1979 Johnson 15hp, yes, another overheating engine.

[Tarinazo]

I got my hands on an overheated Johnson, did my research and found out that this thing tend to overheat. Lots of info in the web on what to do, so I did.
Had no spark, I changed the powerpack (solved it).
Changed fuel pump and fuel lines (inline fuel filter also)
Cleaned carb (although it looked brand new)
Took out thermostat, cleaned it to like-new condition, installed new rubber seal on it.
Cleaned all water pasages, also blew compressed air throug'em.
Dropped lower unit and put a new impeller kit.
Changed the famous upper tube grommet (yes, it was deformed and blocking).

Agter all this done, on my test tank it fired up on the 5th pull, then it would start on tha first pull without even pulling hard. Yesterday I took it out for a spin on my 1436 Jonboat. started with no problems and took for a 5 minute ride to my first fishing point. then another 5 minute ride to another point... wanting to get to my third point I struggled to start it, lot's of rope pulling and it finally started. after this it couldn't start, sore arm and blisters between my fingers.
I took the cover off and noticed that the head cover (not the thermostat cover) had a pink-brown color to it... I assume this is and overheat indication, right?
My guess is that it overheated, blew the powerpack and thus it wouldn'start.

So, now my question is this: Are this engines a sure thing they'll overheat?
Even after all I did to it previously?

 

[Tarinazo]

I forgot to mention that yes, there was always water flow present on the telltale (tale tail, tell tail...), and lots of it! I only bothered to touch it once and it was warm.

 

[racerone]

There are no defects in these engines as far as cooling system design.-----They do not all " overheat all the time "-----How much water was in your test tank ?----Was it 6" above the pump , yes or no?-----If no you may have damaged the new impeller.-----Did you install factory new parts or something else ?

 

[Tarinazo]

Yes, the water pump was properly primed with water while on the test tank. On my test tank the engine did not run more than 2 minutes, it got hot, but extremely hot (I did the can-leave-your-hand-for-a-few-seconds test) and that's why I was confident of taking the engine out for a real spin.
Seals and gaskets I bought from iboats, impeller kit from Ebay... today I'll drop the lower unit and inspect tha water impeller, then the thermostat and water pasages for any debri.

 

[Tarinazo]

I dopped and opened the lower unit to check on the water pump (Ebay bought, and probably chinese quality), surprisingly everything was fine, intact.
I then took the thermostat cover off to check on the t-stat wich wasn't stuck open and could freely open and close by hand.

 

[Tarinazo]

Nothing seem out of order (to the eye).

 

[racerone]

You should not be able to open and close the thermostat by hand.------The cylinder head is guaranteed to be warped ( they all are ) so refinish it and make it flat before installing it.

 

[Tarinazo]

mmmmhhhh, I am including a new tstat just as assurance and was wondering what else coul've caused the overheating knowing that all pasages where clean... I guess the answer was the tstat all the time.
Yes I finesanded the the head and cover on a flat surface, indeed it was somewhat warped.

 

[Tarinazo]

Gaskets and new thermostat are on the way. I put the old thermostat on hot water to see if it would open... it didn't. So for anybody following this, juct leaning your old thermostat to like-new condition sometimes isn't enough, just do as the 1,234,749,566 threads on this matter recomend and buy a new one.

 

[Tarinazo]

Just to wrap up this thread, Yesterday I installed a new head gasket and thermostat, kept the engine running for about 5 to 10 minutes in my test tank, the engine got hot to the point i could keep my fingers on it and the water coming from the telltale was warm all the time.
Next Sunday I'll take it for a spin on the lake.

 

[oldboat1]

A spring and retainer was apparently added to the t.stat assembly in '81. If you have additional overheating problems, you might consider adding those parts. In '89 (?) the t.stat was moved to the top of the head -- another option might be to locate a head and cover from an '89 or '90 J/E 9.9 or 15, and substitute that.

 

[Tarinazo]

Adding a head and cover of newer model will be my next step if I keep having overheating problems, struggling with the bottom cowl to remove the tstat is frustrating.

 

[flyingscott]

Easiest way to change the thermostat on those motors is pull the head. You cannot.jist add a spring to those you also need the waterjacket cover. Did you make.sure there were no blockages in the water system.

 

[thatone123]

I'm suprised no told you that the water tube grommet could be the problem or did they? I've went thru this before and that outboard is right about the age where they do it. If so, engine must come out and make sure you get the right grommet.

 

[racerone]

123 in post #1 he mentions changing the upper grommet !-----How could that motor get to be 40 years old if it was a bad design and just overheats all the time ??

 

[thatone123]

OK, sorry, missed that!