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My 31 is running Yanmar diesels. The starboard
engine's exhaust water is very dark, more so when accelerating. The port engine exhaust water is perfectly clear.
Any idea what the cause is and/or how to fix it

Does it have a sheen? Are you losing any oil? If so, your oil cooler may be shot. I have Cummins and an oil cooler replacement and /or removal for rebuilding is fairly simple.

Sounds like a lot of exhaust soot mixed with the water.

I wonder if your turbo is stuck?

Is the engine developing the power it should under a load?

hb

There is no sheen to the exhaust and we are not losing oil

There is no loss of power

My guess is still Exhaust Soot.

Remove your air filter and see if the condition improves. While the air filter is removed, (and while the engine is NOT running) stick something like a screw driver or a paint stiring stick, or your fingers, into the turbo and make sure its spinning freely.

If you have an aftercooler.....it may be dirty and need cleaning.

Just a guess but you may be either starving for air or overfueling.

I don't know what they are called in todays terms but years ago Mack had a "PUFF LIMITER" on the end of the "Rack" that restricted the pump from dumping lots of fuel on the injectors until the Turbo Boost came up enough to enable a clean burn.......hence the name "PUFF LIMITER".....if the Yanmars have something like this....it could be malfunctioning....dumping excessive fuel during acceleration....

I cannot think of anything else it could be....ain't much of anything else coming out of the exhaust except diesel exhaust and water


hb

I do not have specific knowledge of these motors, but if the exhaust is sooty it usually means there isn't enough air going into the combustion chamber for the amount of fuel.

I had a similar situation with a gasoline turbo charged motor, it turned out to be a leak in the line between the turbo and the intake manifold. This allowed some of the air to escape makning an over rich mixture in the combustion chamber resulting in sooty exhaust.

Two dealers each told me it was variously a blown turbo, or a defective computer module and estimated either fix to be thousands of dollars. A piece of duct tape fixed it until I got a new fitting for a couple of bucks.

I had the same thing happen to my starboard Mercruiser Horizon 350. It was blowing black smoke and the exhaust water was dirty. It turned out to be a small pencil eraser sized filter located under the fuel pressure regulator on the fuel cooler. It was clogged and pushed the fuel pressure in the rails up to about 80 psi instead of the 30-35 psi required. It was obviously running too rich. I pulled the little screen filter out and blew it out with air and solved the problem.

It has been running like a fine tuned machine ever since. I didn't even know about the screen filter until I read about checking it while the fuel pressure regulator was off.

Dick

What model yanmars and hours?
After that info I'll make suggestions but it sounds like incomplete fuel burn.

We had the same problem but it was a valve not seating

Hi. I had the same thing happen to me this weekend. The starboard engine was making soot. The exhaust was a brownish black color i could see in the distance. At slow speed, it had no soot showing, but at between 2000 to 2400 rpm running, i could see it, and my transom was dark when i came in after running it all day.
I have the 3208T's. It runs perfect except for the sooty exhaust. But when I back down on it, i can hear clunk clunk, thru the air filter, until it cools down for about 5 minutes at idle.,Last year i never had this problem but this year I am.
The engines have 2000 hrs. I think Scot here said most need injectors at 2000 hrs. It doesnt use excessive oil and starts right up, and runs smooth. Something is not right in the fuel system, but i would change injectors if i knew one was breaking down at higher speeds. It does come up to speed, but just the starboard has the dirty exhaust, like the Yanmars here do.
Maybe someone could enlighten me on this.
Have a great week.
bob k

Harry: I think they called that an "aneroid" or something like that. it is disconnected on my 1988 Mack R688ST. I like black smoke!!

Rawleigh wrote: I like black smoke!!

And you can certainly feel the difference.....very little TURBO LAG....

Rawleigh, the "B" model is my older truck favorite followed by the "R" Model.....Does your have the 675 or did they have something different in 88......5 Speed???

I can imagine that you too have "Dirty Exhaust".......

hb

Harry Babb wrote:I can imagine that you too have "Dirty Exhaust"......

LOL! Not to mention exhaust slobber after it sits for a while outside! I have the 350HP motor with a Fuller 9 speed. Dual sticks are cool, but require too much practice for me! I really want a B61, but this was available locally for $4,500 and I snagged it for a farm truck.