1968 Johnson 40hp overheating

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  • June 9, 2021 at 7:33 pm #239832
    lindy46
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      Well the poppet can’t completely seal as there is a notch in the hole in the bottom of the housing to let water drain. Protects against winter freezing. So I took a different approach and blocked the hole in the bottom of the housing, left the thermostat out and ran the motor. Water immediately gushed out the exhaust relief hole on the back of the leg. Then the water flow slowly dwindled until it stopped. Let her set awhile and repeated the procedure – same thing – water gushing, slowly dwindled and stopped. When water stopped flowing the head again was hot enough to boil water. I’m at a loss.

      June 9, 2021 at 7:46 pm #239835
      lindy46
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        frankr wrote:

        I sort of don’t think the poppet is the problem. The water pump on that motor sends water up one tube and after passing through the power head the water is presented to the thermostat. If the water is warm enough the stat opens and the water is discharged and replaced by more water from the pump. But if is too cool the stat stays closed and the water pressure unseats the poppet and returns to the pump via the second tube for recirculation. You mentioned the second tube was not connected to anything. That ain’t correct. There were some changes made about that time. Previous models returned the water directly to the pump. Then they made a change that returned the water down to the bottom of the gearcase. Needless to say, the correct parts must be used.

        The second tube was not connected when I pulled the gearcase because it was a long shaft tube. I don’t know what the previous owner was thinking. I replaced it with a correct tube which enters one of the two grommets on top/side of pump housing via #3 (impeller housing cap) on the parts diagram. There is also a third copper tube coming out the underside of the housing via part #3 and going to a grommet further down in the gearcase. Is that correct?

        • This reply was modified 2 years, 10 months ago by lindy46.
        • This reply was modified 2 years, 10 months ago by lindy46.
        • This reply was modified 2 years, 10 months ago by lindy46.
        June 9, 2021 at 8:56 pm #239854
        fleetwin
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          US Member - 2 Years

          Yes, three tubes is correct for 1968…And yes, you are correct, there is a little bleed there in the poppet valve, so I guess the water will slowly drain off like you said.
          Try running the engine with the thermostat cap off again to see if the water flow up there eventually stops like you have reported out of the exhaust relief….If so, it might be time to revisit the water pump, losing its prime for some reason….Are you sure it is deep enough in the tank? Did you have the water pump off when you removed the gearcase?
          Does the engine show signs of previous overheating?

          June 9, 2021 at 9:46 pm #239863
          lindy46
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            US Member - 1 Year (includes $3 online payment fee)
            fleetwin wrote:

            Yes, three tubes is correct for 1968…And yes, you are correct, there is a little bleed there in the poppet valve, so I guess the water will slowly drain off like you said.
            Try running the engine with the thermostat cap off again to see if the water flow up there eventually stops like you have reported out of the exhaust relief….If so, it might be time to revisit the water pump, losing its prime for some reason….Are you sure it is deep enough in the tank? Did you have the water pump off when you removed the gearcase?
            Does the engine show signs of previous overheating?

            I replaced the impeller – that was the reason I dropped the gearcase. Water level in my test tank is up to the middle of the cover on the leg for the shift linkage. So about 3-4 inches above the top of the pump housing. No overheating signs.

            • This reply was modified 2 years, 10 months ago by lindy46.
            June 10, 2021 at 5:33 am #239866
            billw
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              US Member - 2 Years

              Frank and Don certainly know many volumes more about these engines than I do. All I know is what I have personally experienced; and that is problems with cooling, when running them in a barrel. No other models ever gave me any problem but RDs and Big Twins gave me fits. Before you burn any more gray matter, how hard would it be in your situation, to put it on a boat transom and even just back it down a boat ramp on a trailer? Maybe you’re not set to do that yet; but if you can do it, that sure is what I would be trying next….

              Long live American manufacturing!

              June 10, 2021 at 8:15 am #239881
              lindy46
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                US Member - 1 Year (includes $3 online payment fee)
                billw wrote:

                Frank and Don certainly know many volumes more about these engines than I do. All I know is what I have personally experienced; and that is problems with cooling, when running them in a barrel. No other models ever gave me any problem but RDs and Big Twins gave me fits. Before you burn any more gray matter, how hard would it be in your situation, to put it on a boat transom and even just back it down a boat ramp on a trailer? Maybe you’re not set to do that yet; but if you can do it, that sure is what I would be trying next….

                I can see that water circulation would be better when moving down the lake. That may be all there is to it. Might have to be my next step.

                June 10, 2021 at 2:20 pm #239916
                fleetwin
                Participant

                  US Member - 2 Years

                  OK, no signs of previous overheat, only question is why someone removed the thermostat in the past. Like Bill says, probably best to put the boat in the water and try it at this point… Is this an electric start model? If so, check hot light operation before heading out.
                  Forward motion of the gearcase certainly helps supply a pressurized water supply to the water pump. A weak pump will oftentimes slowly stop pumping when in neutral or reverse, but then start pumping again when put in forward gear due to the somewhat pressurized supply to the pump. Did you run the engine before replacing the water pump impeller? If so, did it overheat then?
                  Did you use an OEM impeller or aftermarket? Did you have a good look at the aluminum impeller housing? Pitting and grooves inside the housing will affect pump performance even with a new impeller. Was the upper seal in place on the impeller housing, was it in decent shape? Finally, there is a very small bleed hole in the top of the impeller housing that keeps the system from being air bound, perhaps this is plugged up. The SS impeller base plate must be in good shape also, excessive grooves will affect pump performance. It is important that the SS impeller plate is sealed properly against the aluminum mating surfaces of the gearcase. It is a good idea to use a little sealer/type M/2000 between these two surfaces…Sealing issues in this area will cause the pump to pull air instead of water, especially at higher speeds when the water level drops. Broken screws, gouges, or corrosion/erosion in this mating surface will cause pump issues. It is probably best not to use any sealer at all between the SS impeller plate and the aluminum housing though, even though sealing in this area is just as critical. It is very difficult to apply a small enough amount of sealer in this area, excessive sealer will squeeze out inside the pump and interfere with impeller performance. The aluminum impeller housing should be replaced if there are gouges/corrosion in this area that would affect sealing.

                  June 11, 2021 at 10:05 am #239974
                  lindy46
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                    US Member - 1 Year (includes $3 online payment fee)

                    Electric start. Pump housing was in good shape and upper seal was in good shape. I used an OMC impeller. I did seal the base plate against the gearcase aluminum. Everything I typically do when I replace an impeller. This is a Tennessee motor so never is run in cold water. That’s probably why no thermostat. I don’t have a boat to try it on right now so I’ll have to wait to do further testing. I think it will be ok once it’s run on a boat in the lake. Thanks for all the help and suggestions.

                    • This reply was modified 2 years, 10 months ago by lindy46.
                    July 1, 2021 at 10:35 am #241676
                    lindy46
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                      US Member - 1 Year (includes $3 online payment fee)

                      I think I’ve found my overheating problem. Note the torn up grommet. I connected a hose directly to the intake water tube and the motor ran cool as a cucumber.

                      July 1, 2021 at 12:55 pm #241690
                      fleetwin
                      Participant

                        US Member - 2 Years

                        Hoping that solves this issue for you….It’s tricky to get all that stuff aligned properly… Sand the water tubes smooth and grease em up good… Have an assistant help hold the gearcase while you get everything lined up….Don’t forget to lube up the driveshaft splines and make sure the oring is in place….

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