low idle speed under load Yanmar 3 GM 30

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Oct 2, 2008
1,424
Island Packet 31 Brunswick, Ga
My 1986 Yanmar 3GM 30 with approx 2k hours revs up to around 34K RPM's when not in gear, but only around 2100 in gear. She starts fine and runs smooth. She does not run hot. I have changed the fuel filters (both for inlines, but not the screen in tank). There also is grey exhaust smoke that I have not noticed before. I have cleaned the prop, but there was very little growth on it. It did not make any difference. I have been told that it could be an exhaust problem, but I don't understand why it could rev up when out of gear, and not rev up in gear if it were an exhaust problem. The shaft turns easily by hand, and does not seem to bind. I have noticed that my packing gland does not appear to leak its required on or two drops under way, but it does not smoke or get hot. Could it be binding anyway in spite of the fact that the shaft turns ok by hand?
This is a recent problems as she was running fine in the spring.
So to summarize, does the fact that the engine revs up well out of gear but not in gear help eliminate some of the possibilities with exhaust issues or fuel line issues? I don't find this particular scenario addressed in "Trouble Shooting Marine Diesels".
Thanks
Keith
P.S. I had posed this question in the body of an earlier thread, but since it got no replies, i thought I would repost as a new thread.
 
Oct 30, 2011
542
klidescope 30t norfolk
Engine revs up

Sounds like ya got an injection pump leaking past it's roter what's the oil consumion or is the engine oil level greatly reduced after 10 hours or so of run time if so the engine is running on crankcase oil and blow by is your proplem
 
Jan 4, 2006
7,264
Hunter 310 West Vancouver, B.C.
I'd Still Lean Towards ..................

I have been told that it could be an exhaust problem, but I don't understand why it could rev up when out of gear, and not rev up in gear if it were an exhaust problem.
........................ the mixing elbow as the cause of the problem. It's worth a look at this point in the game

The reason you can reach 34K in neutral is that it takes very little fuel to turn over the engine without a load. Once you add load to the engine, you have lots of additional burned fuel (carbon dioxide) to pass through that constricted mixing elbow. It's this backup which is reducing the loaded RPM.
 

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Oct 30, 2011
542
klidescope 30t norfolk
3gm30

Yea it's exactly what Ralph said sorry had to reread post to realize diagnose wrong problem
 
Oct 30, 2011
542
klidescope 30t norfolk
Yanmar 3gm30

It's defiantly your mixing elbow close water intake remove elbow and inspect I remove mine and clean every other year and get 6 years out of the Oem part gonna try the aftermarket stainless next time
 

Igetit

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May 26, 2011
85
Hunter 270 Lake Monroe, Indiana
I remove mine and clean every other year
Hey whatfiero,
Anything special about the cleaning process for this part? Do you use a wire brush inside, replace any o-rings or gaskets? Is any part of the muffler serviceable?
I'm on a fresh water lake but suspect I may have this problem also.
Thanks,
Paul
 
Oct 29, 2005
2,362
Hunter Marine 326 303 Singapore
Chisel with curve tip and heavy hammer. Replace gasket (p/n 128370-13201) as required. Do note that one end of Joint (p/n 104214-13580) has RH thread to Exhaust Elbow (p/n 128370-13610) and the other LH thread to Mixing Elbow (p/n 124070-13520).
 
Oct 30, 2011
542
klidescope 30t norfolk
Cleaning mixing elbow

Yea chisel scraper and I use a wire brush in a drill to really clean inside then boiler brush (they sell at HD ). I then paint with high temp paint to try to keep corrosion down for the possible month till paint wears off
 
Oct 2, 2008
1,424
Island Packet 31 Brunswick, Ga
Yea chisel scraper and I use a wire brush in a drill to really clean inside then boiler brush (they sell at HD ). I then paint with high temp paint to try to keep corrosion down for the possible month till paint wears off
Do you take it apart every time you clean it, or is it done in place. I will be going to Jacksonville tomorrow to a Yanmar dealer to get the parts. I hope you stay tuned so that I can post my travails and questions under this thread, then conclude with if it fixed the problem. Either way the exhaust elbow needed to be replaced.
Keith
 
Apr 22, 2011
939
Hunter 27 Pecan Grove, Oriental, NC
I bought a set of these picks: http://www.globalindustrial.com/p/t...mpaignId=T9A&gclid=CKT99JP54boCFUwV7AodwCEAHw at Home Depot. The one with the right angle bend seemed to work the best. I removed the elbow from the engine and the exhaust hose for easier access. I found the worst blockage where the raw water enters the top of the riser. Take your time and keep digging away with the pick. It's hard to tell where the crud stops and the metal starts. I then filled the cavity with phosphoric acid but it didn't seem to make much difference. The gasket is part# 128370-13201. You might want to just buy a new riser while you are at the dealer and cleanup the old one as a spare.
 
Oct 29, 2005
2,362
Hunter Marine 326 303 Singapore
Keith you got to take them out and apart to chisel the rust out. I do annual removal and clean out. With this annual service, the Mixing Elbow will last couple years and the Exhaust Elbow and Joint seems to last almost forever.
 
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