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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel > Yanmar 3GMF: Mystery Drop in RPMs
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Thread: Yanmar 3GMF: Mystery Drop in RPMs Reply to Thread
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Topic Review (Newest First)
08-09-2011 06:02 PM
CapitainMike
Quote:
Originally Posted by casey1999 View Post
You are correct on the fuel filter at bottom of the tank. After buying my boat, we moved it to its new slip. We were having problems with the engine stalling. It turned out the screen at the bottom of the tank fuel outlet pipe was totally blocked. I removed the 20 gallon tank and cleaned it. I also cleaned all the filters. The stuff doing the clogging was a black substance (maybe bacteria). I used a biocide with the new fuel. After one year I see the tank has this same black substance on the bottom. It has not made it to the screen. I plan to polish the fuel in the tank to try to remove this material.

As you suggest, I think I will replace my old fuel lines- maybe the inside is falling apart. Any ideas what the black stuff is that I get in my tank?

Also, what effect would a valve adjustment (engine) have if the valve clearances are way out of spec?
Send me your normal email address in a private mail and I will send you two Pdf files that might help you You are supposed to put some in every time you fill up an if you keep diesel in plastic containers them also.

If you are going to take your tank out again fit a small sump and a drain off tap to it.
08-09-2011 02:09 PM
casey1999
Quote:
Originally Posted by CapitainMike View Post
Some fuel tanks have a filter in the bottom of the tank on the outlet fuel pipe. If you can check the filter could be partially blocked. alternatively and may be also you have fuel lines that are breaking up in side causing a blockage.
You are correct on the fuel filter at bottom of the tank. After buying my boat, we moved it to its new slip. We were having problems with the engine stalling. It turned out the screen at the bottom of the tank fuel outlet pipe was totally blocked. I removed the 20 gallon tank and cleaned it. I also cleaned all the filters. The stuff doing the clogging was a black substance (maybe bacteria). I used a biocide with the new fuel. After one year I see the tank has this same black substance on the bottom. It has not made it to the screen. I plan to polish the fuel in the tank to try to remove this material.

As you suggest, I think I will replace my old fuel lines- maybe the inside is falling apart. Any ideas what the black stuff is that I get in my tank?

Also, what effect would a valve adjustment (engine) have if the valve clearances are way out of spec?
08-09-2011 01:20 PM
CapitainMike
Quote:
Originally Posted by casey1999 View Post
I have a Yanmar 3gmd (30 years old). Runs good except even in neutral I only get to 2700 rpm (based on yanmar tach). In gear and moving at 6.5 knot hull speed the rpm is also 2700. Engine has all new filters and exhaust elbow is good. Exhaust color is normal. Not sure what my problem is either. One thing I have noticed is that when the engine is running I do not have fuel being returned via the fuel return line. I need to look into this more. I am thinking my fuel injection pump is not putting out enough fuel. The lift pump on my engine is new.

One thing is the bleed screw on the top of the Yanmar fuel filter housing has a plastic gasket that can crack and let air in. You may want to replace.

Let me know what you find.
Some fuel tanks have a filter in the bottom of the tank on the outlet fuel pipe. If you can check the filter could be partially blocked. alternatively and may be also you have fuel lines that are breaking up in side causing a blockage.
08-08-2011 06:08 PM
casey1999 I have a Yanmar 3gmd (30 years old). Runs good except even in neutral I only get to 2700 rpm (based on yanmar tach). In gear and moving at 6.5 knot hull speed the rpm is also 2700. Engine has all new filters and exhaust elbow is good. Exhaust color is normal. Not sure what my problem is either. One thing I have noticed is that when the engine is running I do not have fuel being returned via the fuel return line. I need to look into this more. I am thinking my fuel injection pump is not putting out enough fuel. The lift pump on my engine is new.

One thing is the bleed screw on the top of the Yanmar fuel filter housing has a plastic gasket that can crack and let air in. You may want to replace.

Let me know what you find.
08-08-2011 02:03 PM
EscapadeCaliber40LRC To make sure it is an engine problem, I'd drop off the shaft behind the clutch and run the engine. If the problem goes away something with the prop is not right. Varible props need to be greased periodically. Do you have a varible prop ?
08-08-2011 11:45 AM
CapitainMike Check the flexible Exhaust Th interior might be de-laminating.
08-08-2011 10:05 AM
aa3jy ..plus check the silencer(air cleaner)
08-08-2011 09:03 AM
cspaniel Bleed the low pressure (screw over the secondary filter) and report what you get. Does it take a while until it expels diesel? does the diesel have bubbles on it?....continue bleeding if the above is true until diesel with no air comes out and try your engine again. Bleed from the screw at "entrance" to injector pump...let us know.
Abe
08-08-2011 12:53 AM
jrd22 What you have described is pretty typical of a clogged fuel filter as Olson says above. I would change both filters and thoroughly bleed the fuel system and see if that makes any difference before digging into anything else.
08-07-2011 03:43 PM
olson34
My guess is....

Based solely on my own limited diesel experience...
I would ask how it's been since the last filter change in your old Racor? (And, did this happen before than time?)

I once had a similar problem that soon progressed to non-starting and it turned out to be a microscopic air leak somewhere in the two O rings or in one of the several plugs in the top of our 1988 OEM Racor filter assembly. Since that model was long ago replaced in the market by a newer model, I replaced it with the current one. Problem solved.

Admittedly this is only a long-range wild guess, but it takes only a very very small air leak to slow a diesel under load.

"Advice virtually rendered to strangers, deposit one virtual cent."

Good luck,
LB
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