2GM20F revving flat out on it”a own and won’t shut off.

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wipe_out

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Hi,
Bit of a scary situation today. All seemed fine for the first 30 min but then for no reason the engine Fuat revved off the scale (over 4000 rpm) and nothing I could do to kill it. Fortunately it died on it”s own eventually.

Returned to the berth by Seastart and when he looked over it he said diesel or oil must be getting into the cylinder.

Anyone had this experience? Any ideas what would cause this?

just hope the engine isn”t damaged from the high revs.
 
Levels were fine on Thursday when I checked everything in preparation for going out today. Haven’t looked again. Was just glad to be back.
Boat is 2002 so assume the engine is original making it 18years old.
 
It sounds like diesel runaway but if it was the amount of smoke which pours out of the engine whilst it consumes it's own sump oil is horrendous and you would have been choking to death. If this didn't happen, and the sump is still full to the normal oil level, it's simply a stuck throttle or governor and the engine is probably OK but it needs sorting to make sure that it doesn't happen again.

Richard
 
I'm guessing it was in neutral and wouldn't go into gear, which would have reduced the revs to the point where they couldn't damage the engine? Presumably the engine stop button also wouldn't function?
 
There was a fair bit of smoke but I would t say it was like a fog.

Was in gear when is started in a narrow channel so pulled back to neutral at which point the revs went through the roof. Pulling the kill cable didn’t immediately help but after about 30-60 seconds it started reducing and then finally stopped.
 
RichardS said:
It sounds like diesel runaway but if it was the amount of smoke which pours out of the engine whilst it consumes it's own sump oil is horrendous and you would have been choking to death. If this didn't happen, and the sump is still full to the normal oil level, it's simply a stuck throttle or governor and the engine is probably OK but it needs sorting to make sure that it doesn't happen again.

Richard
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I agree.. Check the sump oil again before any attempt to start it.
 
wipe_out said:
There was a fair bit of smoke but I would t say it was like a fog.

Was in gear when is started in a narrow channel so pulled back to neutral at which point the revs went through the roof. Pulling the kill cable didn’t immediately help but after about 30-60 seconds it started reducing and then finally stopped.
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If it was fully running away you would be in no doubt and pulling the fuel cut-off would do nothing. I guess that it might be possible that you have just a small amount of piston blow-by and it was on the point of limited runaway, supplemented by the fuel, and cutting off the fuel was enough to slow it down.

The only way to stop a full runaway, which is when the engine burns it's own oil, is to deprive the engine of air. This means holding something over the air intake like a gloved hand or a cushion or similar. Not a loose rag as that might just get sucked in to the air intake. It can be difficult to block the intake because the intake air filter might get in the way so I would remove the filter if you have one so you can see the open pipe and the start the engine and be ready to stop it this way if necessary.

If you can find a large wooden or hard rubber bung which fits snugly into the intake but cannot be drawn in, then that is perfect. At the first sign of uncontrolled running just stick the bung in the intake and it will stop within a few seconds. It won't do any harm at all to run without the air filter whilst you get to the bottom of what is going on.

Obviously, now the engine is stopped, check that the cable from the morse control is actually and reliably closing the throttle arm on the side of the engine against its end stop.

A cylinder compression test would also determine if you have excessive blow-by but you would need an engineer with the right equipment to do that.

Richard
 
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If the engine is a bit old and oil getting up into the rocker box could oil from the breather being returned to the intake not be an issue, or is the possible amount insignificant?
After checking everything perhaps finding an emergency way to block off the air intake would be worth while setting up?
I would be nervous about starting her up again unless I had positively identified the probable issue?
 
I would be nervous about starting it again, unless fully prepared with the oil level checked, the air filter removed and a bung to hand ready to stop the air supply if the engine runs away again.
 
the GM series have a air filter with a narrow intake, away from belts and hot exhaust elbows, so easy to just block with your hand (if there's access in your installation)
but if it stopped pulling the stop cable, shouldn't really be anything THAT bad as some of the suggestions above.
however it's easy to get the lift pump membrane fail and dump diesel in the sump... Easy to check it also.

V.
 
We had exactly this scenario on my mates twister last year with his 1gm . A few times on one trip. Checked everything that was suggested, stuck morse etc etc.
I will speak to him tomorrow to refresh my memory, but I'm sure the basic cause was oil going where it shouldn't, though I think the lift pump was at issue too?
 
The other reason diesels can run away is when there's flammable gas in the air they breathe. Just saying...

A mate of mine had the injector pump of a 1GM put together wrong, so what should have been minimum fuel was maximum.
If the engine can be decompressed, pulling that will stop a runaway.

Blocking the inlet to the air filter on something like a GM series might be an own goal, it will suck from the rocker box breather which will make things worse.
ISTR we've discussed this before and engines for certain applications have an air shut off slide downstream of any breather connections.

I think it can be a bugger to diagnose, because even if the cause is a stuck governor, over-revving will often throw oil into the breather?
 
Thanks all. Some interesting comments. The lift pump diaphragm was not something I had thought of but sounds like something to check. If it is pumping diesel into the crank case then it would certainly create an alternate fuel path through the breather. Will try get to the boat this week and pull the dipstick and see where the level is. If it’s much higher than before it could be the issue. There is oil all over the back of the engine and some under the engine in the pan to suggests oil is being pushed out somewhere and my hunch is it’s likely the breather.
 
When you do restart I would have something ready to block the air inlet - it should be a reasonable seal that will not get sucked into the engine. This should help slow or stop the engine if it takes off again.
 
Believe me a bung of some description will be permanently available from now on even after it’s all resolved. The fact that there is a even a remote chance where the engine can’t be shut off and is in a “full throttle” situation with no rev limiter is not something I plan on leaving open.
My wife and young daughter were on board today and it was not fun. Fortunately my wife was able to stay calm enough to deploy the anchor to stop us drifting into shallow water.
 
I think you need both hands to lift decompression levers, they are two not that far apart and they tend to drop after a few turns (at least mine does) so need to hold them up. If there's space and good access, fine!
 
vas said:
I think you need both hands to lift decompression levers, they are two not that far apart and they tend to drop after a few turns (at least mine does) so need to hold them up. If there's space and good access, fine!
Click to expand...
I thought everyone tied them together with a piece of string led somewhere where it can be pulled without going near alternator belt etc....
Or maybe it's just 3GM owners, who already don't have enough hands.....
 
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