Lugging at medium RPMs - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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Old 07-10-2007
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Lugging at medium RPMs

My Yanmar 15hp is lugging when I go above 2000 rpms (guessing because the tack doesn't work). At any rate, when I'm in gear I can make barely 3 knots before the oil and smoke start coming out. Checked the intake and checked the fuel filter. All seems fine. One thing that occurred to me was, my new oil filter is a bit smaller then the old. I was assuming the one I bought with the engine model number was the right one. Any insight?
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Looked at your prop for fouling lately? Doesn't take much to make your prop not work with a hoot.
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Ya, the prop isn't great. But it still does it even when I'm not in gear.
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I would check your compression first and then the injectors. It sounds like it isn't burning efficiently and those are two likely culprits.

Which Yanmar model is it? If it's the 2QM15, I have the operation and the service manuals on my website, if you'll find them useful.

Here's the link (Yanmar stuff is half way down): literature
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The other problem could be the valves on the engine may need adjusting.
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Too much oil?
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Old 07-11-2007
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How did this begin? Was it gradual or all of a sudden? You say "then the oil and smoke start coming out", what color smoke and what is the oil coming out of? What is the water temp. doing when this happens.
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Your symptoms are probably closer to a blocked exhaust system than anything else. Yanmars that continually burn contaminated fuel ('cloudy' fuel that doesnt burn well in the combustion chambers) will usually wind up with a LOT of soot or coke deposits in the exhaust system ... usually deposited near the water injection elbow. Such deposits will 'narrow' the gas passages and cause high backpressure of the combustion chamber which will lead to reduced rpm and power output ... the smoke and 'oil' (probably soot deposits breaking loose) at higher rpm are the typical clues.

If you have the 'stock' Yanmar cast iron exhaust 'drop' that is fairly easy to unbolt and examine for blockage. However if your engine has been retrofitted with a 'dry stack riser' made of black iron pipe .... you typically have to break the pipe apart to find out if its blocked. If you have the 'typical' clabbered together 'dry riser stack', Yanmar dealers now offer a water injection elbow thats made of aluminum thats easier to remove from 'pipe' than the cast iron one they 'used' to offer.
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Thanks for all your feedback folks. Some great comments. Going to take a look at the exhaust tomorrow. I'll let you know how it goes.
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SH,

The backpressure is easy to check. You can go buy a standard pressure gauge from the autoparts store, with a max pressure of 15psi or so. These usually also have a vacuum side too - but you obviously will not need that. Next, find put a needle adapter on the end of the hose and insert it into the exhaust, right after the mixer elbow, with the needle pointing aft. You should be reading about 2.8 psi with about 3 max (I assume this would be the case on your engine, it is on the 4jh4e). This is taken at WOT (Wide open throttle). Fill the hole when done with High Temp Silicone.

If your pressure exceeds 2.8 -3 psi, you may have more problems than just the exhaust. The excessive pressure causes the cylinders to overheat, and over time will scar them - permanently. THis causes compression failures and many of the symptoms you have. Unfortunately, there is no fix for this short of a rebuild or replace. To check whether your engine is ok, you can compression check the cylinders. Many of the modern diesels are in the mid 500 psi's as I recall. However, the differential may be the most important thing between the cylinders. 10% is typically considered the max.

As far as a high back pressure, this can be cause be a blockage, too small of an exhaust hose, muffeler, riser... or anything else from here to there.

Did that make sense?

I am NOT a diesel machanic. However, this information comes directly from Bob at Mastry Yanmar (Distributor), number: 727-522-9471. I hope that helps. Please verify my information with him as you feel appropriate.
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