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post #1 of 13 Old 09-04-2012 Thread Starter
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yanmar 3gmf loss of power

I've had no problems with my diesel engine for 7 years. It is 14 years old and has 1200 hours and very light use lately. Last time I filled the tank was 2years ago but I still have half tank. Suddenly, last week I took the boat out and noticed I could not get more than 4 knots barely getting 3000 rpm. Usually cruise at 6.5 knots, around 3400 rpm. If I push the throttle all the way I get black smoke.
I dove to check the prop, it was fine. I tried running without air filter, made no difference. I just changed the oil recently, and both oil and fuel filters on schedule. I have a hard time believing that something mechanical would change suddenly. Does anybody have experience of losing power because of old fuel?
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post #2 of 13 Old 09-04-2012
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yanmar 3gmf loss of power

Did you check the shaft and strut too? Use biocides in the fuel?

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post #3 of 13 Old 09-04-2012
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Re: yanmar 3gmf loss of power

I would check the fuel pick-up tube screen (pick up tube is in your fuel tank) and see if it is clogged. In a two year span I start getting black gunk in the bottom of my tank. This can block your fuel pick-up screen. I do use biocide and fuel stabilizer ( I have yanmar 3gmd). As a quick check, try blowing into the fuel pick-up tube and see if it is clear, then try your engine, then clean your tank if that is the problem.
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post #4 of 13 Old 09-04-2012
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Re: yanmar 3gmf loss of power

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Originally Posted by migpraca View Post
Usually cruise at 6.5 knots, around 3400 rpm.
You always run 3400RPM? That's rated RPM for the engine, would be working it a bit hard I think.

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post #5 of 13 Old 09-04-2012
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Re: yanmar 3gmf loss of power

If your getting black smoke you are getting enough fuel,I would suspect some sort of over load,fouled prop/shaft etc or even a very dirty bottem maybe even a aternator bearing trying to seize,a bad injector could be dumping raw fuel instead of a fine mist,I agree 3400 rpms seems like a lot
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post #6 of 13 Old 09-05-2012 Thread Starter
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Re: yanmar 3gmf loss of power

Shaft is fine. There are no unusual vibrations. Biocides... never used them. Does that explain the loss of power? What's the solution? Polish the fuel? Thanks.
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Re: yanmar 3gmf loss of power

Thanks. Sounds like cleaning the tank would be a good place to start.
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Re: yanmar 3gmf loss of power

I suspected that too, but no. Even if I motor sail I can't go past 3000rpm. I'll check the alternator. Thanks.
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Re: yanmar 3gmf loss of power

You're probably right. I may have run it just over 3000 to get 6.5 knots. I'm sure I can go to 3500 on full throttle. Now it feels like there is no power. But it starts immediately and idles very regularly.
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post #10 of 13 Old 09-05-2012
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Re: yanmar 3gmf loss of power

Check where the throttle cable attaches to the engine. Maybe the throttle cable has streched or shifted and you are not really getting full throttle. Try pushing the engine mounted throttle lever to full throttle to see if it is at the end of its travel when remote lever is a full throttle.

You mentioned you just changed fuel filters. Maybe you have an air leak now. I would break and reseal the filter to be sure no air leaks and re-prime.

Bad fuel could give you problems, but I would not think it would suddenly appear- unless somthing is getting plugged (fuel pick up). BTW, when my fuel pick up was clogged, it led to engine stalling.

Last edited by casey1999; 09-05-2012 at 03:02 PM.
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