Engine accelerating out of control

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davidbfox

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I know I've read about this before, but now it's happened to me and I can't find the answer by a search.

Yanmar YSM12 diesel, on last two outings after about 15 minutes suddenly revs out of control. On first occasion, it settled straight down on closing throttle; today it took about 30 seconds with decompressor open to drop to tickover.

Scarey - and I couldn't work out how else to stop it.

Any ideas? Is it coincidence that I've just changed the oil?

After it' settled down, it appears to run fine, but I shoot straight back to my mooring!
 
You have overfilled the oil and it is running out of control on oil being blown into the air intake via the crankcase breather. It's a guess but check the oil level.
 
This is caused by fuel (yes, it could be gas) getting into the engine air supply. It has potential for enormous harm including destruction of the engine. The immediate remedy to a runaway is to starve the engine of air completely - either by engine room shutters (hazardous to anyone in a big engine room, needless to say, so avoid this on medium sized or large mobos if a crew member is down there). On a smaller engine, find a means of blocking the engine air inlet - but ensure that you don't apply anything to the air inlet which could fragment and be sucked in to the engine (so, for example, don't put a towel over the inlet, as fibres will rip off and enter the intake manifold before doing harm elsewhere).

Until you've got to the bottom of this, don't start the engine again! It's very possibly oil related, so that's where to start troubleshooting.
 
I remember in Hull many years ago a compressor running in an underpass on the fish docks exploded killing a couple of men when an LPG delivery wagon with a leak stopped in the tunnel. The leaking gas powered up the compressor uncontrolably.
So Oen's advice is essential. It could just be hot oil vapour (how is it reaching the air inlet?) or a gas leak. Or perhaps a mechanical control problem.
 
Could be dieseling, with a worn engine worn or stuck rings can give crankcase pressure. This allows engine to run on oil vapour, I've got this with an old Merc van at teh moment when it gets hot, one solution I use is either stall the engine, or drive off....or I found it would also stop or idle by taking the oil filler cap off the rocker box cover, releasing the pressure. This will show you if this is the problem, start it up and be ready to take th filler cap off quicly...or use valve lifters as you have already.

Could be over filling but if its an old engine more likely to be worn or stuck rings.
 
Its running on oil. It happened to my last engine in previous boat (Yanmar YSM12) prior to me buying it.

Quickest way to stop it is to put something hard against the air intake. A small chopping board was what I was advised to use. Don't use anything like a rag or cloth - it will get sucked in.

Drain some of the oil out and don't motorsail when its gusty. I think if you heel by about 20 degrees the oil recirculates and starts it off.

It's apparently quite scary.

Donald
 
As others have said, most probably running on crankcase oil, they are notorious for it.

Not totally sure with the YSM but both the YSE and the YSB, the other horizontal single cylinders, actually have the crankcase breather vent from the port side of the engine venting into the air intake, makes it particularly liable to happen if motorsailing on starboard tack.

These engines are also generally mounted tilting slightly back so if you fill to the upper level on the dip stick, you have actually over filled.

Have a flat piece of ply handy and put it over the air intake to stop the engine if it happens again. Don't use your hand or any thing soft that could get drawn in.
 
[ QUOTE ]
I've got this with an old Merc van at teh moment when it gets hot, one solution I use is either stall the engine, or drive off....or I found it would also stop or idle by taking the oil filler cap off the rocker box cover, releasing the pressure.

[/ QUOTE ]
Suggest you check out your crankcase breather system, as it should be more than able to cope with the pressure from worn rings. The next problem will be that the engine oil is blown out of the breathers - preferable to runaway though! Most engines turn the breather into the inlet manifold though, so the oil goes that way, and the engine then runs on that - unstoppably, for around 30 seconds! scarey. I had a Sherpa that did this regularly. Leaving the breather disconnected meant oil everywhere, so I took the breather pipe to an empty oil can, collected the oil (litre in 50 miles or so, and put it back in the engine every 100 miles or so /forums/images/graemlins/smirk.gif Saved a bomb on engine oil..... and the engine was clapped out anyway so poor oil diddnt matter!

As for the Yanmar, make very sure the engine is not running on a gas leak - as has been pointed out, the engine can go wild and explode with great violence. I have seen a flywheel come off a running engine in a workshop, and take off at about 150mph - straight through a brick wall 30 feet away. Not nice at all.

If it is running on lube oil, there will be a great deal of blue/white smoke. If there is no smoke, then the fault is in the throttle/ injector pump. Whichever, it needs sorting as over revving can do serious damage, bouncing and bending the valves, breaking con rods, or bending the crank. Thats the good case scenario. The bad one is the engine breaking up with moving parts flying out with great violence everywhere - and with probably fatal results for anyone nearby.
 
Just for my own knowledge.

If this happens and stopping the engine in the normal way seems impossible, is it feasible to block the air intake, say with the palm of your hand and pull the stop cord on the governor?

Otherwise, how do you stop a runaway diesel?

EDIT>> whoops, just read escargot's post.
 
Thanks for all this advice. I think it was most likely overfilled oil - I measure it with dipstick sat on edge of the filler, like on a motorbike, and it was a bit high. I'll remove some next weekend and try again if I dare. Never thought of calor gas! I realised about 4 this morning that the engine stop I was pulling for dear life cuts off the fuel but isn't the decompressor, as I said in orginal post. So if it's burning something other than diesel fuel it won't stop it. In the absence of a chopping board to stop the engine I assume the real decompressor would stop it. Thanks again.

And yes, there was loads of smoke -.
 
I seem to remember from a diesel course I did ages ago that using the decompressor in that situation or even just to stop a normally running engine can cause lots of damage to the whirly bits inside ,and was told that to block off the air intake with a plate was the answer.Sorry to be so technical but its many years since the course and I try to avoid going anywhere near engines.
Might be an old wives tale but proper engineers will no doubt confirm or deny.
 
Why?

If you can start an engine using valve lifters why can't you stop it?
It isn't going to lift the valves so far they will strike the piston crown, otherwise that would be a bit of a design error.
 
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