Yanmar 2GM20F blew oil/fuel out exhaust

[Jaqun]

I was coming out of a harbor when I cranked up my Yanmar 2GM20F a little higher than normal (cruising at approx 5 knots with growth on hull). at the worst moment (busy inlet) my motor blew a bunch of oil (all over my new dingy but that's another matter ) out the exhaust. I turned the key off and proceeded to put up the sails in about a knot of breeze. I was having trouble finding a breeze to fill my sails, but the boat was inexplicably tracking well. It took me a minute to notice that the motor was still running as I had instinctively turned the key instead of starving the fuel. It was no longer leaking any oil so after a time and enough wind for steerage, I turned the motor off, checked the oil and added a tiny bit, tuned the motor back on and limped home at half speed (3.o knots motorsailing) for about 6 hours.
The motor never gave me any trouble after that. Checked the oil and kept an eye on the exhaust port but the engine behaved fine the rest of the trip.
I am told that sometimes unburnt fuel can build up in the engine and then let go at once? Any other ideas about how oil or fuel can get shot out an exhaust?

[model 10]

What do you consider a bunch? A little goes a long way with oil and water. Sounds like you just blew out the carbon. You should do like some others and run it at full power for a little bit, more often.

[foreverfreedom]

Originally Posted by Jaqun

I was coming out of a harbor when I cranked up my Yanmar 2GM20F a little higher than normal (cruising at approx 5 knots with growth on hull). at the worst moment (busy inlet) my motor blew a bunch of oil (all over my new dingy but that's another matter ) out the exhaust. I turned the key off and proceeded to put up the sails in about a knot of breeze. I was having trouble finding a breeze to fill my sails, but the boat was inexplicably tracking well. It took me a minute to notice that the motor was still running as I had instinctively turned the key instead of starving the fuel. It was no longer leaking any oil so after a time and enough wind for steerage, I turned the motor off, checked the oil and added a tiny bit, tuned the motor back on and limped home at half speed (3.o knots motorsailing) for about 6 hours.
The motor never gave me any trouble after that. Checked the oil and kept an eye on the exhaust port but the engine behaved fine the rest of the trip.
I am told that sometimes unburnt fuel can build up in the engine and then let go at once? Any other ideas about how oil or fuel can get shot out an exhaust?

Definitely sounds like a carbon build-up in the exhaust. We have a 3GM30F, and the manual tells us to run it at just short of max revs for 10 minutes in every hour to keep the exhaust clear.
Also, does your engine has a STOP lever, if so and you just turned the key to turn off the engine, you may have destroyed your alternator diodes.

[Cheechako]

I think the key info is: was it really oil or carbon?

[Jaqun]

I instructed to run at top rev's at the end of a long trip but I will look into doing it more often.
I do not have a stop lever, only the fuel starve pull out. I'm glad the engine didn't stop because I'm not sure I would have had the nerve to re-start it after the incident and I was a long way from home. Noticing that it did not seem to have ill effects from the time it was left on was the reason I felt confident (though cautious) to motor on.
Thanks for the reply.

[foreverfreedom]

Originally Posted by Jaqun

I instructed to run at top rev's at the end of a long trip but I will look into doing it more often.
I do not have a stop lever, only the fuel starve pull out. I'm glad the engine didn't stop because I'm not sure I would have had the nerve to re-start it after the incident and I was a long way from home. Noticing that it did not seem to have ill effects from the time it was left on was the reason I felt confident (though cautious) to motor on.
Thanks for the reply.

"fuel starve pull out" - That's what I mean by a STOP lever. I would now advise checking that your alternator is still charging.

[Jaqun]

Copy. It was only running this way for roughly 10 minutes when I noticed and corrected it but I will check the alternator first thing when I get back to the boat. Thanks.

Cheechako: I think you hit right on. I'll bet it was carbon.

Thank you both.

[jamhass]

Originally Posted by foreverfreedom

you just turned the key to turn off the engine, you may have destroyed your alternator diodes.

No, turning off the "ignition" does not harm the alternator. Removing the load, i.e. turning off the master house bank switch is what can blow the diodes. Turning the key off merely turns off the field, which is essentially what regulators do.

[Elie]

To that, my opinion is quite simple: If it works, don't fix it!... A short misbehavior of an engine should be of no consequences... Just watch it a bit for a while.

[Martkimwat]

Have you checked to see if there is water in the oil? Yanmars are notorious for the exhaust mixer corroding and allowing raw water back into the exhaust inlet on the block. This can eventually corrode through the relatively thin wall between the exhaust port and the pushrod galleries. It can be fixed by getting the exhaust port in the head sleeved. to prevent it happening, check your exhaust water mixing manifold at least once a season and when it is due for replacement, there is a guy on Ebay in the USA who manufactures them in stainless steel for about $100 and ships world wide. much better and a lot cheaper than the Yanmar original.

[wannacat]

My one 2GM20F was hard to start and left a trail of oil/fuel blackish stuff every time I pushed her up over around 2500 rpm. I had the engine tore down and totally rebuilt in Lauderdale. Now she powers up to 3000 rpm easily and no traces on the water running all day long. Starts like a dream and I've suddenly found extra top end speed! I'd have a compression test done if this persists at top rpm.

[Wotname]

Reasonably good advice so far so I will just ask "how many hours on the engine and when was the last time you pulled the exhaust/mixing below off and cleaned it"?

And yes, these engines love being run at near max rpm fairly regularly, just like manual says .