Smoke from exhaust

A

ajames

New Member
Joined
8 Dec 2018
Messages
12
Location
Plymouth
Visit site
I'm hoping someone with more experience can troubleshoot and advise to likely causes. I have a yanmar 3ym30 installed on my boat; when first starting the diesel performs as expected, starts first time with no trouble even in low temperature. however, after the diesel has been ruuning long enough to reach operating temperature white smoke can be seen from the exhaust, it is not excessive, i.e it does not create great billowing clouds, but it is definitely noticeable. Upon increasing revs above ~3000 rpm the smoke turns black, and decreasing revs turns the smoke white again with almost no smoke visible as revs approach idle.

I suspect this is due to incomplete diesel combustion but i don't have a great understanding of diesels, the engine has been laid up and winterised for almost 18 months and has been run for 9 hours since where the smoke has been observed.
 
RichardS said:
Probably the first thing to do is check that the exhaust elbow is not restricted by carbon build-up. It's easy to remove the four nuts and free it from the water and exhaust hoses.

Richard
Click to expand...
Thanks Richard,
I'll give it a check during the week, but i think it might be a cause somehow related due to lack of use as when i last used it in 2019 there was no smoke at all and i'd id expect carbon build up to be more of a gradual thing?
 
If you can reach the exhaust outlet, get your hand down there and feel what's coming out. If it is smoke you will get smuts, if it is steam you will feel dampness.
I had a similar occurrence. The "smoke" turned out to be steam and was caused by a blockage in the raw water inlet reducing the flow of cooling water. While there was water coming out of the exhaust, there was insufficient cooling, causing thr engine to overheat.
 
Simple Diesel engines can emit one of three coloured smoke. White is generally associated with water, blue with oil and black with fuel.

Black smoke is, as you say, associated with incomplete combustion, not enough air to burn the fuel or too much fuel for the available air/oxygen.

Could be an issue with fuel pump and/or injectors or, more likely, air filtration or flow restriction in the exhaust system.
 
Cathy* said:
What was the blockage?
Click to expand...
Mussels!
The water intake is integral with the VP saildrive. When on the hard I usually use a length of rebar to rod up through the body of the saildrive to knock out the collection of dead mussels etc., but on this occasion I seem to have rammed some of them right up the passage leading to the actual seacock valve. When I did eventually deduce what was going on, I tried unsuccessfully to rod down from above (through a length of spare hose attached to the seacock) and found that the solution was to blow down through the tube, which cleared it immediately.
 
Last edited:
ajames said:
I'm hoping someone with more experience can troubleshoot and advise to likely causes. I have a yanmar 3ym30 installed on my boat; when first starting the diesel performs as expected, starts first time with no trouble even in low temperature. however, after the diesel has been ruuning long enough to reach operating temperature white smoke can be seen from the exhaust, it is not excessive, i.e it does not create great billowing clouds, but it is definitely noticeable. Upon increasing revs above ~3000 rpm the smoke turns black, and decreasing revs turns the smoke white again with almost no smoke visible as revs approach idle.

I suspect this is due to incomplete diesel combustion but i don't have a great understanding of diesels, the engine has been laid up and winterised for almost 18 months and has been run for 9 hours since where the smoke has been observed.
Click to expand...

Black smoke is what it is - unburned fuel, usually down to a faulty injector.

White smoke can be more deceptive. If it hangs in the air it is more likely smoke than vapour. vapour is always released on combustion - sometimes it's visible.

In summary, if the engine starts ok runs to max revs and delivers power output, you have a 95+% functioning engine. The tendency to dismantle parts of the combustion and exhaust system to find faults should be resisted. IMHO

PWG
 
I had a similar problem which I diagnosed as being caused by a build up of carbon in the exhaust elbow. In my case the exhaust smoke was not obviously black but it was leaving black deposits on the transom. It's probably the easiest place to start. However if it's obviously black smoke and you have serious reduction in engine revs and performance then it could be something more serious. If it was me I would work back from the exhaust and cooling system as it's a relatively easy and is something you would probably need to do at some stage anyway. After that it's a slightly more daunting job to look at other aspects of the combustion including injectors.
 
Rogershaw said:
What was the temperature of the water coming out of the exhaust while the steam was exiting the exhaust.

I have white smoke and the water coming out is about hand washing temperature.
Click to expand...
TBH I've no idea... we'd motorsailed much of the way from Cowes to Roscoff and were about 30 NM off when it became noticeable - an increasing amount of steam and a decreasing amount of water coming out of the exhaust.
Not being quite on the ball after nearly 20 hours of being awake I actually phoned my Yanmar guy at home in Cowes and 'check the strainer' was his first thought. I stopped the engine, did that pulling a big chunk of weed out (old fashioned brass strainer) and off we went again no problem.
I've checked it regularly since !
 
Mine has let out some white fume in cold weather for years. In my case it is steam and just seems to be a feature of the engine. However if the cooling water is coming out the temperature of a hot hand wash i would say that is too hot and suspect a blockage or damaged impleller. The heat exchanger on the 3YM30 is barely good enough for the job ( even after the upgrade required to some early models) and any lack of flow will likely cause an overheat especialy when the sea water is warmer in summer.
Black smoke is unburned fuel often a blocked air filter ( get yours out and wash it in the sink) or it could be that the vessel's prop is too much. Others have mentioned the tendency of the exhaust manifold to block which could be an issue as could be a blockage in the rubber exhaust pipe.
 
You must log in or register to reply here.

Share this page

Top