Black water from 2gm exhaust

[Aquah0lic]

When trying to get my self off the shoal yesterday, I looked back and saw a little black smoke and lots of black water coming out the pipe. It was so black I thought for a second I had broke the motor and was pumping oil out. I was runing the engine at about 2400-2600 rpm which is where I run when steaming. When I backed off to idle, the black water subsided but I heard what sounded like several muffled backfires. I wasn't aware that deisels can backfire. I've read that black smoke means your pushing the engine too hard but what about black water?

[cabo_sailor]

Black smoke can mean several things. Basically its unburned fuel. You can get it temporarily if you overload the engine, which you might have done trying to get off the shoal. It can also mean that you have a problem with compression in a cylinder or perhaps an injector.

One thing that is fairly common is to have the exhaust elbow plugged up with carbon. Usually when this happens you will notice a decrease in max rpm as well as copious quantities of black water coming out the exhaust.

I'm afraid that if this is a common occurrence you'll need to do some diagnostic checking. The simplest to fix will be the exhaust elbow. Remove, inspect. If filled with carbon gunk, soak in a dilute muriatic acid solution until clean. Inspect for other damage. Mine had corroded a hole the size of a quarter which made for a seriously dangerous source of monoxide in the cabin.

If you can deactivate one cylinder at a time, (my old Yanmar had decompression levers for each cylinder) you can sequentially decompress each cylinder. If the engine sound changes, chances are the cylinder is firing correctly. If decompressing a cylinder makes no difference, then that cylinder is not firing properly and will need attention.

This is not complete by any means but hope it helps.

Rich

[Aquah0lic]

Thanks, that does help. I will look into the elbow. From the outside it looks nearly new but it worth a look. Is that something I can remove with the boat in the water?

My realization that there was no slick told me it wasn't oil. Wouldn't unburnt fuel also leave a slick? Also there was little smoke but the water looked like pure oil pouring out. It was pure black.

Another thought I had, is it possible I was just sucking in black muck from the bottom? I think its all sand there though which made me wonder if I could damage my water pump sucking sand through it.

[never monday]

black chunks in the exhaust water is unburnt fuel. This was from an overload situation.

[DeepFrz]

It very well could have been muck from the bottom and very advisable that you check your impeller and strainer.
Good luck from here on.

[cabo_sailor]

I personally doubt it was muck but it doesn't hurt to check.
There is no problem removing an exhaust elbow with the boat in the water.

One thing you don't mention. Was that the only time you observed this or is it now a common phenomena? One time only, I'd say you overloaded the engine getting off. A common phenomena and you have a problem to solve.

[Aquah0lic]

No, this was the only time I've seen black smoke or water. Its pumping normally now, so no harm done? Maybe I should just leave the elbow alone. This engine runs very good but power does seem to drop off after 2700 rpm. Is that normal. What RPMs do you guys (with a 2gm) normally steam?

[cabo_sailor]

It would sound like you're OK if it was a one time event. The real nasty stuff would be persistent. I'd keep an eye on it and perhaps check your raw water strainer to see what kind of gunk you been sucking in.

I can't help on what's normal for a 2GM engine.

Good luck,
Rich

[never monday]

Originally Posted by Aquah0lic

No, this was the only time I've seen black smoke or water. Its pumping normally now, so no harm done? Maybe I should just leave the elbow alone. This engine runs very good but power does seem to drop off after 2700 rpm. Is that normal. What RPMs do you guys (with a 2gm) normally steam?

what is the maximum RPM it will go?

[Aquah0lic]

Originally Posted by never monday

what is the maximum RPM it will go?

I don't know. In neutral I've pushed it up to 3000 a couple of times before shutting down. Steaming, 2700 is the highest I've run and it uses way more fuel than 2400 for a couple of tenths. Theres a shake at 25 so I run 24 or 26. When I have tried to push it past 2700, it sounds and feels like the engine is really struggling.

[Aquah0lic]

By the way, is that rev it up before shutting down process really beneficial? It seems to my novice brain that reving a free spinning deisel to near full throttle could be bad for the motor.

[never monday]

you need to establish what the number is.
All 2gm's have an inherent vibration somewhere between 177 and 2500. It has to do with the two piston masses 180* out of phase with each other.

[Aquah0lic]

Originally Posted by never monday

you need to establish what the number is.
All 2gm's have an inherent vibration somewhere between 177 and 2500. It has to do with the two piston masses 180* out of phase with each other.

By establish the number, are you saying I should push throttle all the way down and see what happens? Can't you blow up the motor or cause some lesser damage?

[never monday]

Originally Posted by Aquah0lic

By establish the number, are you saying I should push throttle all the way down and see what happens? Can't you blow up the motor or cause some lesser damage?

yes that is exactly what you need to do.
the governor will prevent damage. you as the operator can not damage the engine through normal controls. Except to love the engine to death.

[Aquah0lic]

Should I back off if it starts to smoke real bad? How long should I hold it wide open? I'll do it tomorrow and establish my number. This is done in gear right?