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Yanmar Exhaust Telltail or Blocked?

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    A couple of weeks ago while leaving a marina someone on a passing yacht told me that our exhaust needed to be cleaned.

    This he believed to be a unique Nauticat issue whereby what I thought was an engine telltale stream of water from above the exhaust was actually an overflow due to a blocked exhaust box.

    Water does still come out of the exhaust but I’m wondering if there’s any merit to his comments.

    Photo attached of the box he suggested (not ours, lost that photo..) cleaning out with caustic soda.

    Has anyone had similar experience/issues. I haven’t had the time to pull the piece off but it does give me some concern.

    Many Thanks!

    Alasdair Woodfield

    Hi Duncan,

    Are you referring to the bleed from the anti-siphon’ at top of coolant pipes, see attached ? On my 331 there is a constant stream from this to a hull outlet on the portside. and it has been like this for the last 10 years. The actual exhaust with coolant seems to be discharged in bursts, perhaps your passerby was referring to that ?

    If the exhaust need cleaning surely there would be noticeable reduction in power ?

    Have you tried draining the muffer ? On the 331 the muffler is a fibreglass box with a drain plug on the side which I have periodically undone to see if there is any blockage, so far only clear water has come out.

    Regards, Alasdair

    Piers Covill

    Hi Duncan and Alasdair

    Yes agreed. This sounds like the bleed vent from the antisyphon as Alasdair has pictured. They shouldn’t really emit water but it doesn’t matter if they do. It the a symptom of the non return valve sticking. They are a cheap part to replace if you want to stop it: I replaced mine as I didn’t like the black streaks the water can leave down the hill.

    The exhaust blocking advice is erroneous.

    Best wishes
    NA Tech Sec.


    Afternoon chaps….

    I just wanted to report back on this. After taking the exhaust elbow off it’s clear that it was very much towards the end of its useful life.

    An engineer here in Rotterdam tried to clean it up but it was disintegrating. Hopefully, a photo is attached.

    We decided to opt to replace as this piece failing in any circumstances would be less than ideal. A new elbow was €500 inc gaskets + fitting (we were out of the country..).

    With the new elbow fitted the bleed vent now only pushes water out at high RPM ±2,000. At idle there’s no water coming out of it. From reports on cruisersforum this is much closer to normal behaviour than the jetski like telltale we were seeing.

    The main exhaust also now has a fuller and more constant flow of water.

    @Alasdair “The actual exhaust with coolant seems to be discharged in bursts”
    This is what our exhaust was doing pre-new elbow. Now it’s more constant.

    “If the exhaust need cleaning surely there would be a noticeable reduction in power?”
    It’s quite possible/likely that the backpressure has led to a power reduction. However, as we’ve only had the boat for just over two years, so I don’t have a benchmark to work against.

    When we next take the boat out I’ll see if there’s any difference in water speed.

    If you’re seeing a constant stream at idle I’d consider taking a look at the whole system.

    • This reply was modified 4 years, 8 months ago by 39Schuss.
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