Yanmar 3HN35F OVERHEATING - SailNet Community
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post #1 of 12 Old 05-24-2009 Thread Starter
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Yanmar 3HN35F OVERHEATING

I'm getting an alarm on my engine panel that says cw overheat after running for about a hour at 2500 RPM . If I let it idle for about 10 minutes the alarm goes off , but resumes if I continue at 2500 RPM . Yesterday I nursed her in at 1500 RPM with no resultant alarm . Could someone point me in the right direction for what I should be checking to diagnose this issue ???
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post #2 of 12 Old 05-24-2009
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Two possible reasons:

Cooling fluid level is low. This is cooling water (fresh) inside the engine.
Cooling raw (sea water) is not pumped enough. Possible reasons are impeller and blocade in heat exchanger.

Please also check the V belts. If they are loose the pumping might be less than needed.
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post #3 of 12 Old 05-24-2009
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Could be a bunch of things, e.g., exhaust blockage, intake constraint, pump problem, coolant issues, etc - how many hours on engine? periodic maintenance consisted of? any recent work done? good exhaust flow visible? strainer recently checked/cleaned? hose integrity verified? etc...

It's a wild guess to id an overheat problem without sufficient info
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post #4 of 12 Old 05-24-2009
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Originally Posted by k1vsk View Post
Could be a bunch of things, e.g., exhaust blockage, intake constraint, pump problem, coolant issues, etc - how many hours on engine? periodic maintenance consisted of? any recent work done? good exhaust flow visible? strainer recently checked/cleaned? hose integrity verified? etc...

It's a wild guess to id an overheat problem without sufficient info
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post #5 of 12 Old 05-24-2009
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Here's one more common one for the list - stuck thermostat. Often a stuck thermostat will only open part way, then get stuck. You can run the motor slow with no problems but at cruise it will over heat.
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post #6 of 12 Old 05-24-2009
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Quote:
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Here's one more common one for the list - stuck thermostat. Often a stuck thermostat will only open part way, then get stuck. You can run the motor slow with no problems but at cruise it will over heat.
That comment reminded me of something...........I replaced the thermostat a few years ago just as a matter of performing preventative maintenance and noticed that I had a pretty clear view of the inside of the engine water jacket. That in itself gave me some information as I noticed it was nice and clean.

So that's an idea that is cheap enough: replace the thermostat. I've done it routinely on both my car ('90 model with 400,000 miles) and boat.

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post #7 of 12 Old 05-24-2009
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This happened to me last week. OK below 3000, cw alarm over. Turned out to be a bad gasket on the raw water strainer. At low RPM there was no problem but as soon as the RPM went up and the engine really started sucking water it also started getting air through the strainer - Not enough water going through and it overheated. Throttle back down for a while and it went away. I suppose this could also happen because of a leak anywhere in the system - running the engine at higher RPM puts a lot more suction into the hoses, strainers, everything.
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post #8 of 12 Old 05-25-2009
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First and usually the easiest item to check on Yanmars is the 90 degree elbow that injects the raw water into the elbow, they get restricted with the salt coming out of solution, then check the elbow, they are expensive but they are classed as a 'wear' item that required replacement every few years.
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post #9 of 12 Old 05-26-2009
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Check your fresh water intake. After sitting all winter you probly have growth and water restriction there or some thing sucked into the hole. If you have a water intake filter it may be clogged. It worked fine last year so it should be something along the fresh water side. Your impeller may be damaged. If need a hand let me know. Alan

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post #10 of 12 Old 05-26-2009 Thread Starter
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Thanks for all the input guys . I'm sort of chasing it down . I back flushed from the raw water filter out the through hull intake even though there didn't appear to be any restriction there . I sort of skipped a step in that I've been considering the water pump . I have a new spare impeller so I though to replace the old water pump with a new one & re--build the old one for a spare . The cheapest I could find a new one was $360 , so I'm going to back up a step ( which I should have done anyway ) & look at the raw water strainer even though I had it off & could blow freely through it . Then I'll re-surface the cover plate on the pump & install the new impeller soon as I can find a replacment gasket for it , & hope that cures my problem . Once again thanks guys . Say Al....know anywhere close to here I can find that gasket ???
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