1968 Johnson 40hp overheating

Home Forum Ask A Member 1968 Johnson 40hp overheating

Currently, there are 0 users and 1 guest visiting this topic.
Viewing 3 posts - 16 through 18 (of 18 total)

  • lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26

    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.


    US Member - 2 Years
    Replies: 3829
    Topics: 39

    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.


    US Member - 1 Year
    Replies: 387
    Topics: 26

    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 3 days, 11 hours ago by lindy46.
Viewing 3 posts - 16 through 18 (of 18 total)
  • You must be logged in to reply to this topic.