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Old 03-06-2008, 16:33   #1
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RPM Loss Question

One of my 2005 Yanmar 30's is losing RPMs from time to time. Here's how it goes:

It wavers, losing 200RPM or so for maybe 30 seconds to a minute about 2x in a 12 hour day. After the little waver, it goes right back to full power.

I'm guessing air getting in the line somehow. Anyone experienced this or know anything about it?

The engines have approx 500hrs on them now.

Makes no difference if the fuel tanks are full or empty. Also, it only happens one engine.

Thanks in advance for any ideas.
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Old 03-06-2008, 16:56   #2
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I have had the same problem and have slowly worked my way through everything till the only thing left to do was the filter housing, I had changed filters, o'rings, replumbed the fuel hose etc. etc. I had a racor 2000 that I have now replaced with a Separ 2000, so far the problem hasn't re-occured. Spoke with another sailor with the same filter and problem (racor) and he cured it by putting the pump (electric) on the inlet side so the filter was pressured rather than acting under a vacuum.
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Old 03-06-2008, 17:21   #3
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There was a thread about this that sounded the same not too long ago.
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Old 03-06-2008, 18:55   #4
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Quote:
I'm guessing air getting in the line somehow. Anyone experienced this or know anything about it?
An Air bubble in the line. When the filters start to go it gradually get worse and never improves. An air bubble can last a long time and can be difficult to get out. It can float around a long time and go and come back depening on where it is.
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Old 04-06-2008, 02:12   #5
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OK before getting too carried away, how about something a little simple like, do you have a fridge compressor on that engine? how about a charge controller on the Alt on that engine. Any other load that cycles on and off? Being not big Hp, they will be affected by loads more easily.
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Old 04-06-2008, 04:48   #6
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Quote:
Originally Posted by ssullivan View Post
One of my 2005 Yanmar 30's is losing RPMs from time to time. .

Thanks in advance for any ideas.
I think you're concentrating in the right area. Alan has a good point but he hasn't seen your solar panels or charge controller or taken into account that you have two alternators running.

When this happens to me I change the fuel filter and the problem disappears. If the filters are new, I'd take a look at the fuel return line. This can happen if that line becomes blocked.
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Old 04-06-2008, 07:09   #7
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Sully, I get a similar symptom at times. It goes away when I change the Racor. This happens before my vacuum gage says I need to change Racors.

It happens more often on my port engine than my starboard both of which are fed off the same Racor. If it starts to happen on the starboard, I am about to lose both engines.

George
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Old 04-06-2008, 09:51   #8
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Wow, George, why do you even have gauges ;-)

If what Gord says isn't the case, I'd go for the air leak from a filter that's going south. If it only happens on 1 engine, AND you have a single filtration system, it's probably from the filter to the engine.
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Old 04-06-2008, 13:52   #9
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Rick has a good point. Blocked return line can cause issues. But seeing as the return line is after all filters and pump etc etc, that seems very difficult to block. However, worth a look.
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Old 04-06-2008, 13:59   #10
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A couple of years ago, my Yanmar 4JH3 all of a sudden refused to rev over 2400 rpm. I changed the filters (engine-mounted Yanmar spin-on and Racor 500), but no improvement. Then it got better by itself. Then it got worse again.

It turned out that it was the fuel intake line. There were aluminum turnings from the threaded tank fittings that had got themselves sucked up into the pickup tube. I un-clamped the hose, reamed it out with a length of 10 AWG wire, and the turnings fell out. Also blew bubbles back through the pickup tube in case there were some at the inlet.

Haven't had any problems since.
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