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350 MAG MPI high idle and suction noisie

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  • 350 MAG MPI high idle and suction noisie

    hey guys I have been combing through form after form and spent a lot of money still can't fix the issue. I have a 350 mag MPI tournament ski serial number 0L671154 has 239 hours on it. I can't get this stupid thing to stop the suction noise coming from inside the throttle, before you tell me I need a IAC or the IAC muffler mine doesn't have the muffler.I have replaced the IAC with a new Mercruiser one. still making the suction or hissing noise and she won't go below 1000 RPM. and boggs down when trying to pin it from idle. if I go slow till about 1800 RPM I can pin it and she's perfect no hesitation. I have replaced fuel injectors, fuel filter, fuel pump (ouch) IAC, throttle body, coolant temp sensor, air temp sensor, pvc valve, TPS, MAP sensor, I AM SO FRUSTRATED, BUSTING OUT ANOTHER THOUSAND. been on the phone with merc they said do a leak down test? compression is perfect. it only has 239 hours on it. can yall please help. I can't find a vacuum leak any where. I've sprayed WD40 around throttle body nothing, around PVC valve and hose to the throttle body still nothing. around valve covers nothing. when I pull the vacuum line off on the other side going to the fuel pressure regulator nothing changes nor are there any leaks I can feel a little suction but that's it. that all the vacuum system right?. and no fuel coming out of the fuel regulator either from the line between it and the throttle body? line is dry I think that good....exhaust I can't see being the problem she has a lot more then normal coming out the back... white too not black but does leave black soot on the boat when in the water. please help! I'm stumped.
    Last edited by achris; September 13th, 2019, 05:24 PM. Reason: Capitalise the serial number letter...

  • #2
    Have you put a vacuum gauge on the intake manifold to actually measure to see if you have a leak?
    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)


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    • #3
      This is a long shot, but will not cost any money to check. Remove the PCV valve from the valve cover and plug the hole. On the other side remove the vent hose. Warm the engine up then put your thumb over the grommet where the vent hose was. If the oil filler cap is vented then seal it off also. You should not feel a suction, but if you do then the intake manifold is cracked on the underside. This may not be the problem but worth checking.
      Kenner, Louisiana 1980 Donzi 18'. 2+3 6.2 MPI Alpha 1

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      • #4
        I have not... how do I do that? as In where do I hook it up and what am I looking for I have a vacuum gage. as far as the vacuum system on this the only external parts are the PVC valve and line and then the line going to the fuel pressure regulator correct?

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        • #5
          hey Kenny thank you for the response. I will do this test first thing that there is lite I'm out in Tennessee so its dark now.

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          • #6
            hey Kenny the intake manifold is just where the throttle body is correct?

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            • #7
              All you want to do is seal off the crankcase. Leave the vacuum line to the fuel regulator connected. You do not need the vacuum gauge for this test. You just want 1 vent to the crankcase that you can put your thumb over to feel if there is suction at that point.
              Kenner, Louisiana 1980 Donzi 18'. 2+3 6.2 MPI Alpha 1

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              • #8
                You are correct about the throttle body on the intake manifold. Good luck
                Kenner, Louisiana 1980 Donzi 18'. 2+3 6.2 MPI Alpha 1

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                • #9
                  when I remove the PVC valve the rpm goes up and I can hear the suction go louder.. I am assuming this isn't the same thing as the valve cover isn't blocked and Im not sure on the oil cap but I block it to be safe.

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                  • #10
                    Water temp sensor suspect here.

                    But firstly, have you tried unplugging the iac whilst it’s running ?

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                    • #11
                      I have unplugged the IAC and still nothing changes stays at 1000rpm and suction noise continues

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                      • #12
                        I have replaced the water temp sensor as well

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                        • #13
                          Check if the throttle cable is stucked and pushes the throttle plate . While idling check the cable and see if the rpms drop. Plug a scan tool and check tps and water temp sensor as well. All these assuming the iac you installed is working properly, and you do not have a vacuum leak. Summary I would suggest the followings.
                          1. If you have scan tool access check iac, tps, water temp sensor and MAP sensor. Watch the MAP reading before start the engine ...it should measure equal to baro pressure.
                          2. Plug a vacuum gauge to check the actual vacuum. With operation temp and low rpm (550rpm) you must have above 14-15mmHg and STEADY. With the high idle you have now, if you have below 10-12mmHg then you probably have a vacuum leak.
                          3.Check the throttle cable.
                          I have to tell you that it happened to me to change the iac while I had high idle( I had 2beeps/min) and after the change the beeps stoped but the idle was still high. The new iac was faulty as well......
                          I hope you will find your problem.

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                          • #14
                            Silver power thank you I am in the process of changing fuel injectors out once I get it back together I will start some of the steps you listed what scan tool would you recommend?

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                            • #15
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                              One other thing to look at is the throttle body. Hold it up to the light to see if the butterfly seats completely in the housing. If the housing is distorted that will cause a high idle also.
                              Kenner, Louisiana 1980 Donzi 18'. 2+3 6.2 MPI Alpha 1

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