1968 Johnson 40hp overheating

Home › Forum › Ask A Member › 1968 Johnson 40hp overheating

Viewing 10 posts - 1 through 10 (of 25 total)
1 2 3
  • Author
    Posts
  • June 8, 2021 at 4:34 pm #239727
    lindy46
    Participant

      US Member

      Well, my $50 find is turning into a challenge. She’s still overheating. I’ve changed the impeller, and if I leave the thermostat cover off, water gushes out so the impeller is doing it’s job. I pulled the head and all passages are clear. I ran water through all the holes and water flows freely. The head looks like new. There was no thermostat when I removed the cover so I put in a new one. I’m running in a 30 gallon trash can and the water is well above the water pump housing. Water spits out the exhaust relief hole on the leg, but not as much as I remember from my past motors. Someone said they don’t cool good in a tank but I’ve run other 35 and 40hp motors this way with no problems. Head gets hot enough to boil water. Any ideas?

      June 8, 2021 at 7:54 pm #239743
      fleetwin
      Participant

        US Member - 2 Years

        Is the bypass ball and spring/plastic housing in place under the thermostat? It will overheat if those items aren’t there and working properly. The thermostat was out of this engine when you got it, so the previous owners may have been chasing an overheat issue as well. Is this a fresh or salt water engine? I’m guessing the gearcase has been off before, someone may have botched a water pump job. These engine really don’t spit much water from the upper exhaust relief, usually just a fine spray until the thermostat opens.

        June 8, 2021 at 8:07 pm #239744
        lindy46
        Participant

          US Member

          The ball, spring and housing are there. Looks like the spring holds the ball up into the hole in the bottom of the housing. I can push down on the ball to open the hole. Is that the way it’s supposed to work? That looks like the passage by which cooling water gets into the head. So how does water get in when the ball valve appears to be closed all the time? What opens it?

          June 8, 2021 at 8:14 pm #239745
          lindy46
          Participant

            US Member

            Freshwater motor. I removed the gearcase to change the impeller and only the water pump outlet copper tube was hooked up. The return copper tube was 5″ longer and not hooked to anything. This motor was born a long shaft so someone converted it to short shaft. Don’t understand the longshaft return copper line. I had an extra short copper return line and connected it properly into the return grommet in the pump housing.

            • This reply was modified 2 years, 10 months ago by lindy46.
            June 9, 2021 at 7:49 am #239772
            fleetwin
            Participant

              US Member - 2 Years

              OK, well Frank is the expert on this system, so hopefully he will chime in….But, there were some changes to this pump system, looks like a copper tube was added in 68 to connect the water pick up to the underside of the water pump, instead of through a passage in the casting like in previous years. You have had the gearcase off, did you notice this extra pick up tube? You mention it is a fresh water engine, but were all the sealing surfaces in good shape? No broken/buggered up little pump screws?
              Does it look like the engine has been overheated in the past? Burnt paint, melted wiring?
              That two line water pump system is an effort to help warm up the engine sooner and keep it at a constant temperature. When the thermostat is closed, water is forced by that poppet valve and back down to the pump through that second water tube….Recirculating the water in an effort to warm up the water quicker. So, if the poppet valve is missing, inop, water will just bypass the powerhead and return to the pump, causing the engine to overheat. If that poppet valve is jammed shut, this will just delete the recirculating system.
              http://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1968&hp=40&model=40802A&manufacturer=Evinrude&section=Gearcase+Group
              http://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1968&hp=40&model=40802A&manufacturer=Evinrude&section=Powerhead+Group
              You mentioned being advised that these engines don’t like being run in a tank, which is correct. You also mention that the gearcase is buried deep enough in the tank to be sure it pumps correctly. If the engine does not show signs of a previous overheat, I would be tempted to advise launching the boat and trying the engine in the water to be sure it is not a tank issue. But, you also mentioned there was no thermostat in place, so it would appear someone before you was chasing an overheat issue.
              Are all the pieces of the poppet valve and cup in place, including the seal? Again, if the poppet valve is being bypassed, the water will “short circuit” back to the gearcase without circulating through the powerhead. I suppose you could test this theory by removing the poppet valve/cup, then plugging the return hole with a cork or something…This will delete the recirculation feature and force water through the powerhead…Reassemble the thermostat cap and retest….If the engine seems to run cool now, then we are missing something wrong with the poppet valve system…The return spring has very light tension as I recall, so that is normal. If the engine still overheats, then I think we can safely assume the recirculation system is not to blame.
              You have had the head off, and mentioned this is a fresh water engine, so I am assuming all the sealing surfaces/passages looked OK and were not plugged.
              Finally, perhaps I should have led with this…Is the water relatively clean in your tank? Perhaps the intake screen is just plugged with crud/debris from your tank….I know, you ran the engine with the cap off and saw decent water flow, so this seems unlikely….I am just being hesitant, because I don’t want you to go nuts trying to fix something that isn’t really broken.

              June 9, 2021 at 10:06 am #239785
              lindy46
              Participant

                US Member

                I believe the poppet valve may be at fault. It does open and close when I press on it with a pencil (eraser end) but if I pour a little water into the housing it just runs through the poppet and out the bottom of the motor. Looks like it’s not sealing. I’ll try removing the housing and see if the seal is defective.

                • This reply was modified 2 years, 10 months ago by lindy46.
                June 9, 2021 at 10:10 am #239787
                lindy46
                Participant

                  US Member

                  By the way, there is a third copper tube coming out the underside of the water pump housing and going to a fitting/grommet farther down the lower unit. Fresh way in test tank.

                  • 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 11:37 am #239798
                  fleetwin
                  Participant

                    US Member - 2 Years

                    OK, well I would definitely have a look at the poppet valve seal under the plastic housing….
                    Frank explained why that change was made to the water pick up, adding that copper tube down there…Something about making the housing more durable and distort less? In any event, make sure it is properly positioned in the grommets and the water tube housing that is screwed to the impeller housing isn’t damaged creating water/air leaks….

                    June 9, 2021 at 12:44 pm #239808
                    lindy46
                    Participant

                      US Member

                      There is no seal on the bottom of the housing – the poppet just seats right against the hard plastic of the housing. Parts diagram doesn’t show any other seal – just the rubber gasket (22) which seals the the bottom of the housing to the thermostat housing. The hole in that gasket is much larger than the poppet. I’m trying a rubber gasket with the same sized hole as the hole in the bottom of the housing. The poppet should now seal against the rubber.

                      June 9, 2021 at 6:02 pm #239829
                      frankr
                      Participant

                        US Member

                        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.

                      • Author
                        Posts
                      Viewing 10 posts - 1 through 10 (of 25 total)
                      1 2 3
                      • You must be logged in to reply to this topic.