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  #1  
Old 01-20-2010
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Runaway Diesel

I have a Yanmar YSM-8 that all of a sudden started to rev up in spite of
throttling back the speed control.
It got so bad that we had to plug the air inlet to stop the engine, then get help bringing back to dock.

Now I'm totally confused??
What's wrong, and how can I fix it?
Thanks in advance for any help.

Dick
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Old 01-20-2010
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Throttle cable broken?
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Old 01-20-2010
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On a diesel, your engine will run away because more "fuel" is getting into the cylinders than it should. "Fuel" can refer to anything flamable but the likely culprits are diesel and engine oil although there have been cases of all sorts of other things including propane in the air.

If it is diesel that is doing this, there is something wrong with your fuel system. It has been too long since I have looked at a YSM-8 so I can't remember exactly what can go wrong.

If it is oil, it either comes in through the air intake or past the rings. For it to get into the air intake, usually you have a turbo or supercharger that needs new seals. To get it to blow by the rings, you just need a worn out engine, this was very common on the old cats. Something that would be a good first check is to check the oil level and check for dilution (blowby will cause dilution). If you haven't seen a lot of blue smoke, it is unlikely that this is the problem. I have never seen an engine as small as this run away from blowby.

When you tried to shut it down, did you try shutting off the fuel supply? It is likely a fuel problem and it could be as simple as a broken throttle cable. I would start by disconnecting the throttle cable and checking throttle arm movement. Once you have checked everything out as much as you can with the engine off, try starting it but be ready to shut off the fuel and if that doesn't work, have something really sturdy to shut off the air with. It doesn't sound like it really ran away from your description but if it does, you need to get it smothered fast (I don't recommend a fire extinguisher due to the damage it will cause with corrosion).
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Old 01-20-2010
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I could only find two pictures of that engine online. That flat cylinder configuration is pretty unique. There is a discussion on torrenson that mentions this same problem.

I've heard of diesels being run with 3" ball valves screwed to the intake, so they could be shut off in case of run-away!

Last edited by thinplaces; 01-20-2010 at 08:34 PM. Reason: added link
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Old 01-20-2010
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Just as an FYI, we have a runaway diesel article in our archives:

Runaway Diesel Engine

And a follow-up comment that was recently posted:

Recommend addition to Runaway Diesel article.
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Sincere thanks for the most helpful information, and suggestions.
Am getting right on them.
Dick
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Old 12-14-2012
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Cool Re: Runaway Diesel

I have a YSM-8 That ran away on me today. It scared the hell out of me, but I was seasoned enough to know what to do
Diesel engine runaway is a rare condition affecting diesel engines, where the engine goes out of control, consuming its own lubrication oil and running at higher and higher RPM until it over speeds to a point where it destroys itself either due to mechanical failure or engine seizure through lack of lubrication. For instance, a 1800 rpm engine can run to 4000 or 5000 rpm or beyond.
The reason for the runaway Yanmar Diesel is probably caused by poor combustion rings or from overfilling the crankcase with oil, or certain other mechanical problems such as a broken internal fuel pipe.
A diesel engine will run on this oil mist, since engine oil has the same energy content as diesel fuel, and so the engine revolutions increase as this extra "fuel" is taken in. As a result of increased revolutions, more oil mist is forced out of the crankcase and into the engine, and a vicious cycle is created. The engine reaches a point where it is generating enough oil mist from its own crankcase oil that shutting off the fuel supply will not stop it and it will run faster and faster until it is destroyed.
Several ways to stop a runaway diesel engine are to block off the air intake, either physically using a cover or plug, or alternatively by directing a CO2 fire extinguisher into the air intake to smother the engine. Engines (like my Yanmar YSM-8) fitted with a decompressor can also be stopped by operating the decompressor. The decompressor is much like a "Jake-Brake" or a compression release engine brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves in the cylinders after the compression cycle, releasing the compressed air trapped in the cylinders, and slowing the engine.
Although Jake brake properly refers to the Jacobs brand of engine brakes, the term has become a genericized trademark and is often used to refer to engine brakes or compression release engine brakes in general, especially on large vehicles or heavy equipment.
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Old 12-14-2012
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Re: Runaway Diesel

Sort of funny, but not really, action starts at 2 minutes:

Watch an Old Guy Shut Down a Runaway Diesel Engine While His Youthful Helpers Panic and Run for the Hills! | BangShift.comBangShift.com

Paul T
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Old 12-15-2012
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Re: Runaway Diesel

I would hope it is not due to alternate fuel in the air like a gas can or a gas engine near. A deisel is a low rpm engine. If not shut down it can throw metal parts like a small bomb that is why you need to be able to shut off the air. They have been able to pull engine oil that is needed to lube the parts that are overspeed bad deal. I have seen what is left after a driver got over a leak in a gas line not nice. Regards Lou
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Old 12-15-2012
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Re: Runaway Diesel

did you check the lube oil level afterwards?if it was running on its own oil the level would be low,probalely very low
or it could be something gone awry with the govener causing to hang wide open
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