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Twins DF175A both running rough

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  • Twins DF175A both running rough

    After a 100 hour oil change (on the lift, no sea trial) I ran both engines and after cool down checked oil levels. All good. A few weeks later I was finally able to run the boat. After idling several hundred yards to open water I went to launch and within a few seconds of each other both engines started to run rough. Neither engine will go over 4000 rpm. Normal cruise is around 4000 rpm with top end 6000 rpm. Engines are 2019's with +/- 210 hours. Both engines run rough through the full throttle range. Idled back to dock and began to troubleshoot. I have changed fuel filters, drained each vst, no water no debris in either. Fuel water separators clean and free of any water. The only common elements is the fuel source ( single tank with two pickups) and boat ground system. No codes or errors indicated on C-10 gauges. The engines act as if they are in a fail safe or limp home mode... The fuel tank was filled last September (150 gallons) with a couple of partial adds of 45 gallons or so once in Sept 23 and again in November. So while the fuel is not fresh out of the pump, it has not gone stale yet. I only purchase ethanol free fuel. Any thoughts?

  • #2
    Common vent on tank?
    Common battery isolation switch?
    Just asking?


    • #3
      - agree with Solarman, check the tank vent in case those mud wasps have made a nest there and blocked it up
      - seen a few reports of small birds making nests inside the cowl thus blocking the air intake, so check that. Sounds odd but it does happen.
      - check fuel lines between tank and engine for kinks, damage, loose, and tighten up all hose clamps.
      - check that all batteries are fully charged and in good condition


      • #4
        Each engine has its own battery with individual switches. The batteries are maintenance free marine starting batteries, both are less than 1 year old. All batteries are maintained by an onboard charger/maintainer. Batt voltage reads 13.6 volts. I also during troubleshooting switched the battery switches 1 to 2 and 2 to 1. The tanks vents through the cap and filler tube. I did not see any obstructions along the vent.


        • #5
          Possibly due to being on same boat, and same environment check both intake breather covers and cowlings for nests/blockage restriction of air flow?
          Then check TVS on both motors, as they are in same environment, might be sticking/ not moving freely?
          Then maybe inspect/check other sensors (map, oxygen, knock)?
          Emergency cut-off switch, single or duel?
          Have motors been serviced (approx 5yrs old)?
          Quality of fuel? Trustworthy station/ truely marine fuel?

          Just thinking of common areas/ common environment?


          • #6
            Sounds like old gas, idles and low speed fine once you push the engine it falls flat on it’s face. Try running one engine on a portable tank with fresh fuel, drain the VST on one engine and hook up a portable tank.


            • #7
              They do not spit and sputter, nor do they stall out. Both runs as if a cylinder is disabled. This apparent through the full throttle range. Even in this state I can reach 4000 RPM and get on a plane. I do not believe the gas is bad, but I will connect a small can of fresh gas to eliminate or confirm bad fuel.


              • #8
                We are not sure what all was done at the time of oil change? Was it done by you, or where boat is stored/Marina, or Suzuki shop?

                With 4k rpms or above, it's not sounding like "fail-safe".

                Then it could be plugs, plug wires, or a coil?
                Any chance a plug(s) on each motor is bad, gap is wrong (have had new plugs bad from factory). Another possibility is poor connection on coil wire/ground?

                Could pull plugs (use a piece cardboard to keep in same order -top, bottom/ right bank, left bank- to compare side by side) to see if any plugs are black/wet or clean/dry (all plugs should look the same).

                Get back to us to let us know a little more of the details.


                • #9
                  For lack of another cause. I'll go through this. It is just so odd that both engines were affected within minutes of each other. And the odds of the same failure on two separate engines are pretty low.
                  I changed the oil. I cleaned and inspected per Suzuki's recommendations as presented in the owner's manual.


                  • #10
                    Ran new gas through it. No change


                    • #11
                      This was the next morning after changing fuel filters and basic rroubleshooting.


                      • #12
                        I'm guessing both motors are ~ same age, built at same factory, have operated for about the same hours, in the same environment, have about the same wear, etc.

                        How long since the last qualified service? Valves checked, cam chain & tensioner checked?

                        Your video tells me that a qualified Suzuki technician should do maintenance on both motors? Are the motors still under any warranty? They are about 5 yrs?

                        Good luck, post back on what you find out.


                        • #13
                          that is really rough!!!
                          question-- do you use the engine interface cable to monitor rom fuel usage etc?
                          mine apparently went bad .. mine was throwing all sorts of codes and would run about 4000 rpm and no more
                          disconnected cable and problems went away
                          a couple of things to try
                          remove the fuel tank cap -- if there is a problem with the vent you will know right away
                          when running- observe the small fuel filter on the back of the engine .. should not have air bubbling through it. if there is air you probably have a bad fuel hose connection or a bad fuel bulb... and the fuel bulb should be positioned vertically and double check the fuel is flowing through it correctly as there is a one way valve inside
                          and I agree with youthat the possibility of two engines having the same problem is very remote..
                          really leads to just about one thing in common / fuel
                          Last edited by artdf175; 04-24-2024, 10:58 AM.


                          • #14
                            Ok here is the latest. I connected Suzuki SDS to laptop. No codes, but I was able to determine that there is no fire to number two cylinder. This is the same for both engines. This has to be something the ECU has done...I have exhausted my expertise, so it is time to haul from the water and take to a Suzuki Certified Mechanic. I may be able to update in about 6 to 8 weeks.

                            For what it is worth. The engines are at 211 and 212 hours. Purchased at teh same time, came over from Japan on the same boat. So I assume manufactured around the same time. Suzuki mechanic did the install and first service. I did the 100 hundred and the 200 hundred hour service. While not a certified mechanic, I am mechanically inclined and can handle the lower tier service requirements. Over the winter break I take the boat out regularly. It did sit for about 4 weeks prior to the 200 hour service.


                            • #15
                              I take it that you have since pulled #2 spark plugs and leads to check them out, and at least make sure all connections are good?

                              I will be interested to hear the results when the pros get their hands on them.

                              If this forum is still alive then…..