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  1. #1
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    Default outboard motor discharge water - how hot?

    my 4hp, 1990 yamaha discharges cooling water that is too hot for my hand (not boiling but a wee bit too hot) - stream is not super strong but still good

    is that normal?

  2. #2
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    Hi,

    I don't know about Yamaha, but my Tohatsu 5hp tender outboard cooling steam is quite cool. When I put my had in it, I'm always surprised by how cool it is, just a little warmer than the sea temp, but I've never tested it when the engine is running hard, so it may be hotter then??

    Hope that helps!!

  3. #3
    Dabhand is offline Registered User
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    I had a similar problem on a 50Hp Mercury i used to own, turned out to be the thermostat, also had the over temperature alarm go off - not suprisingly. Not sure if a small engine such as a 4hp Yamaha would have a thermostat or an over temperature alarm, but it otherwise could indicate some sort of blockage.

  4. #4
    jfm's Avatar
    jfm is offline Registered User
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    Quote Originally Posted by old school View Post
    my 4hp, 1990 yamaha discharges cooling water that is too hot for my hand (not boiling but a wee bit too hot) - stream is not super strong but still good

    is that normal?
    That's too hot. Should be about 50deg rise on the seawater temp, so discharge is 60-70 deg C typically, which is quite hot (hot tap at home is about 65deg).

    On an outboard the temp of the water rises if the flow rate is too low. You mention a not-strong flow. It might need a clean out, or a new pump impeller, which are easy enough jobs (you drop the gearbox off to reveal the pump and impeller)

  5. #5
    Fantasie 19's Avatar
    Fantasie 19 is offline Registered User
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    I agree with JFM .. when was it last serviced as that may confirmt he problwm

    I have a 4HP Yamaha same age as yours - tell tale water is tepid at best...
    Never knowingly undersailed...
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  6. #6
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    Quote Originally Posted by old school View Post
    is that normal?
    Hard to tell just by your way of measuring, I know folks who can handle forever hot water, oil, etc. at temperatures much higher than I could stand for a few seconds. But it's also important WHEN you touched the water.
    If after some time of running at idle, that'll definitely be hotter than in normal operation - counterintuitive as it might sound - due to the lower water flow. Try using the outboard for a while at normal speed (not WOT, mind), and touch the water during that kind of usage (or immediately after, if it's difficult to reach the flow with the boat cruising).
    If in those conditions the water is less hot than at idle, the cooling is probably working fine.

  7. #7
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    Quote Originally Posted by jfm View Post
    That's too hot. Should be about 50deg rise on the seawater temp, so discharge is 60-70 deg C typically, which is quite hot (hot tap at home is about 65deg).
    I hope not! Most thermostatic mixer taps have a detent at 38* and won't usually go higher than 45* or maybe just over. Our hot water tankstat is set to 45ish from memory as you run the risk of scalding from non-mixer taps otherwise. Even the calorifier on the boat has a regulator on the hot water outlet...
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  8. #8
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    I would say that sounds too hot and too light, you want it to be warm but not hot and a good squirt. If you have not already done so pop the gearbox off and have a look at the impeller, not a five minute job but well worth it.

  9. #9
    Join Date
    Apr 2010
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    Default great suggestions!

    I'm going to inspect the impellor - cheap safe fix. I sure do not want to "fry" my dependable outboard

    Thank you all again!

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