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Water Pressure dropping

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  • Water Pressure dropping

    Hi everyone, I have been reading these threads for years and found them very useful, this is the first time I have needed to post as I have always found my answers in previous threads, unfortunately not this time.

    I have a Yamaha 115 2006 2 stroke. I have been running it on the same custom built speed boat for 10 years. Had a problem with the gears recently so had them rebuilt, while it was apart I replaced the water impeller.

    I am not sure if it was directly after this repair or shortly after my engine began to overheat. The 2000RPM limiter kicked in and it took me a while to figure out it was overheating as there was no audible alarm. Checked timing and fuel supply, cleaned carbs, filters and replaced fuel line but no luck. When I realised it was over heating I checked the tell-tale.

    In neutral the tell tale acts normally, increase throttle and it shoots out a good solid stream, then I tested out on water. Same problem with overheating. Rebuilt the water pump again, checked the poppet valve and removed thermostats but no luck. I noticed that when running in gear the tell tale would start good but then drop pressure, still water coming out but a very low stream. As soon as I saw this I stopped the engine rather than letting it get hot and engaging the rev limiter.

    Tested a few different conditions and found that if I was trimmed all the way down the pressure stayed good, if I trimmed up at all it would start good for about 30 seconds then drop. My boat handles terribly when trimmed all the way down and the steering gets really heavy, also I use the boat for wakeboarding and it puts out a better wave when trimmed up a little. So annoying that I can't use it at the correct trim. I noticed that it would almost always lose pressure when turning a corner but seemed ok when in reverse although I couldn't really open up the throttle in reverse to properly check.

    Whenever water pressure dropped all I had to do was stop, rev the engine in neutral and it would go back to normal and I could start up again.

    Seeing as nothing has changed in 10 years I can't understand why this is happening, the anti-cavitation plate is about level with the bottom of the boat and the water inlet seems to be fully submerged in the water. I even changed the strainer on the inlet to see if that helped.

    I also cleaned the hull, which was pretty dirty of barnacles, weeds and algae. There are quite a few dings in the hull from chipped paint and epoxy, so maybe there is some turbulence created under the hull that is causing air bubbles to get into the inlet. But I have seen my hull in a lot worse states than it is currently and never had a problem like this.

    Is there anything I am missing? Has anyone experienced anything like this before? Could I have made some elementary mistake when rebuilding the lower leg?

    Thanks in advance

  • #2
    any air turbulence can cause lowered water pressure.
    when you had the pump apart did you remove the rubber damper and holder and clean the corrosion?
    keep the t-stats in place.
    they are also part of the pressure control system.


    • #3
      Thanks for the reply. I have ordered new thermostats so will put them in when they arrive, but the problem was there when they were still in.

      Not sure what you mean by the rubber damper? Is this the bit under the wear plate? The circular washer type part?

      Anyway no I didn't clean it but it didn't look corroded. Worth a shot anyway.

      I am thinking of taking the boat out the water tomorrow and cleaning up any dings or scraps, then re-antifoul. Can't think of what else to do??

      Another option would be to lower the engine a few centimetres but it has run fine for 10 years so can't imagine the mounting height would be the problem.


      • #4
        that damper is below the wear plate beside the ex cavity. also any melting of the pump housing scraps the housing.


        • #5
          Just curious, did you (prior) when turning fairly hard with speed, have any issues with overheating? Or were the turns were short/fast enough not to overheat/cause issues?

          Plus 1 ^^^ if the outer pump housing is warped at all, it'll cause pumping issues

          And just for S&G's, was the impeller, placed in the housing, installed in the correct direction?
          Last edited by TownsendsFJR1300; 04-08-2016, 08:06 AM.
          1997 Angler 204 CC, 2006 F150 TXR


          • #6
            Never had an overheating issue before. The pump housing isn't warped at all and I have already replaced it with another housing to check that out. I replaced the metal impeller casing too. The only parts of the water pump I haven't changed are the wear plate (which is pretty new and in good condition) and the woodruff key.

            I usually have the engine trimmed up pretty high, around half way on the gauge. Never had any issue like this.

            I don't understand why the pump is working good if trimmed all the way down. I guess the intake is sitting deeper in the water so the only thing I can think of is that all the dings and scrapes in my hull are causing turbulence that are making air bubbles close to the surface. I'm taking the boat out this morning anyway to re anti-foul and clean it up as best I can. At least that will eliminate one more possibility. When I have it at the house I'll take some pictures of the water pump.


            • #7
              From what you describe, you were ok (or borderline) and perhaps that additional bit of turbulence (as you said) took just enough water away from the intake at exact conditions to affect the cooling system.

              I'm assuming the boat isn't any lighter(less fuel, gear), running higher/faster in the water??

              As already mentioned, if the bottom is a bit rougher with barnacles, growth, etc, and your already that close (to the edge of overheating already), its now enough to tip the scales if you would...

              As it seems your close to the edge anyway, follow up on your clean up of the bottom and you might consider lowering the engine just a notch. You'll lose a hair on the top end but it won't likely be as temperamental (trimming it). Just a thought..
              1997 Angler 204 CC, 2006 F150 TXR


              • #8
                Yeah I thought about dropping it down a couple of centimetres. The anti-cavitation plate is level with the bottom of the boat at the moment though. I'm worried that if it goes too low the steering will get heavier than it already is.


                • #9
                  On my boat, I know the engine is set too low, (probably 2-3"). From the factory, they drilled the transom holes (top set and bottom set) at the FAR END OF EACH OTHER!!

                  There is NO adjustment unless I drill NEW holes in the transom (and I think the rear "water well" is right smack where the holes SHOULD GO...

                  With that said, trimmed down, there's no slippage if I nail it from idle (and it'll get on plane very quick) and trimming it once on plane is fine and NOT heavy.

                  To get it to run straight (little to no steering input), I do have to fine trim the engine as my F150 is a bit heavier than the old two stroke, so I have to compensate for that.

                  Per the spec's on the Yamaha site and what the old 150 2 stroke ran, top end is about 45 MPH(true-and normal for the set up) and is sketchy at that speed anyway.

                  Even if I raised the engine somehow, the hull (fairly deep vee) doesn't like going that fast.

                  I usually cruise around 35 MPH.

                  Just something to ponder...
                  1997 Angler 204 CC, 2006 F150 TXR


                  • #10
                    Any updates?
                    1997 Angler 204 CC, 2006 F150 TXR


                    • #11
                      Hi guys, sorry been offline for a few days while I was painting the boat. Got the bottom freshly painted and ready to go back in the water tomorrow and see if it makes a difference.

                      Made one other change on the water pump but thought I would test it as good as I could before going back in the water.

                      Not sure if this is a reliable test but I stuck a hose into the water outlet of the water pump with the leg off. When I put water into the outlet there is water getting through the pump and coming out of the drive shaft. Is this normal? I was thinking that it would have to be sealed?

                      I replaced the small o-ring on the inside of the pump as the old one didn't fill the gap properly. Still getting water coming out the drive shaft? Any ideas if this is supposed to happen?


                      • #12
                        Notwithstanding the above, I'm thinking also turbulence but also pump internal cavitation/aeration. I'd also try running with the screens removed, in clean water of course, and see if problem alters in anyway. Look for obstruction in or behind the screens.


                        • #13
                          I had a good look behind the screen, removed them and the pump housing and it all looks clear with no obstructions. I thought about running with them off but worried about sucking anything up, even a small stone could block the cooling passages. I guess out in the deep water it will be ok though.


                          • #14
                            leave the screens on.
                            did you pull the damper and the holder and clean the corrosion off of them?


                            • #15
                              Originally posted by ThaiWake View Post
                              I had a good look behind the screen, removed them and the pump housing and it all looks clear with no obstructions. I thought about running with them off but worried about sucking anything up, even a small stone could block the cooling passages. I guess out in the deep water it will be ok though.
                              I should have added, the screens may have oil or grease on their surfaces. Those holes are small and may, just may, restrict just like a water repellent sieve. Some fuel funnels with a screen, when new let water through, but once fuel and particularly fuel and oil mix has gone through this screen, will repel water from getting through (a purpose of the screen as well as preventing dirt and anything else getting thru).

                              A bit left field, I'd scrub these filters with soap and water. And inside behind. Just thinking a rebuild may required a petroleum dip/clean.