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Old 02-04-2008, 21:02   #1
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Drop in RPM'S IN diesel engine

I have 2 Yanmar 3 GM 30 F diesel engines in my multihull. At times, there is a slight drop in RPM's in my port engine compared to my starboard engine. Both engines are fueled from one fuel tank.
What should I be checking out and can this lead to a major problem.
Thank you very much.
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Old 02-04-2008, 21:17   #2
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How much of a drop...any smoke...is it a "flick" or longer lasting?

The first thing I would suspect is the tach(s)
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Old 03-04-2008, 05:35   #3
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A slight drop now and then followed by resuming could be a lot of things. A better description might help some of our more engine savvy gurus. A strobe light and a sticker on the flywheel can measure the RPM's optically and get the more exact numbers.
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Old 03-04-2008, 05:43   #4
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is there a corresponding change in the engine sound?

probably have a small air leak on that side.
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Old 03-04-2008, 05:52   #5
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I've had the same probem for 5 years. Spend thousands on injector overhauls and replumbing the fuel supply and still It will faulter and sometimes stop for no apparent reason.
I suspect its a simple problem but just can't find it - a small air leak is favorite.

Good luck and I'll follow the thread with intest.
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Old 03-04-2008, 09:22   #6
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The drop is slight- about 200 rpms-a little longer than a flicka- just a couple of minutes than goes back up. Maybe 20 minutes later it will happen again. No smoke and no strange engine noises. Thank you
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Old 03-04-2008, 09:30   #7
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By "no strange engine noises" do you mean no change in engine sounds at all? - or no noises inconsistent with engine speed? In other words, is any engine noise change consistent with a drop of 200 rpm or is the rpm drop just an indicated drop on the tach?

Do you have vacuum gauges on your filters?

Did you clear that line wrapped around the prop?

Dave
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Old 03-04-2008, 10:15   #8
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No changes in engine sounds at all. Yes we have vacuum guages on filters and 2 days ago the diver cleaned the hull and checked everything so no line around prop. We have certainly had that before
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Old 03-04-2008, 10:20   #9
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No changes in engine sounds at all.
In this case, you have a tach issue. Disconnect it and your problem is solved.

Dave
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Old 03-04-2008, 10:24   #10
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We are bringing boat out this afternoon; thanks for the advise and will let you know results.
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Old 03-04-2008, 10:39   #11
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If you can borrow an IR tach, see if you can replicate the drop with the tach looking at the crank pulley. At least then you will see if its your ears or the tach that are mis adjusted.

(They are a lot of fun on their own right... point at the ceiling fan, and anything else thats spinning. )
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Old 03-04-2008, 13:47   #12
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Try a good fuel addiative before you do anything.
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Old 03-04-2008, 18:01   #13
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was not able to get to tach disconnect today but will also use a good fuel additive.
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Old 03-04-2008, 18:06   #14
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I've had the same probem for 5 years. Spend thousands on injector overhauls and replumbing the fuel supply and still It will faulter and sometimes stop for no apparent reason.
I suspect its a simple problem but just can't find it - a small air leak is favorite.

Good luck and I'll follow the thread with intest.
In those thousands have you changed the tach itself or the wiring?
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Old 04-04-2008, 07:27   #15
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was not able to get to tach disconnect today but will also use a good fuel additive.
cc - I hope you understand that "disconnect the tach" was a joke meant to down play your problem. A faulty tach is WAAAAAY less signifcant than a real engine problem. The engine doesn't care what the tach says. The engine would be quite happy without any tach at all. So, don't disconnect your tach. Perhaps check wiring connections as a possible solution to the erroneous indications.

Dave
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