Engine revving

R

richardsnowstar

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I motored out of the harbour the other day and put the sails up. I put the engine into neutral and it was ticking over while we started to sail. The revs then suddenly shot up to maximum. The only way to stop it was to cut the fuel supply. The engine is a Bukh 36.

Can anyone tell me why a diesel engine would suddenly rev like this? Any advice would be appreciated.
 
None of the diesel boffins has replied yet, although I'm sure they will. In the meantime this is the worst cause of a diesel revving as you describe:

http://en.wikipedia.org/wiki/Diesel_engine_runaway

Basically, the engine is 'feeding' off its own lubricating oil. It's scary. I'm not saying yours was caused by the same thing, but it will caution you to trace the fault pdq. Good luck.
 
Probable cause is that engine oil is getting into the combustion chamber and igniting.

How it's doing that isn't so simple. How old is the engine, and how many hours do you think that it has run? Has it a full history of oil changes? Has it possibly been over-filled recently?
 
macd said:
None of the diesel boffins has replied yet, although I'm sure they will. In the meantime this is the worst cause of a diesel revving as you describe:

http://en.wikipedia.org/wiki/Diesel_engine_runaway

Basically, the engine is 'feeding' off its own lubricating oil. It's scary. I'm not saying yours was caused by the same thing, but it will caution you to trace the fault pdq. Good luck.
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Not pretending to be a diesel expert...

That can happen (according to my RYA diesel course!), but I doubt it would be sudden, there would surely be signs beforehand, like being reluctant to drop to tickover.

Does the throttle still work?
 
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It is not the oil, as it would not stop by using the stop control, or shutting fuel supply, but just run away, fueled by the oil .

so check the engine controls arnt holding full power, be ready with the stop control on next start. if runs full speed then, disconnect controls & check the governer etc.
 
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Hi, has happened to me twice. First time was due to it feeding off its own fumes and the air intake was wrongly turned towards the oil filler encouraging this. Second time was due to an air leak from a crack in a CRV filter and it revved high for about 5 seconds every 30 mins or so. Cheers. Colin.
 
Thanks for all the replies. The engine is 27 years old and has done a bit over 4000 hours. It has been serviced regularly and was not overfilled (I did the last oil change). The engine was ticking over in neutral just before it started over-revving so I don't think it was the throttle linkage. There had been no signs of the engine misbehaving before this incident. I have used the engine a fair bit over the weekend after this incident and there has been no repeat or any other problems. The other factor that may be important is that as we were sailing we were heeling a fair bit just before it happened. Is this important?
 
I'm not particularly familiar with your engine but it sounds very much like a governor failure. If its a mech governor perhaps a sticky fly weight?
 
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Gas would also be likely to give a un controllable with fuel shut off run away.

how low was the fuel tank as some injection pumps miss behave when fed airey fuel
 
richardsnowstar said:
...The other factor that may be important is that as we were sailing we were heeling a fair bit just before it happened. Is this important?
Click to expand...

Most likely more oil than is normal got past the rings due to the angle of heel and it ran on this. The throttle and stopper won't work if an diesel runs on its own oil and it will usually rev up to max rpm or until it self destructs.
As a precuation, stop the engine if you expect to be heeled well over!
Ian
 
As others have pointed out, unlikely to have run up on lube oil as shutting the fuel supply worked. But in case this is the problem the only way I know of stopping the engine is to choke it. That is, block the air inlet with something like the sole of a shoe. Do NOT use your hand.
 
The Bukh has a decompressor, so that would do it more safely. I understand not all marine disels have them though!
 
richardsnowstar said:
Thanks for all the replies. The engine is 27 years old and has done a bit over 4000 hours. It has been serviced regularly and was not overfilled (I did the last oil change). The engine was ticking over in neutral just before it started over-revving so I don't think it was the throttle linkage. There had been no signs of the engine misbehaving before this incident. I have used the engine a fair bit over the weekend after this incident and there has been no repeat or any other problems. The other factor that may be important is that as we were sailing we were heeling a fair bit just before it happened. Is this important?
Click to expand...

The angle of heel may have been sufficient for more oul to enter the bores than the oil control rings could deal with. The engines come with a warning of the maximum angle at which they should be run usually.
 
There are a number of things already suggested which could have caused this and happily, the engine stopped when the fuel was cut off! As I understand it, an old engine which could be prone to run-away will have reduced, even marginal, compression and is likely to exhibit starting problems as a consequence. Due to leakage between the combustion chamber and the crankcase past the pistons, not only can oil vapour be drawn up from the crancase to be burnt as fuel, but a part of the charge of diesel can be blown down into the crancase. It would be a good idea to check your oil level - has it increased or decreased significantly? Also does the oil feel right when rubbed between your fingers, or has it been diluted by diesel? Don't panic, I'm just suggesting you check these things to put your mind at rest.

Another thing that irks me is that the dipstick in an engine is calibrated for level usage, but the manufacturers never address the question of whether the oil should be a fixed amount or filled to the mark regardless of the inclination of the engine to match the run of the propshaft. It could lead to either overfilling or aeration around the pick-up depending on the relative positions of the dipstick and pick-up.

Rob
 
If it ran on its own engine oil it would have made an awful racket, very load diesel knock type of racket and there would have been clouds of whitish bluish smoke from the back. Perhaps a bit of air was trapped in the filter housing and the angle of heel caused it to be sucked through, although should have cleared after a minute at most. I would just keep an eye on it, but as has been said the governor could be playing up, but would be hard do diagnose unless it does it all the time.
 
This is a reasonably common problem with diesel car engines and is caused by the Turbo leaking oil into the inlet manifold. If you get this problem with a car quickly put in top gear and let the clutch out. Does your boat engine have a Turbo, if so that's probably the cause of the problem.

Glen
 
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