory. Growing up is optional."

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        • #5
          How are you confirming the engine speeds? Dash tacho or diagnostic software?

          Chris.....
          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


          • #6
            None of the two engines should be at those rpm with any of the GM engines, never mind the MPI 5.0.
            Instant thoughts are that your boat being a 1996 and your engines being MPI, would tell me that your boat has been re engined. Are the tachometers set up right ?
            other possibility might be the troublesome idle air control valves. Perhaps unlikely on both engines and that you have no bother going into gear. I think if you were genuinely at these rpms at idle, you would know all about it. My single engine 5.0 mpi and boat are fiercely pitched prop wise for top end pace...but at 1500 rpm, would be trying to plane almost. Haven’t checked the manual, but the actual idle rpm on mine is around 650-680 rpm, from memory at the smart craft readout.

            Comment


            • #7
              Thanks for the responses. First off I am not sure if they have been re engined, but am sure somethings have been done To the engine. I am not sure about how to tach’s are set up, but like I mentioned I am replacing the AIC first on both of them. My mechanic hooked up a Mercruiser error code scanner (not familiar with proper terminology) and it didn’t give us an error with the idle, of that’s even possible.

              Ughhh sorry for being so vague, it’s not my intent. How would I know if the tachs are set up properly/accurately or not.

              Comment


              • #8
                Originally posted by Birdman34 View Post
                .... How would I know if the tachs are set up properly/accurately or not.
                Back of the tacho is a switch. It may be labeled with letters, which each represent the number of cylinders of the engine, or with the actual numbers. However, if you are using the dash tachos, that basically invalidates the readings. For an accurate reading, either the diagnostic software (on a laptop) or the digital tacho in a timing light. Dash tacho is not anywhere near accurate enough...

                I have another question. Since Merc didn't produce a 5.0MPI until after 2001, what is the year (or better still, the engine serial numbers) of the engines? (You said the boat was 1996. Which is why we don't care about the boat, only the engine)

                Chris........
                Last edited by achris; February 9th, 2019, 11:14 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


                • #9
                  You should normally have 550rpm@idle without gear and 600rpm@idle with gear engaged.
                  First of all you must be sure that the rpm readings you say are accurate comparing your tacho with a scan tool. Normally with this idle you have real problem when engage and disengage gear......
                  If your reading are accurate and you trully have this idle, then you should check the followings.
                  1. Vacuum leak (check the hose coming up of the intake)
                  2. Check IAC - maybe replace
                  3. Check the throtle plate while idling to be fully closed or not. It shoud be fully closed,....maybe the throttle cable is somewhere stucked and push the plate to partially open position.
                  4. Check for trouble codes with a scan tool like Diacom.
                  I hope you will find the solution to your problem.
                  Last edited by silver_power; February 13th, 2019, 03:41 AM.

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                  • #10
                    Also, not sure about the MPI engines, but when setting idle on a carbed engine the engine needs to be under load, , boat tied to a dock or outdrive in a test tank. MPI could be different being that they have the idle air control valve.

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                    • #11
                      There is no idle adjustment on MPI engines. The computer does that for you. Have you checked the idle speed with an accurate tacho? (Dash tacho is NOT accurate enough)..

                      Chris...
                      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


                      • #12
                        Still think you would really notice it if your engines were actually idling at that speed. Seems unlikely, especially on both. They would hammer into gear and have you away a a rate of speed. My boat is trying to plane not much above these rpms, haha.
                        As I mentioned before..the issue likely lies somewhere between your boat being of a vintage before the mpi was around and your tachometers. If you like, you could buy a VesselView mobile to check and gather all accurate engine data.

                        Comment


                        • #13
                          If you can find someone with a "secondary pickup tack" they'll work on anything with a spark plug wire.
                          "Growing old is mandatory. Growing up is optional."

                          Comment


                          • #14
                            Ok everyone. So I replaced the IAC on both engines. That took care of the high idle. Now at 700rpm on both, so within spec. I didn’t mention but I had some milky oil as well in both engines, I thought it was odd that both engines had exactly the same problems. I noticed I had condensation under both oil caps, the boat had been sitting for a while in a humid environment and was rained on repeatedly. I saw that the seals around the engine hatches were old and needed to be sexure so I actually siliconed the parts that needed it. All good there.

                            I then performed two oil changes, launched the boat and ran it for about 1 hr, brought it back in and checked/changed the oil. Milkshake is gone in both. Ran the boat another 1hr and checked oil when done again. All looked good but it was checked at operating temp. I still have to check the oil when at room temp to look for milkiness.

                            Now I have a trim issue on the port Outdrive (Bravo 3) engine won’t trim up but trailer function works.
                            sorry to change the subject, but any one have any ideas with this new issue?

                            Comment


                            • #15
                              Sign up today
                              Originally posted by Birdman34 View Post
                              Now I have a trim issue on the port Outdrive (Bravo 3) engine won’t trim up but trailer function works...Sorry to change the subject, but any one have any ideas with this new issue?
                              Sounds like you may have an issue with the trim limit switch on the port side of the gimbal ring either shorted out internally, misadjusted, wiring issue inside the boat or operation switch at the controller.

                              Here's a video to help get an idea started for you..., https://www.youtube.com/results?sear...Part+1&sp=mAEB
                              OEM Quality Gear Lubes And Greases Available @ iboats.com 'By Bel-Ray Marine Lubricants -- BOAT SPECS -- FORUM GUIDELINES -- Volvo, OMC & Merc Stern Drive Technical Information For DIYers. -- OEM VOLVO PARTS POWERED BY iBOATS.COM -- All OEM Parts / Parts Lookup Now Available Through iboats.com --

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