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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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  #1  
Old 05-03-2012
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Is this a Stupid Thought?

So I gotta Yanmar YSM8 that until the last time I ran it under load has always ran-away on me at some point forcing me to shut the engine off via the compression chamber to avoid a full-blown run away situation.

Now, Twas thinking- Since I have replaced the tranny and governor mechanism that leaves only a leak somewhere internally (piston, gasket, o-ring, ect) that is letting (what I think to be) fuel, not oil, somewhere it is not supposed to be causing combustion and then run away.

So here is my thought. The engine knocks a good deal at lower RPMS being just a single clutch old 8 hp engine- simple as can be.
So, my thought was that usually when the engine runs away its when I am at low RPM in forward. Maybe, I think, the extra rattling and vibration is allowing some fuel to work its self into an internal chamber. When the engine is revved up it purrs more or less and is pretty stable- just a small vibration throughout.

So here it is- Do you think that running my diesel at a higher RPM could help prevent fuel or perhaps oil from leaking into a combustion compartment or something? Like, could the knocking at low RPM be A REASON (not THE REASON) that the engine is getting excess fuel/oil inside?

Obviously either way I need to get inside and replace what is worn/ no good. Just wondering if until I get to that if this could be a sort of ghetto-rig fix that will hold-out during the occasional usage I put on the engine.

If this post has been entirely asinine and you feel I have stolen a portion of your life you may never recover then my apologies. Twas the magic dragon befuddled me mind inta conjuring such fools talk.
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Old 05-03-2012
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Re: Is this a Stupid Thought?

Small single cyl diesels do tend to seem to 'knock' at low revs.. your runaway problem is a bit of a mystery, esp if the governor has been replaced.

There should be no path for fuel into the cyl besides the injector lines and the injector itself, so it's difficult to imagine a vibration scenario that might cause 'extra' fuel to enter the system. Also, any fuel not properly atomized by the injector would itself 'knock' or tick, and cause brownish/gray smoke, as the piston tries to compress the non-compressible still-liquid fuel.

If this is a low rpm phenomenon then maybe raising your revs slightly will avoid the problem but this could be problematic while trying to dock the boat....
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Old 05-03-2012
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Re: Is this a Stupid Thought?

lol, yes docking has been fun. I usually just build my speed up then kill the engine and use momentum to carry me in. If I am going against the wind then I keep my jib up until the last second to keep momentum.
This is a mystery... I took off the entire front of the engine and took out the entire governor mechanism and fuel regulator. They were very gunked up but other than that ok.
I do notice that a minute or so before the engine runs a large amount of white smoke begins coming out of the exhaust (which means fuel I read).
I do not suspect oil because I cleaned out the filter and the oil pressure regulator valve is in good shape. I would know if the oil pump wasnt working n a hurry so there is not much else that could result in oil as the culprit (Unless I am missing something.)
The only place where fuel is "let out" is at the fuel injection pump which is what connects to the fuel control rack which is regulated by the fuel injection limiter which attaches direct to the second arm of the governor control mechanism. The second arm attaches to the first which then connects to the regulator spring and finally the lever. The amount of fuel being let in obviously depends on throttle.
I have not actually SEEN my fuel injector pump while it was working- So I suppose it could not be regulating fuel discharge properly? Like it could be streaming instead of spraying? idk..
It doesn't make since to me. The only part of the fuel system where the fuel is put into the engine is at the injector and regulator which I have taken out and cleaned completely.
I thought it must be something in the crank case but I am pretty stumped and out of money for diagnosing.
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Old 05-03-2012
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Re: Is this a Stupid Thought?

I want to note that the engine never runs away in idle or in reverse. Also, its unexpected and unpredictable. Sometimes I run under load for half an hour before it starts to run away. Other times it happens after 2-3 minutes.
Usually the events are as follow:
Cruising under load. Throttle position locked. However, notice that engine revs up and down few hundred RPM after a few minutes--- Like suddenly fuel stopped then went..bad description.. Ok so its like your going forward but then you feel a slight back pull but then your forward again. Like I said bad description but just know that even though the throttle is locked the engine will rev up and down a few hundred RPM (can feel it slightly in the boat.)
Next starts the increased smoke out of the back. I think it is white but I suppose it could be greyish white. Not blue though.
So yea now a good bit of smoke is billowing out of the back... another minute or two and she runs. Then I kill her.
Then, I could restart the engine and do the same thing. It will not immediately run away again but will go through the same routine. The only thing you cant predict is how long each cycle will be. Sometimes half an hour, sometimes 3 minutes.
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Old 05-06-2012
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Re: Is this a Stupid Thought?

Sounds like is a fuel control problem, not lube oil getting into the cylinder. Note that the injector does not regulate the fuel - the injection pump does. The injector basically is a a relief valve with a nozzle on the outlet.

Stopping the engine using the compression release is not a good idea. Most engine manuals warn against this. Best way to stop a runaway engine is cut off the air by plugging the air intake.
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Old 05-06-2012
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Re: Is this a Stupid Thought?

This motor has a thumb screw on the front of the engine to turn to clean the oil strainer, if you remove the strainer you could clean it . Might help. Dale P.S.[ it's by the hand crank]
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Old 05-06-2012
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Re: Is this a Stupid Thought?

It really does sound like a governor problem.
Run-away typically is very violent and difficult to stop unless you can shut off the air supply to the combustion chambers.


....and this is what happens if you cannot stop it. These motors are burning lube oil. They will not run for long like this....



I liked the CO2 fire extinguisher idea as a method of killing the motor. I wonder would it be reliable???
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