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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 06-05-2014
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Yanmar 3GM30F

Engine runs good for about an hour...3,000 RPM's...then starts to "hunt" and RPM's drop to 1,000..have replaced everything...lines, filters, injectors etc but problem persists.

Last edited by blavin; 06-05-2014 at 06:54 PM.
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Old 06-05-2014
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Re: Yanmar 3GM30F

How's your fuel? 30,000 is a bit high for a 3GM30F maybe 3000.
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Old 06-05-2014
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Re: Yanmar 3GM30F

sounds like you sucking air, re-bleed fluel lines check for air leajks around filters, etc
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Old 06-08-2014
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Re: Yanmar 3GM30F

How bad is your hunting? Can you see an actual change on your tach? I sometimes get a hunt (same motor) but it's barely noticeable and can be eliminated by changing rpms just a smidge. I think sometimes sympathetic vibrations can be mistaken for the engine hunting. Have you checked your coupling alignment lately? Prop for dings? Cutlass? I run at 26-2800 rpm and feel 3k constantly is a bit high but a lot depends in the individual set-up.
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Re: Yanmar 3GM30F

Will check the coupling...good suggestion ! Hunting at 2800-3000 then drops to 2000-1500 after an hour of motoring. Thanks for your input...
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Re: Yanmar 3GM30F

"Hunting" usually refers to cycling rpm up and down. If you are just dropping rpms, that's a lot different. Can you make it regain rpm by throttling up? Maybe just linkage? If you can't get it above 2gs, there's something wrong with the engine itself, probably a fuel blockage or air leak. Did you check all the bleeders? There is one behind the rubber water line by the injector pump. Make sure all the banjo fittings are tight, fuel filter cup is seated well, filters are clean. Air leaks can be insidious problems to find. Is your tank clean? If it has a screen on the end of the dip tube, losing rpms from fot clogging it is a classic symptom.
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Re: Yanmar 3GM30F

Yes, am losing rpm's....after running at 3,000 drops to 1500 then rises again and repeats...only happens after running for about an hour.....all the other issues you've mentioned have been attended to...more then once...it's driving me crazy plus missing out on fabulous boating weather !!
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Re: Yanmar 3GM30F

Tank vent clear?
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Re: Yanmar 3GM30F

Blown out and cleaned....done everything...some twice !!...Thanks
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Re: Yanmar 3GM30F

I've had two instances of similar problems. The first one drove me nuts. There was a hairline crack in the crappy plastic dip tube elbow that goes into the fuel tank. It's not in a spot where it's easy to see and the crack was almost invisible. I wrapped the entire thing with epoxy and fg cloth neatly and it's better than new. The second instance was from fot in the fuel tank. The fill tubes on some tanks have a screen over the end at the bottom of the tank. It clogs easily. I took the fill tube out and poked much larger holes in the screen. Some people recommend taking the screen off altogether. Your regular filters catch dirt where it's easy to remove. Taking the dip tube out also let me siphon out any remaining contamination. Your problem sounds VERY much like this second one because the engine does not die, it just loses rpm. Hope this helps. These things are like a puzzle
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