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7.4 Mercruiser: Surging Engine. HELP!!!!!

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    7.4 Mercruiser: Surging Engine. HELP!!!!!

    Hi All,

    I was using the boat for 3 months without any issues. Then I was out cruising along approx. (24 MPH @ 3400 RPM) and the engine seemed to start surging when I went to WOT as if I was losing a cylinder or two. I checked the engine and as I was looking it over, checking the wires, distributor cap etc. while it was running and I just barely touched the top of the Cap and the engine died. I opened the cap and checked the Rotor. I found the little slot in the inside of the Rotor which fits into the top of the distributor, was sheared off and the Rotor was just spinning freely. We got towed in.


    I ran around the corner and bought a new Sierra Cap & Rotor kit. I replaced the Rotor since I replaced the Cap last Fall. I went out to TEST RUN the boat and it seemed fine, went to WOT and back to cruise, after about 2-3 mins. around 3400/3600 RPM it started to surge again. I stopped running the boat and sat in the water for about a few minutes. Then I took off and opened her up and again it ran OK. While running now WOT for approx. 45 seconds and bringing it back to cruise approx. 3400RPM now after 3-5 mins. of running fine, it starts to surge again. Back to the dock again I go.

    I replaced the Cap now that came with the New Cap & Rotor Kit and back out I went. Seemed to run OK for about 5-6 mins. then it started to surge again. Back to the dock again. While it was running I tried to wiggle it and see if there was any play and I heard a click. It was the Rotor hitting the inside of the Cap again.

    I had my mechanic come take a look at it. We found that one of the screws that locks down one side of the cap was slightly loose. We turned the screw and although it’s tight it never stops turning. We found that while pushing down on the screw hard while turning it got it to really bite hard and now it was very tight.

    Out again I go to TEST Run it and long & short of it is, it is still surging after a few mins. of running. Again it seems to run OK for a few 4-6 mins. and then it’s surging again. It may run OK and then surge and then run OK several times every time I TEST Run her. The issue seems to be intermittent as far as the times it runs before it surges and if I go to WOT as well.

    My Mechanic said I should try replacing the Cap & Rotor again since the previous time out, I must have shave a little bit off the inside of the cap when I tried to wiggle it a bit to see if the cap was loose and it was. But he says I should try replacing the Cap & Rotor again since when I tried to wiggle the Cap the Rotor tapped the inside of the Cap. We saw evidence of some very fine curly, hair-like shavings before we realized there was a slightly loose screw. Since we know that now, he says to get a New Cap & Rotor and make sure the screws are tight and try it again.


    We did inspect the Distributor for any sloppy play and it seemed very tight with NO Play in it. Unless the play happens while its only running? The one screw hole may be slightly stripped as these plates the screws go into are extremely thin.

    Also just another Note: There seems to be a slight surging effect while Idling down the canal. I can also see the Tach at 800 RPM and it bounces up to 900 RPM and back down to 800RPM and I can hear it pick up and drop slightly as well. Again similar to a Cylinder cutting in and out.

    Does anyone have any suggestions or ideas of what may be causing this issue? Since the first day we got stuck and I found the slot was broken off the inside of the Rotor that fits into the slot in the top of the Distributor we feel that we are on the right track and do not think it’s the fuel injection related etc. Stumped on this one......

    One more Note: The Plugs & Wires were replaced the year before last.

    Thanks & Look forward to your responses.

    Eddie


    #2
    Some rotors got to be glued down.
    Be good, be happy, for tomorrow is promised to no man !

    1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

    '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

    Manalapan N.J

    Comment


      #3
      if its not actually misfiring, im going to say you have a fuel delivery issue... probably a plugged filter which is not allowing enough fuel thru to keep the engine running steady at WOT....


      NU LIBERTE'
      Salem, OR

      1989 Bayliner 2556 Convertible
      5.7 OMC Cobra - 15.5x11 prop
      N2K equipped throughout..
      2014 Ram 3500 crew cab, 6.7 Cummins
      2007 M-3705 SLC weekend warrior, 5th wheel
      '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
      Heavy Equipment Repair and Specialty Welding

      Comment


        #4
        What year is the engine? Fuel pump mounted on the raw water pump? If so has the oil chamber been maintained? If not the fuel pump arm may be warn preventing proper fuel pressure.
        1997 Maxum 2400 SCR 5.7LX Bravo II

        Mike

        Comment


          #5
          Originally posted by Nauti_Mike View Post
          What year is the engine? Fuel pump mounted on the raw water pump? If so has the oil chamber been maintained? If not the fuel pump arm may be warn preventing proper fuel pressure.



          Hi Guys,

          The Boat is a yr. 2000 Ciera Sunbridge 285.. 7.4 Mercruiser.

          As far as the fuel - I replaced everything that should be done in the Spring and then in the Fall. I am usually very meticulous about the proper care. I have it in a Marina and have them work on it as well as having a private Mechanic. I always have it maintained properly. But this one has us scratching our heads.

          I always replace the Oil and filter in the fall and the Fuel filter in the Spring, along with all of the proper winterizing / fogging of the engine in the Fall and getting it ready and cleaned up in the Spring..

          The Plugs and wires were replaced 2 years ago.as now I am on the 3rd Cap & Rotor. Now going for the 4th dues to it getting scraped while out last time.

          I am replacing the Cap & Rotor and adding a new screw with a Lock nut and washers to the underside of the Distributor plate on the screw that appears to be slightly stripped , (just for safety)..

          Thanks for the responses so far guys.

          I will post how I make out... (If the rains ever stops) LOL...

          Ed

          Comment


          • Nauti_Mike
            Nauti_Mike commented
            Editing a comment
            That sounds great but you did not answer my question, do you have a fuel pump mounted on the raw water pump? These are know to be troublesome.

          #6
          Does sound like a possible cap/rotor issue. I’d also replace the plugs just cause that’s cheap and easy. May find s plug doesn’t look right.
          Dave
          Edmonds, WA
          "THE FIX"
          '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
          (.030 over-Vortec top end-part closed cooled)
          The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
          Misc. projects thread
          https://www.baylinerownersclub.org/f...56-gctid789773

          Comment


            #7


            Hi Guys,

            This has been an ongoing saga now for about 6 weeks.

            I have replaced the following items in effort to fix this Surging throttle effect.:

            I am not going to explain all of the specifics as to why certain items were replaced, but just know that these items were all replaced or adjusted in hopes of solving the issue. Also it would take several pages to explain I detail.

            Items & Work Done:

            All done in the last 2-3 weeks
            • 3 Rotors and Caps.
            • New Electronic Ignition System.
            • New Fuel Filter.
            • 80Gals. of New fuel

            Done last Spring in 2017:
            • New Plugs & Wires - in 2017
            • All the regular things - Fuel filter, Oil filter, Oil, Lower Unit Oil, Power Steering, Power Tabs etc. Always done every year. - From2009 up until 2017 & 2018, every year.
            • I really try to be meticulous and stay on top of the maintenance.

            ***************So Here's Where We Are Now*****************
            • So I replaced the Fuel filter and there was a fair amount of sediment in the 1/2 Gal. plastic sherbet container I used to dump the fuel from the filter into. Then I replaced the Fuel filter and went for a test ride. I figured that I should get some fresh gas since I was close to the bottom of the tank with about 20-25 gallons of fuel left in her out of 100 gals. So I filled her up and went for a ride and there was no difference.
            • I then had a mechanic friend of mine take a look at it and we ran some tests. We checked the Fuel pump pressure and it was in the right spot.
            • Then we checked the Timing. At fast Idle @ 1000RPM's and running her up to 2000RPM's and then 3000RPM's we found that the timing advancement was working correctly. It was moving up 10, 20 and 30% as I advanced the throttle to the higher RPM's.
            • So we went out for a ride and checked the Timing while under way. I opened her up WOT & after about 30 - 45 seconds she was up on plane and at 4200RPM's. She ran fine and all was well, however as before after about 2-3 minutes of running she starts to surge again sounds like a cylinder cutting out and it drops to 3600 RPM's. At 20-30 second intervals it would start to kick in again and she was back up to 4200RPM's which is the top end RPM while at WOT. Then again 20- 30 or 45 seconds back to 3600 RPM's. This is how she continues to run.
            • Now I can get her from idle up to 3400 - 3600RPM's which is approx. 23/24 MPH , which is my normal cruising speed and she will run at that speeds OK. Sometimes for 5 - 10 minutes, but again as soon as I go to WOT she starts up again with the surging. Then we noticed from this 23/24 MPH Cruise range, I went to WOT and there was a Clicking sound which turned out to be the KNOCK sensor, while this happened we also still had the Timing light out and ON the marks and we noticed that as the surging started the Timing which was normally advanced about 30%. was now retarding back to 15 to 20 % and surging.
            • So now we are thinking that there may be an issue with the Gas. From what I have been told these engines really like to run on Premium Fuel. I have had the boat 7 seasons in the water and I have never put in anything other then Regular Fuel. So As A sort of a Test I added 3 bottles of STP Octane booster which my mechanic said to put in 4-5 bottles but the store I got them from only had 3 bottles. They say they can each treat 28 gallons of fuel so I thought I was pretty close to the Manufacturer's Spec. Either way it made no difference.
            • So now at this point I am just trying to burn off some of the fuel so I can fill the tank with Premium fuel. Unfortunately I do not know if this will make any difference.
            • Now one other thought was, I had a friend tell me a buddy of his had a similar issue. He found that the fuel line from the main tank was collapsing when he went to WOT. But since the demand for the fuel was not as great at 2/3rd's or 3/4 Throttle it was able to keep up and supply the needed fuel to the engine without surging. He replaced the fuel line and the issue went away.
            • Has anyone had or heard of this happening with ta fuel line?
            • Sounds to me like the Knock Sensor is triggering the Timing to be Retarded due to the Fuel possibly not able to be coridinated with the Firing and the pumping of the fuel into the cylinder simultaneously by the distributor (If my Theory is somewhat correct) causing it to surge? If so, it sounds like the only thin g to do is to burn off the fuel and try the Premium fuel.
            • But I still don't understand why it ran OK for the last 6 years OK on the Regular fuel. Unlees the fuel the Gas Dock has is tainted.
            • Also does anyone else have any othere suggestions or comments now with the new info. in regards to the Fuel Pressure Being OK and the Timing?


            Unfortunately I can only take her out on the weekends, so this process is taking way longer then I wanted it to take.

            Thanks Again Guys for all of the Feedback and can certainly use any suggestions and / or comments

            Eddie

















            Comment


              #8
              WOT for this engine is 4400-4800 so you’re a little low. Secondly one should not operate the boat at WOT for extended period of times, no more than a few minutes. Seems to me you’re running the engine too hard and it’s tells you it doesn’t like it.
              1997 Maxum 2400 SCR 5.7LX Bravo II

              Mike

              Comment


              • Centerline2
                Centerline2 commented
                Editing a comment
                this is true, but it should be able to run smooth for extended periods at WOT if one wants to push it that hard..

              • Nauti_Mike
                Nauti_Mike commented
                Editing a comment
                I agree but the knock sensor is being triggered and retarding timing. While this could be bad fuel it’s odd to only happen at WOT. I would if a valve starts floating at this rpm due to a weak spring or something else with a little too much play.

              #9
              fuel vent line obstructed or plugged? try running with gas cap loosened or off, see if it makes a difference. Otherwise, collapsing fuel line sounds like a possibility as mentioned above
              Ted
              2005 288 Command Bridge
              350 Mag MPI / BRAVO III

              Every Day Is Saturday

              Comment


                #10
                is this engine carbereted, TBI or EFI?..... a surge and a misfire are two very different symptoms, but when they seem to merge together, there are only a couple things that can cause this..

                I have seen a problem like this a couple times on street engines, once it was due to a faulty lifter. at the high rpm (more oil pressure), the hydraulic lifter would pump up too much and hold a valve open... once the oil pressure dropped when the engine rpm dropped, the lifter would relax and it would run fine again...

                the other time is was carboned up valves and a couple of them moved sluggish in the valve guide, and at high rpms one of the valve wouldnt close quick enough... but it it ran great at the lower rpms...

                during both occurrences, there never was a real strong misfire, but just a surge and the engine ran weak and without the power it should of had.

                both problems are VERY DIFFICULT to diagnose using conventional methods, but using an IR thermometer and some contortionist skills, you can attempt to see what cylinder is running cold at WOT when underway.... this can be dangerous, so take the proper precautions and have someone very competent and responsible running the boat while you are standing on your head in the engine bay getting these readings from the underside of the exhaust manifolds...

                and if the engine is carbereted, a float that is set too low can cause the engine to run excessively lean at higher rpms, which can cause surging or slight misfireing...

                a spark plug or plug wire can cause a misfire at a higher rpm when it may run great at the lower rpm range..., but a misfire due to plugs, wires or other ignition issues is almost always prominent and obvious to someone who has any engine savvy... a misfire due to insufficient fuel can be much less prominent (but not always) and can manifest itself in different forms....

                if it IS a valve train issue, everything can be working perfect with the only problem being that the cylinder is not holding its pressure, and so is not contributing to the power output...


                NU LIBERTE'
                Salem, OR

                1989 Bayliner 2556 Convertible
                5.7 OMC Cobra - 15.5x11 prop
                N2K equipped throughout..
                2014 Ram 3500 crew cab, 6.7 Cummins
                2007 M-3705 SLC weekend warrior, 5th wheel
                '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
                Heavy Equipment Repair and Specialty Welding

                Comment


                  #11
                  Compression test on a warm engine all plugs out keep track of what plug came out of what hole.

                  post results here, let's stop guessing and do some diagnosing .

                  we will go from there.

                  vacuum test next.
                  Be good, be happy, for tomorrow is promised to no man !

                  1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

                  '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

                  Manalapan N.J

                  Comment


                    #12
                    I didn't see that you changed the plugs since Spring of '17. I had a similar problem in our 2000 7.4L MPI and we diagnosed it down to a plug (the plug was breaking down at high RPM, but the engine ran smooth as silk at lower RPMs). Cheap fix, might be worth getting a new set and installing them.

                    Comment


                      #13
                      Originally posted by Nauti_Mike View Post
                      WOT for this engine is 4400-4800 so you’re a little low. Secondly one should not operate the boat at WOT for extended period of times, no more than a few minutes. Seems to me you’re running the engine too hard and it’s tells you it doesn’t like it.
                      Hi Mike,


                      This is an Bayliner Ciera Sunbridge 285, a fairly good size boat with only one engine which I think is underpowered or should I say really needs twins.

                      However for this boat, the Max rpm for a Mercruiser 7.4L MPI engine should be between 4200 and 4600 rpm according to Merc service manual. It has always toped out at 4200RPM's never hire since I got it which equates to 33MPH according to the GPS. That's with the lower Unit Trimmed up at the optimal angle and the Trim tabs set up all the way.
                      The problem first started when I was cruising at 3600RPM's which is also 23-24 MPH.

                      I never run WOT for minutes..... Never anymore then 30 -45 seconds at a time, and that's just to get it up on plane faster. I could bring it up to 3400 - 3600 RPM and it will plane eventually but it takes some time. Also requires re-triming the Lower Unit and Trim Tabs down. Then once on plane re-trimming again raising them a bit.

                      A funny thing with this boat is it needs the Trim Tabs to Trim the boats roll angle at all speeds except for WOT. Even at idle 5MPH down the canal I drop them a bit to help with the slight drift from Port to Starboard since these and a lot of boats develop this loose steering effect over time and in some cases even when they are brand new.

                      Ed

                      Comment


                        #14
                        Originally posted by Saturday288 View Post
                        fuel vent line obstructed or plugged? try running with gas cap loosened or off, see if it makes a difference. Otherwise, collapsing fuel line sounds like a possibility as mentioned above
                        Hi Saturday288,

                        Thanks for the suggestion. I will check the vent line as well.

                        Ed

                        Comment


                          #15
                          Originally posted by Centerline2 View Post
                          is this engine carbereted, TBI or EFI?..... a surge and a misfire are two very different symptoms, but when they seem to merge together, there are only a couple things that can cause this..

                          I have seen a problem like this a couple times on street engines, once it was due to a faulty lifter. at the high rpm (more oil pressure), the hydraulic lifter would pump up too much and hold a valve open... once the oil pressure dropped when the engine rpm dropped, the lifter would relax and it would run fine again...

                          the other time is was carboned up valves and a couple of them moved sluggish in the valve guide, and at high rpms one of the valve wouldnt close quick enough... but it it ran great at the lower rpms...

                          during both occurrences, there never was a real strong misfire, but just a surge and the engine ran weak and without the power it should of had.

                          both problems are VERY DIFFICULT to diagnose using conventional methods, but using an IR thermometer and some contortionist skills, you can attempt to see what cylinder is running cold at WOT when underway.... this can be dangerous, so take the proper precautions and have someone very competent and responsible running the boat while you are standing on your head in the engine bay getting these readings from the underside of the exhaust manifolds...

                          and if the engine is carbereted, a float that is set too low can cause the engine to run excessively lean at higher rpms, which can cause surging or slight misfireing...

                          a spark plug or plug wire can cause a misfire at a higher rpm when it may run great at the lower rpm range..., but a misfire due to plugs, wires or other ignition issues is almost always prominent and obvious to someone who has any engine savvy... a misfire due to insufficient fuel can be much less prominent (but not always) and can manifest itself in different forms....

                          if it IS a valve train issue, everything can be working perfect with the only problem being that the cylinder is not holding its pressure, and so is not contributing to the power output...


                          Hi Centerline2,


                          I hear what you're saying and they are valid points, but I find that this Knock sensor going off at the same time as the Distributor timing starts to retard the engines timing odd. You can hear the Knock sensor going off (at least we are 95% sure that that's what we are hearing). The timing gets retarded about 10 - 15 %.

                          Although it is random as to speed and run times when it starts to surge, whether it be 2-3 mins. into a run or 8-10 mins. into a run and it really does not seem to matter, it stills surges. It runs pretty smooth at idle in Neutral and while in Neutral if you run it up to 2000RPM and even up to 3000RPM it still runs very smooth and the timing is OK. It seems that its only while under way or its just that it does not appear to surge since I am only bringing it up to 2000 & 3000RPM for 20 - 30 seconds at a time, if that long.

                          Remember we checked the fuel pressure and we are good at all speeds. We have checked the timing with a timing light while at the dock in Neutral and while underway running her up thru the RPM range from 2000RPM to 4200RPM. My buddy is actually in the hatch, laying over the engine with the timing light as I di\rive her the RPM range.

                          One other thing that I have thought of is the sound of the knock sensor also sounds very similar to a spark plug wire that I am imagining got a chunk taken out of it or was melted away and the Spark plug wire is Arcing. But the only issue I have with that theory is then wouldn't it do that steadily ? Again unless due to vibration or something it is only happening intermittently. Also would the timing get retarded if the Spark Plug wire was Arcing? I don't think if the Spark plug wire was Arcing it would retard the timing.

                          Thanks

                          Ed

                          Comment

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