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Diesel generator rough running

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  • Diesel generator rough running

    I seek your input to a problem I have with the rough running of my 15KVA diesel generator.

    On startup it runs very smooth; deviation in displayed RPM is within 1 or 2 RPM of 1550 and only changes slowly over that range.

    My observation is that it runs smooth for around 10 minutes then commences to run rough; the motor rocks on its mounts and the displayed RPM bounces around in about a 25 RPM range.

    The transition from smooth to rough is consistent. That is, it always runs smooth on startup then after a while goes to rough mode.

    The unit has only 35 hours on it. It ran smooth when first purchased 18 months ago.

    I have made a short video so that yous can see and hear what I am talking about.

    It was suggested that the fuel maybe "old" or just bad so today I put the pickup and return hoses into a container of fresh diesel. The fresh fuel made no difference ... still ran rough after warmup.

    Running from the container of fuel did allow me to establish that there is good pressure on the return line and that the return line is not blowing bubbles.

    My apologies for having the camera (phone) upside down when displaying the control panel at end of the video.

  • #2
    Tell us about the exhaust; does it smoke when cold, hot, in between, never only when running rough? I think that you need to eliminate a few things. You've done the first step, no bubbles in the return fuel flow. a check of valve clearances is also in order. If you have a thermometer, check each cylinder at the exhaust manifold area for operating temp, it may help isolate which cyl is malfunctioning. Cracking the fuel line at each injector while the engine is running rough may also show which is at fault.

    Beyond the simple tests to isolate the problem, I'm guessing that there is a fuel delivery problem due to a sticky injector or a delivery valve problem at the injector pump. The tolerances in diesel fuel systems are extremely tight and sometimes microscopic contamination will give you grief.

    I couldn't make your video play, so I'm shooting in the dark


    • #3
      Originally posted by Tim Clarke View Post
      I couldn't make your video play, so I'm shooting in the dark
      Same experience, video won't play. 35hrs on the clock and it's out of warranty?


      • #4
        Says video is not available.

        Does it have a water separator? Have you changed or drained the fuel filter in the past 18 months?


        • #5
          If you haven't haven't changed the fuel filter I'd consider that. Also check the air filter. After that I'd look at Tims suggestions. When checking the injectors do crack all the lines at least once to be sure its just one cylinder.


          • #6
            Am not sure what is problem with the video ... was my first upload to YouTube and it works for me from the link.

            Here is a link to the raw mp4 ... beware it is 250mb.

            Reggie; unit came with a 12 month warranty. Supplier was most cooperative during the warranty period for a couple of other issues.

            Tim: your shots in the dark feel good to me. I have been feeling that the "intake or exhaust valve adjustment" is out due to some settling (perhaps poor initial adjustment) such that on warm up the stem grows. I do have an IR Thermometer; will gather detail of your points and post back in next half hour or so.


            • #7
              I know this is obvious but whatever changes over at 10 minutes will tell what the problem is. Is there a pre-heater until it gets warm or something electronic changing the fuel flow? I'd call the company because if it runs smooth at start & warm up it's not too serious but without knowing what brand or what changes you'll just get a bunch of guesses here. One call should do the trick.
              One other thought, are you running off other power & have a auto switch that lets the genset warm up for about 10 minutes & then switches the load to the genset? I had that on a couple of mine, just a thought.
              Last edited by flylo; 12-26-2017, 10:41 PM.
              "Let me recommend the best medicine in the
              world: a long journey, at a mild season, through a pleasant
              country, in easy stages."
              ~ James Madison


              • #8
                Change the secondary fuel filters. They should be inline close to the fuel rail near the injectors.


                • #9
                  Ok; here are my stats thus far.

                  On startup the smoke is white for just a puff or two ... then goes pretty much clear. My recollection is that it ran more black in the early days with a solid stream of some blackish colour pretty much continious.

                  The internal of the pipe is a dry black.

                  I measured the exhaust temps with an IR Thermometer pointed at the exhaust manifold flange (near head) of each cylinder.
                  103 degC, 115degC, 110decC, 101degC.

                  Have cracked the injector pipes for each cylinder where they come out of the injector. For each of these, once cracked to release fluid, the motor ran smoother and there was a spurt of diesel fluid. To my eye the spurt was without bubbles and was consistent for each. On re-seating each pipe the motor returned to rough running.


                  • #10
                    Originally posted by flylo View Post
                    ... are you running off other power & have a auto switch that lets the genset warm up for about 10 minutes & then switches the load to the genset? I had that on a couple of mine, just a thought.
                    The tests are being done with the auto switch in manual mode ... the voltage output is commenced near immediate.

                    There is a warmup mode but it is set at 10 seconds. Likewise cool down etc are all 10 seconds.


                    • #11
                      The link to the YouTube video should be good now. For me (the publisher) there is need to click "Publish" after the upload streams.

                      Last edited by Norman Bain; 12-26-2017, 11:41 PM.


                      • #12
                        Did you change the secondary fuel filter? It's the smaller blue filter. When the secondary starts going the injectors don't get full pressure and it will shudder like that.


                        • #13
                          Well, it sure does shake. It doesn't sound like it is rough or misfiring at all to me. I believe that it's a governor issue, dropping and gaining RPM quickly. Does this change at all with load variations? The engine doesn't look familiar to me, but the fuel system does, especially the nozzles. Reminds me of the British C.A.V. systems, or the Kiki systems of Japan. It sounds like it is "hunting" for it's setting, also called Hi Idle, which is controlled by the governor based on RPM and load. While it won't hurt to change fuel filters, this does not look like a fuel starvation issue to me.

                          Also, I have no experience with power plants, only heavy and medium truck engines. I wonder if generator issue could cause this? ( rapidly changing load)

                          It's getting along in the evening here, I'll check back in the morning


                          • #14
                            Change that secondary filter then show us how beautifully she runs afterwards.


                            • #15
                              I have to agree with Tim Clarke. I also don't believe this is a fuel starvation issue. The engine would exhibit the same running condition hot or cold if it was starved for fuel or if you had an air leak on the lift pump side, (sucking air) this too would not be temperature specific and would be present warm or cold.
                              The injection pump hunting for a steady idle is what it sounds like to me. What does it do when subjected to a heavy (full) load? Or when manually set to 1,900-2,000 rpm?

                              Another factor could be that the injection pump's timing could be of the type that changes injection timing when cold and then switches to a hot running condition timing setting. I see a lot of newer injection system with this feature. Emissions are one of the key motivators for this change although more optimal timing to suit operating conditions are also driving factors. This would definitely be one of those things that change when warm and I have seen it affect diesel engines if not functioning correctly.
                              Not knowing the intricacies and specifics of your fuel system I can only hazard a guess. I'm not sure what if any engine management systems are in place but sometimes one can fool the pump into thinking that the engine is cold by defeating it's temp signal. If it has one.
                              Home, down in the valley behind the Red Angus
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