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  1. #1

    Default 89 Johnson overheats

    This is an '89 Johnson 70 hp VRO. It usually starts and runs well. Not this summer though.
    Recently on the water the alarm went off at high speed (constant alarm, high heat). I shut down immediately, restarted and was able to run for quite some time at low speed back to the dock, with no problem. The tell-tale seemed ok at all times, but I can't say for sure I looked right away when I shut it down.
    My mechanic put a new t-stat in and the same thing happened. I called him on the water and he suggested taking it out to get back to the dock. I did that and it ran fine even at high speeds. The tell-tale was ok at all times without a t-stat. After doing some research it sounds like that is not a long-term solution.
    I took both old and new t-stats and put them in a pot of water. They both seemed to open at roughly the same temp (not exactly sure what that was though, under 180 I think).
    So this morning I put muffs on, the old t-stat in and started it. It idled fine. I ran it up to 4500 or so (in neutral) and after a minute or two the stream started spitting and slowing. I shut down immediately, before the alarm could go off. I put the new t-stat in and exactly the same thing happened.
    It doesn't sound like there is much in the way between the water pump and t-stat on this motor (but I'm not sure). Would replacing the water pump and impeller likely solve this? I know I haven't replaced it in the 6 or so years I've had it, and the previous owner didn't seem like the type to do much preventative maintenance. I'd sure like to get on the water before summer is over, and not worry about alarms going off.
    Thank you in advance for any advice!. I really tried to figure out things from this forum and others, but I didn't find anything that fit my circumstances.

  2. #2
    Join Date
    Jul 2014
    Location
    Buffalo, NY
    Posts
    50

    Default Re: 89 Johnson overheats

    I would replace the impeller and it would also be a good idea not to run at 4500 rpm on muffs.....

  3. #3
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    14,945

    Default Re: 89 Johnson overheats

    Does the tell tale come off the side of the block or the top ?-----The latest factory waterpump kit comes with a few pieces and instruction to help with overheat problems.----Compression test numbers should be posted here as well.

  4. #4

    Default Re: 89 Johnson overheats

    The tube comes off the rear side of the block. As for the compression, I don't know. I always thought that was something complicated a mechanic had to do. Apparently it's not. I'll have my new tester on Friday

  5. #5
    Join Date
    May 2013
    Location
    Scarborough, ON
    Posts
    101

    Default Re: 89 Johnson overheats

    From what my memory serves, it's recommended to replace the water pump every 100 hours of operation, which is about 3 years for many people.
    No point in trying to save $100 on a $2000 engine.

    CarMech1969

  6. #6
    Join Date
    Nov 2005
    Location
    Mt. Zion, Illinois,
    Posts
    670

    Default Re: 89 Johnson overheats

    CarMech is right. You have a water pump that is at minimum 6 years old and, as you said, probably older than that. Spend the $$, install a new water pump, then go from there. Be very careful to make sure the old impeller fins are in tact because if there are pieces missing they could be lodged somewhere in your cooling system.

    http://www.marineengine.com/newparts...num=OMC0432955

  7. #7

    Default Re: 89 Johnson overheats

    Warmed it up, disconnected lanyard, wot, took out plugs and tested...From the top down, 110, 115 and 115. I tested each cylinder twice, so I guess I'm happy with the results. It'll be getting a new water pump tomorrow morning, hopefully.

  8. #8

    Default Re: 89 Johnson overheats

    Compression is fine.


    Although a water pump may go for quite a bit longer reliably, it is something that often gets replaced yearly (and preferably in the beginning of the season so you start with a new pump not one that has taken a set from sitting for the off season).. Some newer models of engines/designs truly need them replaced yearly, otherwise they will likely overheat.

    Something else to be aware of - many outboards run hotter at idle and speeds < 1200 RPM or so than they do at ~1200 to ~4000 RPM, and they heat up a bit at ~4000 and greater speeds. When you shut a hot engine down, the water drains out, and things somewhat heat up a bit more for a bit shortly after. If you shut it down to stop an overheat you gotta let it sit there awhile to make a difference - ie maybe 15 minutes with the hood off. If an engine is pumping water and overheats I will usually pull the throttle back to 1500 RPM and see if it cools down on it's own and would avoid idling it since that often makes them heat up normally.

    Generally trying to test much on a flush is a bad idea. Many engines do not quite cool the same on a flush, depending on your flush and water pressure, and there is no back pressure on the exhaust so they tend to run a little different. For instance I've seen new engines that after 10 minutes overheated on a flush - a flush is mostly for flushing.

    A better method is get yourself a plastic 50 gallon barrel, cut it to size, put a drain hole in the bottom the same size as your boat bilge plug - now you have a little test tank you can fill up with water and will better simulate how the engine is going to run during normal conditions. Now you can't go putting it in gear at any speed otherwise you will blow all the water out - but you can even idle in gear for a bit to get a ballpark in gear idle adjustment even this way. The biggest benefit though is when revving the engine in neutral or running it for extended periods of time, you are simulating more normal conditions and not what happens on a flush.

  9. #9

    Default Re: 89 Johnson overheats

    We think this (the attachment) may have had something to do with it. I took it tubing and it ran like a champ. Idle was too low, so my neighbor showed me how to adjust that a bit. All in all, I'm glad it's fixed, and I learned how to check compression and adjust idle.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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