Exhaust Soot (lots of it)

[DAW]

My Yanmar 35gm puts out an ungodly amount of soot through the exhaust underwater. It leaves a large trail through the water but never surfaces like an oil slick or sheen. It blackens the transome, but the major amount just streams behind the boat. I mean it is VERY visible. This problem comes and goes, but every time it does happen I can only get the RPMs up to 1100-1200. Has anyone else had a similar problem or even know what is causing this? She is a 1991 37.5 with 438 hrs. on her. Thank you.

 

[SteveC]

Are you able to get high RPM's

out of the engine?? If you can't get the max 3200?? or so you may have a corroded exhaust elbow. If you haven't replaced it, it may be constricting the exhaust.

 

[Bryan C.]

Not breathing or under load

The black soot indicates an engine under load -- too much fuel being processed relative to the RPMs that the engine can produce. If your bottom and prop are fouled, it will create excess load. If it is smoking that badly even in neutral, I agree with Steve, your exhaust system is coked up or clogged up, and the mixing elblow is the probable culprate.

 

[Bob England]

Other possibilities

The previous two posts cover the most likely problems. Otherwise, it could be caused by bad injectors. Another more esoteric problem could be the way you advance the throttle (fuel) lever. Diesels don't have the fast throttle response that gasoline engines have. I have found with my 2GM20F that if I advance the fuel lever too quickly, the engine will go into "smoking mode", and leave a black trail in the water like you described. If I advance the level slowly, letting the engine build RPMs gradually, this is less likely to happen. Just a thought.

 

[Thorp]

We've been here before

With only 438 hrs on a 9 year old boat indicates you don't use your engine much. Diesels hate this kind of neglect and is sending you a message. I'll bet your fuel has gone bad which probably fouled your filters as well as the injectors and the mixing elbow. You should search the archives for the discussion we had a while back on this subject. In a nutshell, you should never put your engine under a load until its warms to normal operating temperature, keep the fuel fresh and stabilized and run it allot... You'll never wear it out by running it, but you'll pay the price for too little use.

 

[Mick Boland]

air filter

Check your air filter first.