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Discussion Starter #1
I have about 500 hours on my 3GY20 Yanmar. I've had no problem with it. I've always been able to run it up to 3400 - 3600 RPM. I prefer to run it about 2700 RPM for comfort and vibration, and that seems to optimize speed/efficiency. I try to run it at 3000 periodically to clean the cylinders.

Today I was running it in the bay about 2500 RPM, pushed it up to 2800 as I headed back to the marina. I felt it slow down, looked down at the tach and it was 2500 again. Increasing the throttle made no difference--it wouldn't go past 2500. I can put it in neutral and rev it to 3800, but in gear nothing past 2500.

Any ideas where to start looking?

Thanks.

Steve
 

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Discussion Starter #2
I should add that I can see the prop in the water and it doesn't have anything wound around it. At least nothing that's visible.
 

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When was the last time you changed a fuel filter?? Could be the early sign of pluggage.. If that's not the issue for sure, perhaps your fuel tank pickup has a screen and it's starting to get gummed up..
 
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Fuel starvation probable
Low compression possible
 
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I concur likely fuel problem filter or possible injector issue check filter then lift pump lastly the injectors
 

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I also think it sounds like fuel starvation.

When was the last fuel filter change? If the filter is dirty or fills with water the engine will begin to lose power and be unable to achieve full RPM.

Might check the fuel pickup clear (blow it out using LP air?). Check for a kink in a fuel line.

Good luck.
 
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Steve,

Once you have checked out the fuel supply, check your exhaust mixing elbow.

It clogs up slowly over time and restricts the engine due to back pressure buildup.



Greg
 

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What was the exhaust like when you only achieved 2500 rpm in gear? Any black smoke or soot? If so the exhaust elbow's restriction is a likely reason.
 

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I just serviced one of our charter boats a 3ym30, it had the same issue, I sent the injectors out to be serviced and replaced exhaust elbow now it runs like a champ
 
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Discussion Starter #11
Thank you all very much.

I went through the fuel system thoroughly this summer and put in a new filter. It was running well after that, and it does rev up fully in neutral which would seem to discount the fuel line, wouldn't it?

Injectors and the exhaust mixing chamber are possibilities. Is there a test that can be done on injectors? If the engine is revving in neutral could it still be injectors?

I'll do some more checking based on the suggestions given.
 

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Discussion Starter #12
I didn't notice any smoke when it would only run 2500 rpm in gear. I'll run it again tomorrow to verify.
 

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Look for black soot in the exhaust. If it is present it can indicate that the engine doesn't breathe well enough to burn the fuel needed for full rpm. This leads directly to the exhaust elbow being restricted, a very common occurance as the elbow is where the cooling water is injected. The crud cools and lines the elbow until the engine is not able to exhaust the air needed for combustion.
 

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Revving in neutral won't necessarily rule out fuel supply issues.. takes more fuel to push a load... but if you've changed filters recently then look elsewhere, esp if you know your fuel tank is not full of cooties...;)
 

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Look for black soot in the exhaust. If it is present it can indicate that the engine doesn't breathe well enough to burn the fuel needed for full rpm. This leads directly to the exhaust elbow being restricted, a very common occurance as the elbow is where the cooling water is injected. The crud cools and lines the elbow until the engine is not able to exhaust the air needed for combustion.
Brian is correct when he fingered the mixing elbow which has a life span of about 500 hours on that engine. (It's gotten to the point where I'm thinking of carrying spare mixing elbows for both the 2GM and 3GM engines given the number of times we've helped people with the same issue with these engines while on cruises. Selling new replacements with a modest mark-up would certainly be preferable to helping people dig crap out of their clogged elbows with a dremel tool!)
 
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At 500 hours, your diesel is just a baby! I think it sounds like classic fouled prop putting a load on the engine.

Tod
 

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I had a similar issue with my Yanmar this summer...and prior to the trip I had replaced both the primary and secondary fuel filter so figured that part had to be good at least. turns out, there was a lot of sediment in the primary bowl that I had neglected to clean out. We got into some waves and the sediment got bounced around plugging the brand new filter. I cleaned it out, put in a new filter and off we went with no problems.
 

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Discussion Starter #19
Brian and svHyLyte give good explanations answering my puzzlement at how the exhaust elbow could be giving this effect.

I did go out and verify that at low rpm the exhaust is clear, and as I rev up and it stops at 2500 rpm there is black smoke and a black streak in the cooling water coming out of the tailpipe.

I'll start with that elbow. (It isn't coming off easily.) Does anyone have a recommendation of where to get the elbow?

Thanks again.

Steve
 

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Either a Yanmar dealer or one can be fabricated - either welded or from black iron plumbing parts. Stay away from galvanized as it can give off noxious fumes if it ever gets hot enough.
 
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