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Mercruiser 350 Mag MPI Low Fuel Pressure

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    #16
    Glad you located what seems like most of your problem!

    I’m waiting on the distributor cap, rotor, and wires to come in to try on mine. (New plugs in already) While waiting, I’ve been reading through the service manual #24. I discovered that the “alarm” (buzzer, horn, whatever you want to call it) is mounted but both leads are cut. Also learned that despite all my gauges reading normal operating, there are also switches for various items that *might* interrupt normal operations? Although I’ve never seen high temps, low oil levels, or low pressures (aside from the initial low fuel rail)- I’m wondering if maybe I have a bad switch in the system. I have not found a specific readout in the service manual that states “holds engine at XXXX Rpm” it does state it would interfere with normal operation. I would think I’d see a trouble code if a switch was shot but I haven’t seen any. I’m planning to at least check the Transmission temp switch (normally open) for continuity, as it is the only switch on my 350 black scorpion called out in the wiring diagram. I also need to somehow figure out how to wire back in the alarm buzzer- it could tell me something I’ve been missing! Any other thoughts on things to check when I replace the ignition items listed above?

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      #17
      !
      Fixing everyone elses boat just so I can use mine...

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        #18
        Was finally able to get the boat on the water again. Replaced the distributor cap and rotor, along with spark plug wires and checking continuity on those couple of switches. Everything went good in the driveway, but same problem once we got on the water. I’m at a loss now, planning to take it into the shop and go from there. So much for a Labor Day weekend on the water now!

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          #19
          Finally found the problem! Took the boat to the shop, as I mentioned. They were no help- said that everything tested as good as a brand new boat. I started thinking about it- and had already checked inlet air, verified fuel, and verified spark. I also checked any switch/sensor that could give a limp mode. So what else could it be? The ONLY issue I could see is an exhaust restriction.

          I picked up the boat and pulled the whole system. I thought it could have been a flapper wedged shut in the riser. (It was not). So I kept pulling parts out. Turns out that my muffler has a moving float/baffle inside That had cracked in half and wedged itself in a way the restricted nearly 3/4 of the exhaust flow out of the boat. I couldn’t believe it! Can’t make this stuff up. So I took a 3” hole saw and cut through the baffle on each outlet. Took it to the lake and the boat ran perfect. We will eventually replace the muffler but for now it’s fine. There is a small increase in exhaust sound at lower speeds but barely noticeable.

          So glad she’s up and running great again! Wish I had thought to start the whole process by checking my muffler fluid, would have saved weeks of diagnosis.

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            #20
            Well played sir!!! That's an anomaly if ever there was. Good job!!!
            Fixing everyone elses boat just so I can use mine...

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              #21
              We're not just about installing fire alarms—we're dedicated to ensuring your safety with a deep understanding of New York’s diverse environments. Whether you're in a busy city area or a peaceful suburb, our team brings expertise and personalized solutions to meet your needs and comply with local regulations. Your well-being is our top priority!

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