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Yamaha 1992 90HP overheat - possible impeller?

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  • Yamaha 1992 90HP overheat - possible impeller?

    Hi all

    I am having an overheat problem on my 90hp Yamaha outboard, particularly when revving over 3500 rpm. History of the problem is as follows:

    had a winter service done with new impeller put in, then took it out and had overheat issues at around 2500 rpm. Noticed the tell tale was very weak in the water but strong with the muffs on.

    I then replaced the head gasket and water jacket gasket and poppet vale and spring. Thermostat was opening fine with warm water test and cleaned out all water passages that had a bit of sand and corrosion build up but nothing too bad.

    took the boat out and now only get the overheat alarm at about 3,500 rpm with steam coming out of exhaust ports. Cools itself after a minute and then can carry on cruising below 3,000 rpm. Although I do notice a bit of steam coming out the exhaust port but no alarm. If I try and push it further then the alarm will go off again.

    I called the first mechanic who did the impeller change and he said it needs another new impeller. Not sure if he is taking me for a ride or not!

    Like I mentioned with the muffs on the tell tale is very strong but not in the water where it is weak at any rpm range. Could it be that it needs a new impeller so soon? On the muffs the tell tale is cool to the touch and water jackets remain cool to me touch.

    any help would be appreciated!

  • #2
    I have had a new impellor lose a vane before shortly after replacing it.
    I now always replace with a kit so the cup and wear plate are all new.
    Just in case the impellor was a new old stock that was sitting on a shelf for years.
    it never hurts to pull it and inspect it to see for sure if a new one is needed.


    • #3
      It seems like you could check the impeller yourself? If you did head gaskets and all the other stuff you mentioned, you can certainly do an impeller job.

      Inspect not only the impeller but also the housing/plate for wear or grooves. Don't overlook small grooves - if you can get your fingernail caught in the groove, it's too big.

      Otherwise, look for clogs/buildup in other water passageways.
      2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
      1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)


      • #4
        Best guess, cleaning the water passages is in your future, try all the miracle flushing with vinegar, salt away, acid and everything else, then…….fix it properly.


        • #5
          Thanks all

          As mentioned I already did the water passages when I changed the head and water cover gaskets. I followed everyone's advice and changed the impeller, the old one didn't look bad at all, but the guy never changed the gaskets around, so wondering if that was causing the issue.

          I changed the wear plate, housing, impeller and all the gaskets. Managed to get the lower leg back on myself using the ratchet strap method, was easier than I thought it would be!

          tried it on the muffs and water pumping out strong, now have to wait to next weekend to see how it goes on the water.

          If this doesn't fix, what other water passages are there to clear? anything on the exhaust plate side?

          Thanks again for all your help!


          • #6
            yes there are, but using some Ridlyme marine descalier pumped thru the motor for a couple of hours might help and keep you from breaking bolts removing other things.

            but if the water pisser is a lot better now, hopefully you have taken care of the problem


            • #7
              Originally posted by 99yam40 View Post
              yes there are, but using some Ridlyme marine descalier pumped thru the motor for a couple of hours might help and keep you from breaking bolts removing other things.

              but if the water pisser is a lot better now, hopefully you have taken care of the problem
              I concur. I have used Rydlime or barnacle buster before and was surprised at the amount of crud that came out. I let it circulate for 4 hours I think. Just make sure you have a big enough pump to circulate that water all the way to the top of the motor.. plenty of good vids on youtube covering it. It is worth a try for the cost....

              But like 99yam said, if you have a lot better flow out the telltel now, hopefully that is a good sign.


              • #8
                So took the boat out this morning, same issues were presenting, would see steam coming out of the exhaust port after the alarm went off. Checked the temps of the thermostat housing, cylinder head and water jacket housing and all were around 60-65 degrees C when the alarm went off and the exhaust plate was 55 degrees. Water was flowing from the telltale. Am thinking that there may be a blockage in the exhaust plate side now given that the water jackets are all clear on the head side. Will probably strip them down now and try clean them out. Hopefully don't break any bolts when doing it!


                • #9
                  Here is a photo and link to the video of the motor after the alarm went off


                  • #10
                    Does not seem like it is pissing hard to me. The old two strokes pissed like a race horse. Did you take the thermostat and housing cap off while running and see if you are getting lots of water up to thermostat? Cover up any electrical stuff as everything will get soaked.


                    • #11
                      Well I dropped the leg again and tried backward flushing from the stat, not a lot of water came out the inlet pipe so I’ve run some whipper snipper line up the water inlet tube noticed water start dripping faster after using it and a little bit of sand stuck to the trimmer line.

                      put everything back together and ran without the thermostat housing and poppet valves in place to see if water flowed and it did reach up there after about 10-15 seconds of running the motor. Will give it one more shot out on the water when I can take it before I think of removing exhaust plate which looks like it would be a PITA to change.


                      • #12
                        dropping the lower unit, hooking up a hose to the pipe heading to the power head, and turning on the water hose full blast. might help some.
                        and rig up an air hose to the water line to inject compressed air from time to time also might move things around.
                        need the stat out also to give it all some place extra to go


                        • #13
                          I just wanted to check whether this is enough water flow at the top of the powerhead whilst on the muffs or does there seem to be a restriction somewhere?

                          video, sharing, camera phone, video phone, free, upload


                          • #14
                            It looks about like my 90 does. However, your older video of the overheating...., it is definitely not peeing nearly as hard as mine


                            • #15
                              So tried to descale today but still no luck, bit the bullet and pulled the exhaust plate off but snapped three bolts, one is accessible but two down the bottom would either need a hole drilled through the cowling or pulling the power head to remove.
                              I’m sure this is my problem as there was so much crud and bits of broken old impeller from who knows when in there. Am cleaning it up now but thinking of how to remove these bolts.

                              one thought I had was given the low torque value would just sealing the holes with permatex no1 be ok? I think this will be the last time this job will be done for this motor before it becomes an anchor for a four stroke upgrade in the next year or so.