Best way to check water passages 79 Johnson 15hp...blockage?

damien99

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Joined
May 12, 2010
Messages
5
I was running my motor last night and it was overheating when running at high speed. 100psi in each cylinder and New impeller this year...tell tale blasts water. I was getting water out the top exaust once it warmed up, but even that got super hot. I'm thinking water passage plugged somewhere...not entirely sure though

Thanks for any help!
 

bktheking

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Jul 29, 2008
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5,057
Re: Best way to check water passages 79 Johnson 15hp...blockage?

It should be super hot, it's water coming from the exhaust area, water helps cool the exhaust. How did you determine it was overheating, did it quit running? Did you measure the head temp with a thermo?
 

damien99

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May 12, 2010
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Re: Best way to check water passages 79 Johnson 15hp...blockage?

I didn't take any temps, Noticed it because of how much steam was coming out if it as it was running. I pulled the cover and put my hand on the head and burnt my finger. dropped a little water on it as well and it was instantly bubbling. I know it should be hot, but this was too hot.
 

Cofe

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Apr 23, 2009
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Re: Best way to check water passages 79 Johnson 15hp...blockage?

Sounds like your thermostat could be the problem. The thermostat lets water circulate threw the head. A tell tale stream only tells you if your water pump is working.
 

bktheking

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Re: Best way to check water passages 79 Johnson 15hp...blockage?

Agreed, that was my next suggestion, T-Stat
 

JTROCKS1

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Jun 9, 2009
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Re: Best way to check water passages 79 Johnson 15hp...blockage?

Man am I the person to reply to this thread. I have the same motor same year and spent 4 days running down an overheating issue. If the impeller is relatively new I would also agree that the thermostat may be clogged. Especially if the water comming out of the pee hole is warmer than that of the exhaust. If that doesn't do the trick you may possibly have a blockage elsewhere. My issue was with a partially plugged tube into the bottom of the motor itself. The copper tube from the water pump where it goes into the motor had some sort of partial blockage. I took a length of coat hanger and made a very slight bend in the end and put it in the end of my cordless drill and "roto rooted the blockage out. Also if you have to drop the lower unit blow some compressed air up the copper tube and also the pee hole if you can. You may also want to spray some water up there as well. Go to the Dollar store and get a water ballon kit. The fitting that screws onto the hose just fits nice into the water tube. Just don't use a lot of pressure.
 

Cofe

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Apr 23, 2009
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1,883
Re: Best way to check water passages 79 Johnson 15hp...blockage?

When the lower unit is removed, here is a test tube connection I made with an old copper toilet water supply connection. Soldered and old hose sprayer on the end, and put an extra grommet on the top. With this rig it I can tell if the head gasket, or exhaust gasket is leaking by removing the plugs, and if one of them is leaking, water will come out of the plug holes. Propeller is to show size of the rig.
 

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mikesea

Lieutenant Commander
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Oct 1, 2006
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1,830
Re: Best way to check water passages 79 Johnson 15hp...blockage?

a very common problem on the 9.9 and 15 is a collapsed grommet on top of the watertube.Im real curious as to wheter JTROCKS1 ever read my fix.I did the identical deal with a wire coat hanger,but,smashed the end down and filed it into a knife edge,worked great many times
 

JTROCKS1

Cadet
Joined
Jun 9, 2009
Messages
16
Re: Best way to check water passages 79 Johnson 15hp...blockage?

I absolutely did read your fix! I don't think I would have ever figured out what my problem was unless I read about your battles and what you shared. I absolutely owe you a beer.
 

damien99

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Joined
May 12, 2010
Messages
5
Re: Best way to check water passages 79 Johnson 15hp...blockage?

Awesome stuff guys, I pulled the cover and removed the stat and did a test in a pot of water...no good. Bad thermostad!! When I first got this motor I had to replace the impeller...it fell apart and pieces of it broke off. I had done all the stuff mentioned with the lower unit off. I just hadn't used the motor enough at full throttle for the thermostat to kick in after all the other work. There isn't anything left for cooling to go wrong...I hope. I've replaced it all.

Almost back on the water to do another test run and hammer a bunch of largemouth hogs.

Thanks...great site
 
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