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Old 31-05-2016, 17:49   #1
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Low rpm and used up lots of oil in short time

Here's what happened today. Yanmar 2YM15 with 700 hrs, boat recently launched. Motoring at about 2800 rpm when the max rpm suddenly dropped to about 2000. Thought it might be air in the lines or clogged filters. Then I looked at the exhaust and the water was coming out black with white or blue smoke - didn't take the time to analyze it for too long at that point. Put up sails, went below, and found that the oil was about 1/3 to 1/2 a qt down from where it was when we started that day and the engine had been running for less than 15 minutes at that point.

Mechanic will be looking at it shortly, but it seems that oil is getting into the cylinders, yes? Besides rings (which is difficult to believe in a engine with 700 hrs with regular oil changes) what could this be?

Nice sail though.
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Old 31-05-2016, 17:54   #2
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Re: Low rpm and used up lots of oil in short time

Sometimes the worst thing for boat equipment is low hours over a long time. A truck diesel may go 400,000 miles with a lot of idling time to boot. But if you ran it for 10 minutes only, but many times.. it wouldn't! Good luck, let us know what they find.
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Old 31-05-2016, 18:06   #3
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Re: Low rpm and used up lots of oil in short time

We do lots of cruising - put 100 hrs/year at least and running full days with the engine going - we're in Maine, so the season's short. It's a 2006 engine. I could see carbon buildup, but many sailboat engines are used to get on/off the mooring/dock, and last for quite a bit longer. I'm more looking for what to expect from this. Is it a catastrophic engine failure or something in between?

And it has never used oil in the past.
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Old 31-05-2016, 18:48   #4
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Re: Low rpm and used up lots of oil in short time

The RPM drop sounds like a clogged Exhaust elbow or a delaminated exhaust hose.
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Old 01-06-2016, 05:44   #5
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Re: Low rpm and used up lots of oil in short time

things break unexpectedly; oil ring, valve guide, seal between any oil gallery under pressure and exhaust. a compression test will test rings, a look at the exhaust water under operation( collect in a bucket) , oil leak into turbo or manifold should give significat smoke.
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Old 01-06-2016, 09:42   #6
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Re: Low rpm and used up lots of oil in short time

Worst thing it could be would be a cracked block.
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Old 01-06-2016, 10:16   #7
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Re: Low rpm and used up lots of oil in short time

My first look would be to see if the breather hose or vent was clogged, take it off from the filter and blow into it, there should be little to no restriction, if it isn't real clear, then 99% of the time where it connects to the valve cover the screen wire or whatever is used as an oil / air separator is clogged. Taking the dip stick out when it's running is also a good check, if there is pressure in the crankcase, you will know in a hurry. Just slide the stick up a little so you can out it back in a hurry, no need to take it all the way out.
Step two, take oil filler cap off with engine running, there will be a pulsation of air as the pistons move up and down of course, but there should be no real flow of air. If there is a lot of airflow, not just a pulsation, then that points to blow by, stuck or broken rings or glazed cylinder walls maybe.

Never forget the exhaust elbow and or has already been mentioned the exhaust hose, the hose in particular can really fool you, it can look just fine, but the inside can collapse when running, uncommon, but it happens once in a blue moon.


Did it start running a little rough, or just a drop in RPM, but still smooth?
You may have "lost" an injector, lost as in clogged and lost that cylinder, the way to check that is to loosen each injector line at the injector one at a time, you should get a significant drop in RPM and it should get a little rough as you loosen a line. I put a rag around the line as a little fuel will spray or leak out when loosened.


Next is a compression test, but that requires equipment your unlikely to have, so maybe hire a mechanic who has it.
If I had to guess, your symptoms seem to point to a cracked piston, but it could be other things as well, hopefully something simple.

Check the oil level again now that it has had a chance to settle, often checking right after shut down it will show low as there is much that hasn't drained back into the pan yet.
Half a quart in fifteen minutes is a tremendous amount
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Old 01-06-2016, 10:45   #8
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Re: Low rpm and used up lots of oil in short time

Thanks - that's good info. I did check the oil twice, but will check again with a cool engine. When the rpms dropped, it was like someone throttled down.

Can't get there today, maybe tomorrow.
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Old 01-06-2016, 11:38   #9
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Re: Low rpm and used up lots of oil in short time

Cause could perhaps be that something has wrapped round the prop; seaweed, a bit of rope or fishing line.
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Old 01-06-2016, 13:40   #10
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Re: Low rpm and used up lots of oil in short time

We have a winner! The mixing elbow was clogged - at least that's the first thing found. According to the mechanic, the resulting high pressure in the crankcase blew oil past the rings, resulting in that observation.

Interesting to note that you could wreck your engine by running out of oil when the mixing elbow is clogged up, and there is still water coming out of the exhaust. Glad we shut down and sailed back onto the mooring instead of limping in at reduced rpms.

Thanks everyone for your thoughts.
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Old 01-06-2016, 14:43   #11
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Re: Low rpm and used up lots of oil in short time

Good, mixing elbow lots better than most of the other possibilities


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Old 01-06-2016, 22:15   #12
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Re: Low rpm and used up lots of oil in short time

Do you run it at full throttle occasionally? some say 5 mins every hour to help blast out the elbow My yanmar manual (YSM8) recommends doing this before shutting down. FYI they have stainless steel mixing elbows on Ebay which I imagine would last longer but have no personal experience with them. Personally will fabricate my own out of ss when mine craps out as dont like the Yanmar design which can lead to the salt water getting in the head when it fails. Better to keep the salt water injection on the downhill side of the exhaust IMHO.
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Old 02-06-2016, 05:01   #13
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Re: Low rpm and used up lots of oil in short time

I started doing that last year after reading about it. Too little too late. The boat's for sale, so I don't expect to encounter this problem with this boat again.
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Old 02-06-2016, 08:02   #14
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Re: Low rpm and used up lots of oil in short time

700 hours is the rated life on the exhaust elbow. It will blow the head gasket if you go to long .
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