Black Exhaust water

[Nan Z]

Our '86 Pearson 28-2 has a Yanmar diesel 2GM20F that suddenly is spewing black water at higher rpm's (2000-2400). Under this rpm, the exhaust is perfect with good volume. Any ideas? Thanks.

[NYSail]

You have either a fouled bottom or prop OR your mixing elbow is getting clogged. On my old 3HM35f yanmar, I would see black when the elbow was clogging. Easy replace..... however check bottom and prop first.

Good luck!

[jamhass]

Black exhaust indicates a very rich mixture, common when the engine is under a very high load. Watch a semi when it floors it trying to pass uphill.

If only a momentary issue (as a sharp increase in power, but settles) don't worry about it. If it persists, then check bottom, etc etc. It it has "always" happened at higher rpm, you may be over-pitched.

[Wotname]

Nan Z, nice first post after 8 years; good to see you posting .

You have received good advice in the above replies so let me add this observation.

2,000 to 2,400 rpm is not high for your 2GM20F. With a clean prop and bottom and calm water, it should run up to 3400 - 3600 at WOT (without smoking). If it doesn't get anywhere near this, you are certainly over propped and then any fouling of the prop / bottom will only exacerbate matters.

[RaymondR]

Not a good idea to run the engine with it doing this for very long. I had a 3GM30 in a boat which was over-propped and it eventually blew the head gasket.

[sailormed]

Run your engine at WOT for 20 min and see if the black smoke disappears. You could have accumated soot in the exhaust system. Running at WOT will clean it out.

[NYSail]

Nothing will clean out a coked elbow except a chisel and hammer. Or simply a new mixing elbow......

[gmakhs]

High load or you need to clean the injectors .
If your engine isn't loaded , remove the injectors and get them to a store for cleaning an adjustment , check the elbow as well

[jamhass]

Originally Posted by NYSail

Nothing will clean out a coked elbow except a chisel and hammer. Or simply a new mixing elbow......

Not so.

You can very nicely clean an exhaust elbow electrolytically.

A 5-gal bucket, water, baking soda, some rebar, PVC pipe and a battery charger sets you up. Soak overnight and done. Cleans everything out, but even leaves the paint.

Google "electrolytic rust removal".

One improvement:

take a piece of PVC pipe, drill it full of holes and use it to insulate the rebar from the inside of the elbow.

[Nan Z]

Thank you for your explanation. I plan to have the boat hauled shortly so will see about the bottom and prop.

[Nan Z]

Thank you. Very good information. We purchased this boat last year and was only able to use it late last sailing season. The previous owner put a three-blade prop on it, through Mack Boring and Yanmar, to give him more maneuverability in his tight marina. I have been concerned about it and was planning on asking the yard to put the original 2-bladed prop back on. The stern dips when motoring so that is another sign that it is over-propped, correct?

[Nan Z]

This is good to know: I do not want to cause engine damage. I am having the boat hauled shortly so will definitely have the yard check it out. Thank you.

[Nan Z]

Just had that done at the end of last year. Thanks for helping me rule out causes.

[RaymondR]

Originally Posted by Nan Z

Thank you. Very good information. We purchased this boat last year and was only able to use it late last sailing season. The previous owner put a three-blade prop on it, through Mack Boring and Yanmar, to give him more maneuverability in his tight marina. I have been concerned about it and was planning on asking the yard to put the original 2-bladed prop back on. The stern dips when motoring so that is another sign that it is over-propped, correct?

In the circumstances I think you can be reasonably certain that over-propping is the cause of the problem. I cured my problem (which had come about by using an incorrect gearbox reduction ratio in the prop sizing computer program) by decreasing the prop diameter by 1 1/2" and reducing the pitch by one inch.

[NYSail]

Sure many ways to clean Coke’s elbow...... how much is it worth to risk water in your engine. With the coke comes layers of metal and if there is a breach, expect water flow back. For me...... every 10 years or so spending a couple bucks getting a brand new part and a couple hours labor was worth it.
Good luck

Greg