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Old 01-09-2014
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Re: Yanmar 1GM10 runaway?

It's not just fuel that is being drawn in, engine oil can combust too.
Your crankcase is vented through a crankcase ventilation tube that returns crankcase gases back into your intake.
If the engine is worn enough to create blow-by via the piston rings or you have too much oil in the crankcase or worse fuel in your oil via a fault in the fuel rail. You have a recipe for disaster.
The crankcase gases will feed into your intake via the crankcase ventilation tube. Once they become combustible you have a runaway condition.
You can shut off the fuel but the engine with keep running off of the crankcase gases. It will start to increase in rpm which will promote this situation and so on and so on. The engine will start to create so much oil mist that it will continue to increase in rpm until it has a mechanical failure or runs out of oil and seizes. You got lucky somehow.
I'm going to take a guess and say that there is no fuel in your oil and your oil level was a bit to high and/or your rings are worn.
If it happens again you can smother the intake, you can decompress the engine with the decompression lever or you can give it a shot with a fire extinguisher into the intake and put the fire out.
One way or the other you are going to need some engine work done.
Sorry to hear this Paul.
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