73 Johnson 85HP Cooling questions

[ezmobee]

I got my boat all ready to go this year and started it in my driveway. I really wanted to make sure the motor was pumping water properly so I didn't use the muffs. Sometimes it seems like the garden hose pressure forces the water where it needs to go so you can't quite tell if the water pump is working correctly. Instead I filled this giant truck box I have up with water and lowered the motor into that. Well near as I could tell, it wasn't pumping water AT ALL. Unfortunately it took me a while to definitively determine this with the exhaust stirring up the water so bad. Just about the time I was coming to the conclusion that it wasn't at all pumping and that the block was getting very hot, the overheat horn sounded. I immediately shut it down. My first question is do you think I did any damage to the motor with this quick overheat? It probably ran for 5 minutes or so total.

Anyway, I assumed my impeller was toast so today I dropped my lower unit. My impeller looks to be in OK shape. Not perfect but good enough that it should have been pumping. But what I did see that was weird is I am missing one of the plastic tubes that connect the impeller housing to the tubes that run up the midsection. I'm assuming that is my problem. (I'll be replacing the impeller anyway of course since I have it apart.) What could have happened to the tube? Or how could it have worked without it? I've been using it for 2 full seasons since I last had it at the shop to have it "awakened" when I bought it which included an impeller change.

 

[ezeke]

Re: 73 Johnson 85HP Cooling questions

Well, you know from the service manual's water flow diagram that the water goes up one tube and back through the other and that both tubes need to be in good shape. The fact that the overheat went off is a good indication that the wrong tube went bad, or that you have a thermostat problem.

Assume that both are bad and fix 'em.

BTW, when a overheat condition occurs, most mechanics insist on replacing the head gaskets.

 

[F_R]

Re: 73 Johnson 85HP Cooling questions

The plastic tubes serve no function once the lower unit is installed. They are only guide tubes which you stick into the pump housing so you can hit the holes with the real water tubes which you cant see when you get that far. Afterwards, they have a way of working their way up the water tubes. Look up higher on the water tube.

 

[ezmobee]

Re: 73 Johnson 85HP Cooling questions

F_R, yeah I was just reading my manual again and saw they were referred to as "guides". I think I may have figured out my problem. Upon further inspection it appears that the plastic of the impeller housing got melted somehow and the melted plastic is about 70% blocking the output stream. I think that's my problem. It must have been run without proper water flow at some point. I have junky earmuffs which I don't think I'm going to use anymore. I'm going to pick up a complete impeller kit tomorrow and I'll let you know how I make out.