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2016 DF300 Fuel Problem

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  • 2016 DF300 Fuel Problem

    2016 DF300AP 600 hours

    Symptoms Day 1

    1. First occurrence under load - after running WOT for 20 minutes, engine RPM's shot up extremely high and engine lost significant thrust. Reduced throttle down to idle and put in neutral.
    2. Every time after under load - place in forward gear and slowly increase throttle which provides direct slowly increasing effect on RPM and thrust until I get to ~1900 RPM's. Around 1900 RPM's while slowly increasing throttle, RPM's begin to exponentially increase to red line despite not moving throttle. During this phenomenon, the engine loses thrust to the point that it feels like the propeller is cavitating. From start of 1900 RPM's to red line is about 2-3 seconds.
    3. Not under load in throttle only mode - slowly increase throttle which provides slowly increasing effect on RPM's. When I get to ~3000 RPM's, the engine starts to run rough and shake.

    Symptoms Day 2

    2. Under load - same symptom but occurs between 2350 & 2700 RPM's.
    3. Not under load in throttle only mode - same symptom same RPM's (3000).

    Items Checked or Replaced

    1. Replaced fuel lines and fuel clamps from tank to engine
    2. Replaced Racor, primer bulb, & Racor filter
    3. Occurs on both fuel tanks separately and together
    4. Occurs with gas tank caps separately open and together open
    5. Replaced fuel filter (15412-93J10)
    6. Replaced low pressure fuel filter (15643-93J00)
    7. Replaced high pressure fuel filter (15440-93J00)
    8. Occurs on multiple separate batteries
    9. No pickup tube filters in gas tanks

    Observations & Next Steps
    1. I believe I hear the low pressure fuel pump turn on when ignition is turned on
    2. Check for symptoms while running engine from a 3 gallon can right at the engine
    3. Purchase and install fuel pressure gauge to test fuel system at engine - low pressure fuel pump, high pressure fuel pump
    4. Remove VST, clean & inspect, replace high pressure fuel pump pickup filter screen (15642-90J00)

    Thoughts on other things to check and next steps?

  • #2
    Sounds exactly like what happens when you have spun the hub in the propellor! I would say with 99% certainty that this is the problem.

    What happens is that the rubber hub bush deteriorates and loses grip on the prop. When there is low load on the prop, it has enough grip to hold on, but as soon as the load increases, the rubber lets go and the revs spike but the prop doesnt spin. And you get that sudden massive over rev happening.

    Your description of what is happening is a classic description of a spun prop hub.

    Bit surprising that someone hasnt already pointed this out and saved you all the time and expense of throwing those parts at it.

    You can get the prop re-bushed, although some people find that if the prop hub has been damaged the problem can then be recurring. They actually freeze the rubber bush in liquid nitrogen, press it into the hub, and when it expands it then has a very high grip on the prop hub.

    I understand that Suzuki’s new Watergrip props have a different hub/bush system that provides greater resistance to propellor slip and can be replaced without needing a press.


    • #3
      Moonlighter, appreciate your perspective. When in throttle only mode (no load to prop), would you expect the engine to run rough at 3000 RPM’s and above with a spun hub? Or maybe 2 problems, spun hub and fuel?

      My initial feeling was also something mechanical, until I ran it in throttle only mode at higher RPM’s.

      Would love to share a quick video of both. Do you need to host them on another site and just share the link?
      Last edited by Yosuthnmasa; 09-13-2021, 09:54 PM.


      • #4
        Regarding your point #3, when the engine is in neutral gear, and you lift the throttle lever to increase rpm, the engine’s ecu will limit rpm to 3000rpm. At 3000rpm the ecu cuts fuel, restricts spark and advance, and the engine will shake, rattle n roll! Same as a race car that is on the pit lane limiter. It runs BAAAAAD!

        So what yours is doing when revved in neutral is exactly what its expected to do, and nothing to worry about. It is designed to prevent engine over revving in neutral and causing damage.


        • #5
          Consider this …..

          Say you are running along at max rpm. Been going for some time so the engine and gears have all heated up much more than they would at a normal cruise speed. The boat hits a wave just big enough so the engine comes out of the water momentarily, the prop loses grip, and then its back in the water and suddenly gets 100% grip again. And on the 2 hours trip home you repeat this same event 50 times….

          Think for a moment just how much shock load that puts into the propellor, hub, gearbox and drive train. (Not to mention the boats transom). And with the gearbox being hotter, so is the prop shaft and the heat transfers to the prop hub and bush…..

          Its a massive, sudden shock load. Repeated again and again. Like an impact diver.

          That shock load is similar, but perhaps not quite as bad, as the shock caused by a single event such as if you hit a log.

          Remember when you had a bolt that wouldnt undo? You heat it up and its then easier to undo…. A hot prop hub is more susceptible to spinning out.

          Outboard engineers design in a weak point to the drivetrain that will give way before the more expensive parts are damaged. In this case, its the prop bushing that is deliberately designed as the weakest link.

          There are some messages to owners in there about how they drive their boats….. If its being driven on the limit, where the props are frequently coming out of the water at high revs, its hard on the drivetrain components. So boats driven that way will unavoidably have to replace and repair driveline components more frequently.


          • #6
            Here’s videos of both scenarios:




            • #7
              After watching your video I have no doubt at all that your prop has spun its hub bush.

              Take the prop off, take it to a prop shop and ask then to rebush it. It will get progressively worse the more often the prop spins in the hub.

              If you inspect the prop hub you may see evidence of the slipping. You can scratch a line across the hub and bush and see if it has moved after the next attempt to get onto plane. But really, there is no doubt that the bush has failed.
              Last edited by Moonlighter; 09-14-2021, 01:39 AM.


              • #8
                Moonlighter, much appreciated on your advice!


                • #9
                  Wanted to follow up with the results.

                  I replaced the OEM stainless steel prop and hub with a Solas stainless steel prop with Rubex 104 hub.

                  • Need a 27mm socket for the prop nut.
                  • Original thrust washer stuck hard. Hammer, tapping, prying, etc did not work. Trick to get off - hardened steel punch with hammer placed on surface of thrust washer, tapped fairly hard with hammer towards the bow of boat. 2 good taps and it broke it loose.
                  • Remove all hardware (washers & prop), replaced with everything provided with the Rubex hub kit including supplied bronze spacer nut.

                  Took it our for a quick test and no more surging high RPM’s under load at medium to high speeds. Plan is to try and see if my original prop can be rehubbed with a Rubex hub? Then carry prop and hub as a spare.

                  Thanks for your guidance Moonlighter! Hope this thread helps someone in the future.


                  • #10
                    To prevent frozen prop shaft washers apply water resistant grease to the prop shaft. Save a bunch of pounding next time.


                    • #11
                      Just be aware that even though the Solas prop might be the same diameter and pitch as the original Suzuki OEM, the blade shape always varies between brands so it is rarely “apples and apples”. So performance is likely to be different.

                      Would be worth doing some test runs to check that full power rpm’s are still hitting around 6000rpm, and that performance generally is still OK.

                      There is a “sticky” thread in the top section of this form that I put together some years ago that gives some suggestions on prop testing etc. Recommend you have a read of that too. Here is a link

                      Let us know how you go getting the Suzuki prop rebushed. Will be useful info for other members.