Yanmar 3YM20 won't rev past 1000 RPM - SailNet Community
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post #1 of 13 Old 08-25-2014 Thread Starter
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Yanmar 3YM20 won't rev past 1000 RPM

So I was out sailing and my usual routine is to drop the sails near my slip and motor in. So I fired the motor and it ran, but felt sluggish. I looked at the throttle and noticed it was close to full throttle, yet my speed was about 2000 RPM. I know it goes easily to 3500 and have done that in a pinch, so that was odd. I powered the motor dow and sailed a bit more. When I fired it back up (and it fired up with ease) it now seemed to only go up to maybe 1500 RPM. No warning lights or anything. So I was avoiding paddle boarders and electric boats which i the norm in the harbor this time of year and I had the motor idling. No issues at idle. When I needed to have some thrust, engine went into gear but never got over 1000 RPM. It never died or stalled and it fired right up. My biddy at the dock said he saw dark smoke and when I fired it up in the slip about 30 minutes later, it started right up but I saw a dark slick behind the boat. My initial thoughts are to check the air filter. Clogged air filter sounds like it would have those symptoms. Wont rev and black smoke from unburnt fuel. I have also read about carbon buildup in the exhaust causing it to not be able to breath. Engine has about 300 hours on it. I have had the boat 3 years and I have not changed the air filter at all. Its also due for an oil change. Ideas?

S/V Cuajota - 1975 WD Schock Santana 30

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post #2 of 13 Old 08-25-2014
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Re: Yanmar 3YM20 won't rev past 1000 RPM

Hey,

Check your prop / shaft. Perhaps you have something wrapped around the shaft or prop. That would certainly slow the engine down.

Barry

Barry Lenoble
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post #3 of 13 Old 08-26-2014 Thread Starter
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Re: Yanmar 3YM20 won't rev past 1000 RPM

A little more info....

This happens in neutral, so that takes the prop and prop shaft out of the equation.

I cleaned the air filter and tried it and no difference. I removed the exhaust elbow and poked around a bit and here is what it looked like.



Looks like there is some buildup inside.

Once I put the elbow back on it revved to about 2500 RPM and idled with no smoke.

I think the clog isn't in the elbow, but the pipe connected to the elbow. After running the engine for a few minutes, I felt the elbow and it was hot to the touch. the pipe connected to it was cold. Barely warm. Here is a pic and you can see the elbow and the pipe connected to the elbow.


S/V Cuajota - 1975 WD Schock Santana 30

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post #4 of 13 Old 08-29-2014
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Re: Yanmar 3YM20 won't rev past 1000 RPM

Hey,

Your boat has a water injection exhaust system. The cooling water that was sucked up from the sea and run through the heat exchanger is then injected into the exhaust. This cools the exhaust and quiets the exhaust. The small rubber hose going to the exhaust elbow carries the water. That's why the elbow is hot on one side and cool on the other.

It does look like the elbow is coked up.

Barry

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post #5 of 13 Old 08-29-2014
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Re: Yanmar 3YM20 won't rev past 1000 RPM

Without wanting to state the obvious, have you checked that full throttle on the lever actually gives you full throttle at the engine end?

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1974 C&C27 MkII

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post #6 of 13 Old 08-29-2014 Thread Starter
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Yanmar 3YM20 won't rev past 1000 RPM

Yes Paul. I am indeed getting full throttle at the lever. Thanks Barry I did figure that out as well.

Update...

I pulled the exhaust again and got tons of built up carbon out. Huge pieces. BUT not the blockage. My guess is the blockage was there for a while and causing the carbon buildup. I poured water into one end and it didn't go through the pipes. So I read about the vinegar soak. So I decided to try. Put the pipes in a bucket with 2 gallons of white vinegar. After 8 hours liquid flows through the pipe. Also there is so much soot and carbon in the vinegar that it's black. I'm soaking it until the morning then gonna put it back on the boat and see it it's a temp fix or one that may last a year or so. Looks like I found the culprit. Blocked exhaust.

Here is SOME of what I got out before the vinegar bath.

Yanmar 3YM20 won't rev past 1000 RPM-imageuploadedbytapatalk1409300889.316020.jpg

S/V Cuajota - 1975 WD Schock Santana 30

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post #7 of 13 Old 08-29-2014
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Re: Yanmar 3YM20 won't rev past 1000 RPM

I had the same problem with my 3YM30. I removed the exhaust mixing elbow and everything looked good. On a whim, I unscrewed the small elbow that brings raw water into the exhaust, and there was a blockage just like yours. I just chipped out the deposit with a small screwdriver and that solved the problem. This situation was preceded with a problem with the raw water pump. Have you checked yours?
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Re: Yanmar 3YM20 won't rev past 1000 RPM

I replaced the raw water pump last year.

S/V Cuajota - 1975 WD Schock Santana 30

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post #9 of 13 Old 08-29-2014
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Re: Yanmar 3YM20 won't rev past 1000 RPM

I had a similar problem on a 4JH a couple of years ago. Would fire and run fine but would not get past 1,600 rpm, forward, neutral, or reverse.

Eventually sent fuel pump out for a rebuild. They said nothing wrong but dirty. That said, it then ran fine and still does.

Perhaps the pump has a governor in it, mine does. I suspect it got stuck.
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Re: Yanmar 3YM20 won't rev past 1000 RPM

FIXED!!!!

The vinegar did the trick! I soaked both the elbow pipe and the pipe that comes off the engine together. I didn't have a vice to separate them so I just dunked them connected in a bucket of vinegar. Let it sit for 24 hours in that. Then shook out the remnants of the carbon deposits. Even as I was installing it large clumps were still coming out! It was amazing how much crud was in that small of an area. Originally I poured water into one end and nothing came out the other. Clearly blocked. After the 24 hour soak liquid freely flowed from one end to the other. Put it all back in and fired it up. No smoke at all. Good sign. Let it idle for a few minutes to get the oil pressure up. Then I put it in neutral and revved it. When this was happening before it wouldn't rev past 1000 RPM. Now it freely revved up to 3700 RPM. I wasn't at full throttle yet and it was revving freely. No smoke or black soot in the exhaust. Idling smooth. I found this tip on a forum somewhere and saw that some include this in the yearly maintenance of he boat. All in all I am very happy with this. $15 dollar fix vs. $300.

S/V Cuajota - 1975 WD Schock Santana 30

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