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DF90 RPM Varies During Beginning Of Trip

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  • DF90 RPM Varies During Beginning Of Trip

    I need some direction, please. 2011 DF90 with 275 hours which was running flawlessly until recently. The last three trips out now include the RPM varying seemingly a couple hundred RPM. I don't hear it until I get it in the range of 2000 - 5500 RPM. It may happen below 2000 or above 5500 RPM, but I can't detect it. I can't ever feel it (as a surge), but can hear it and do see it on a Simrad digital display. Cannot see it on an analog gauge. Motor doesn't really run rough, but as if I am moving the throttle deliberately slower and then back every 1/2 second or so. I have held the throttle lever steady, so it's not that it is actually loose. I did not think about running it up to 2000+ out of gear.

    If we go at least a few miles across the lake the past three trips (different distances each time), the varying RPM goes on continuously until we get to our spot and stop for a couple of hours. The return trip is back to flawless - RPM is steady. We have ran on for many more miles on the return trip without any issues. The only thing recently was a refuel from a marina with labelled non-ethanol gas which I have used in past years without issues. Past history may not matter, though. I added a heavy dose of fuel additive yesterday before this third trip, but did not have the chance to run it today.

    I replaced plugs, fuel filter on motor and inline fuel/water separator filter at about 250 hours. Water separator window is clean. I don't find any gauge warning indicators on. Oil level indicates at the upper end of the stick's range. Water temps run approximately 138 degrees. Pee stream is normal. Running voltage is just under 14 volts and alternator is right at 15 volts both per Simrad display. Normally cruise at 4K RPM.

    What could this possibly be? Thanks!

  • #2
    Trim the motor a little deeper & see. My DF175 does this if trimmed a little too far up. Engine loads rpm down, then then (for lack of proper term) unloads/coasts rpm increases.

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    • #3
      Thanks for reply. Motor is trimmed all the way down. Any trimming up underway and SOG decreases. This is on a pontoon.

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      • #4
        How old are your batteries? These engines are sensitive to good power supply and can play games if they dont get it.

        Its possible that a battery that’s not great and is not holding charge well may charge up over the course of the trip - Which could explain it running better later on in the day?

        Also find the white wire, it should be connected to the start battery + terminal, or, of you have a battery switch, to the + side so it gets powered when the switch is on. It also has a fuse in this wire. Check the wire, its connections, and take the fuse out clean it check the fuse holder. This wire powers the ecu so its very important.

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        • #5
          Thanks Moonlighter. This makes great sense, Battery is from 2014, I believe. I was going to change it earlier only as a precaution (I have a jump box onboard) and really don't use the main battery for more than the Simrad when the motor is not running, but world events +life took over. I will replace the battery and check the white wire/fuse before the next trip out.

          Thank you very much!

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          • #6
            Fingers crossed that is the issue.

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            • #7
              Thanks again for the replies!

              Moonlighter, you need to PM me the bill for your diagnosis. Paypal? New battery and my problem went away. Previous battery was from 2013 - I should have changed it sooner, but apparently learned a lesson how the motor was giving me a warning. I probably won't remember this in ~7 years, but, still....

              I did not reinstall the old battery to confirm, but I'm satisfied.

              Thanks again, Moonlighter.

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              • #8
                Originally posted by urbnsr View Post
                New battery and my problem went away. Previous battery was from 2013 - I should have changed it sooner, but apparently learned a lesson how the motor was giving me a warning. I probably won't remember this in ~7 years, but, still....

                I did not reinstall the old battery to confirm, but I'm satisfied.

                Thanks again, Moonlighter.
                Hey, do you still have the old battery? Can you measure the voltage that is still coming of her? It would be interesting just as a reference.

                Thanks, Urban.

                Suzuki DF90A '08, model year '09 with 1502 ccm

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                • #9
                  Hi Urban,

                  Shoot. Sorry, I don't have it. But I never noticed a hint of trouble when starting. While running and looking at a Simrad display, line voltage was 13.8 volts with alternator just under 15 volts.

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                  • #10
                    Possible update. Surging RPM returned after new battery was installed. This is the first time we just ran with it surging and after about 4 miles, the surging RPM settled down (and the digital fuel efficiency returned to it's higher value). A few hours later, we were returning back to the docks when the motor just quit running. No warning signal, no spuddering (sp?). The analog fuel gauge indicated 1/2 tank. I squeezed the primer bulb which did seem a little soft. Water/fuel separator inspection bowl looked clean.

                    Attempts to restart timed out each cycle without any firing. After each cycle, I can hear a double beep. I assume that is the indicator that it timed out. We were close enough to paddle in with low winds and calm lake, but I called the marina and they pulled us in. Mechanic hooked motor up to the computer, but no useful information from it as to where to start looking. He did pull the cam position sensor and cleaned it. He said the motor attempted to start, but ended up not. He is going to swap out that sensor with a known good one to see if that is it. He's not sure at this time what to do if that sensor doesn't fix it.

                    He did mention that the computer diagnostics did indicate that the oil pressure triggered two times (oil level was fine or a bit above the full mark as I check it often). While he was trying to start the motor after cleaning the sensor, he mentioned that the oil pressure triggered >200 more times. He wasn't sure that the multiple attempts to start caused those many triggers in error.

                    So, maybe the computer is either going bad or the power to the computer is not to spec. Something else I noticed a week or so ago: The digital fuel gauge on the Simrad is fairly accurate and at approximately 5/8 of a tank full when I noticed the digital gauge indicated that I had a full tank again (without me setting it to full). Maybe also related to my problem...??

                    More later.

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                    • #11
                      How did the white wire its connection to the battery and its fuse (and wires into the fuse holder) look when you checked them?

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                      • #12
                        Well, it's a red wire, but I doubt that matters. Battery terminals, wire connections and fuse all look very clean with no corrosion. I pulled and replaced the fuse multiple times in an attempt to scrub the terminals in the water resistant fuse housing. Pulling the fuse kills power to the dash and the motor trim switch on the throttle handle.

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                        • #13
                          Originally posted by Moonlighter View Post
                          How did the white wire its connection to the battery and its fuse (and wires into the fuse holder) look when you checked them?
                          The wiring is a little different than the earlier engines, even though there is provision in the loom to connect a separate 15 amp fuse to the battery, there is a red wire from the starter that goes to the fuse box where it is redirected to the main control relay then to the ignition key.

                          His mechanic should be able to diagnose whether his cam sensor was faulty or not, not swap it out, it would have brought the check engine light on if the sensor stopped switching. When the engine is in a no start condition it should be a lot easier to find the problem than an intermittent problem.

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