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post #31 of 50 Old 05-22-2013
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Re: Runaway diesel

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Originally Posted by dvuyxx View Post
I wouldn't mind practicing shutting off the engine by snuffing the intake. Can you harm a diesel by doing that?
only if you managed to get something sucked into the engine.

Generally only 4 things harm engines.

Low/no oil
Overheating
foreign objects ingested
overrunning.

The rest is just wear and tear

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post #32 of 50 Old 05-22-2013
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Re: Runaway diesel

I got to wondering about the decompression lever....

From Yanmar:
Important
Do not stop the engine with the decompression lever. If the decompression
lever used to stop the engine, fuel will spray out and accumulate on top of
the pistons. This will create the danger of an explosion the next time the
engine is started.

From Universal:
DO NOT USE THE DECOMPRESSION LEVER TO STOP THE ENGINE, THIS WILL CAUSE SERIOUS DAMAGE TO THE EXHAUST VALVES.

HANUMAN
'88 C&C 30 MKII
NOANK, CT
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post #33 of 50 Old 05-22-2013 Thread Starter
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Re: Runaway diesel

Sorry about Harborless' problems but I certainly learned about "running away" diesels from this. I like to learn from other peoples problems and not my own.
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post #34 of 50 Old 05-22-2013
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Re: Runaway diesel

I can see the decompression lever causing fuel issues. All that nasty (and diesel is a nasty fuel) has to go somewhere when you hit that level... and it's under a lot of pressure too, so it is going to go -everywhere-

But I cannot see how it can hurt the exhaust valves. You spin the engine over with the lever open to start, why would it hurt the valves to open it when running?

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post #35 of 50 Old 05-22-2013
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Re: Runaway diesel

a) Sounds like part of the runaway problems being discussed here may be specific to 2-cycle diesels which have ports in the side of the cylinder and therefore a potential path for oil to come up from the crankcase

b) Also sounds like checking compression and making sure the rings are sealing nicely would prevent oil from getting up into the combustion area

But I'm just theorizing, my experience is only with gas engines (excepting my straight-6 ford when I was a teenager - used to diesel for a few seconds when you shut it off warm :-) )
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post #36 of 50 Old 05-22-2013
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Re: Runaway diesel

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Originally Posted by mad_machine View Post
I can see the decompression lever causing fuel issues. All that nasty (and diesel is a nasty fuel) has to go somewhere when you hit that level... and it's under a lot of pressure too, so it is going to go -everywhere-

But I cannot see how it can hurt the exhaust valves. You spin the engine over with the lever open to start, why would it hurt the valves to open it when running?
Well, I'm just guessing here, and I'm sure that it depends on the particular engine, but the decompression lever probably holds the exhaust valves open (so that the piston doesn't generate any compression when it strokes upwards, pushing the air and fuel mixture out), and so if you actuate it when the engine is running at speed then it could either A) cause the valve to hit the piston or B) strip out whatever gear/sprocket that is used to actuate the valve to decompress. When you are starting the engine (especially by hand) you are only cranking a few RPMs, when it is running (even at idle) you are at least 500 RPMs.
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post #37 of 50 Old 05-22-2013
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Re: Runaway diesel

I'm not seeing the fuel issue with the decompression lever - the deal is it's a run away engine, you've already pulled the fuel shut off and that didn't work.

Ergo, no diesel getting sprayed anywhere unless it has a magic source.

Lessons learned are opportunities earned.
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post #38 of 50 Old 05-22-2013
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Re: Runaway diesel

Can we start at the beginning. I don't know what running away means. I was picturing like a clutch slipping. He revved it but it didn't move forward. Like slippage. Press the gas and it revs but no movement.
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Re: Runaway diesel

North, it means a diesel that runs and runs and runs on its own - so how can you stop it? Remember, diesels ignite by compression, no spark plug. So opening the ignition circuit to stop the engine doesn't work. The case they're discussing is where there's too much oil in the bottom of the crankcase, so it's coming up the cylinder walls as a nice froth, which combusts and keeps the engine going...faster and faster ...

Since it's got a source of fuel and a source of heat, only way to kill it is to cover the air intake, which they're discussing the ups down ins & outs of doing ...
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Re: Runaway diesel

Oh right! So when it runs away is it stuck in gear or just revving in neutral?
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