1968 Johnson 40hp overheating

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  • lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239727

    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?


    fleetwin

    US Member - 2 Years
    Replies: 3829
    Topics: 39
    #239743

    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.


    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239744

    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?


    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239745

    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 6 days, 1 hour ago by lindy46.

    fleetwin

    US Member - 2 Years
    Replies: 3829
    Topics: 39
    #239772

    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.


    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239785

    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 5 days, 11 hours ago by lindy46.

    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239787

    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 5 days, 11 hours ago by lindy46.
    • This reply was modified 5 days, 8 hours ago by lindy46.

    fleetwin

    US Member - 2 Years
    Replies: 3829
    Topics: 39
    #239798

    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….


    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239808

    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.


    frankr

    US Member - 1 Year
    Replies: 5706
    Topics: 49
    #239829

    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.


    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239832

    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.


    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239835

    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 5 days, 1 hour ago by lindy46.
    • This reply was modified 5 days, 1 hour ago by lindy46.
    • This reply was modified 5 days, 1 hour ago by lindy46.

    fleetwin

    US Member - 2 Years
    Replies: 3829
    Topics: 39
    #239854

    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?


    lindy46

    US Member - 1 Year
    Replies: 387
    Topics: 26
    #239863

    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 4 days, 23 hours ago by lindy46.

    billw

    US Member - 2 Years
    Replies: 1483
    Topics: 45
    #239866

    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!

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