Yanmar blowing black, then eventually stopped

[Jimmy_G]

I had my Yanmar 2GM20 serviced a few months ago and then found it blowing black smoke and carbon spots on the water (not oil) when over 1800 rpm. It seemed to run fine below those revs. The hull is quite clean but the prop it accumulating a few crusty critters.
I noted that the mechanic overfilled the sump so I got him back to bring it back to full. that made no difference to the smoking above 1800.
My mechanic suggested removing the air filter in case it was air blockage via the intake, but that didn't change it.'My next move was to look at the air exit for a narrow or mostly blocked exit, but I haven''t done that yet.
Yesterday we went out, but after about 4 h running of the engine it lost revs then died after a few minutes.
Blowing out the carbon accumulation was an early suggestion, but there was too much black smoke for that scenario and running it harder didnt make things any better. So I have all but ruled that out.
I have had injectors suggested, but not much else to go by.
I am not aware of the engines history other than it was installed in 1990 and underwent some sort of rebuild about 8 years ago.
I am about to try and find a good quality marine diesel mechanic in the Brisbane area (SE Queensland) (any suggestions?), but was looking for any expert advice that may be out there.
I recently read a long blog (~27 pp) on this channel about a guy on holidays
with a similar problem and don't want to go through the semi-blind, 'lets try this' style of approach to finding the problem, which eventually ended in him having to buy ad install a new engine, plus costs of all the experimentational, but unsuccessful attempts at diagnosis and work ($25K).
Ideas welcome.

regards,
Dave

[Compass790]

Blocked exhaust elbow is most likely cause if boat bottom & prop clean clean.
When it was serviced were the tappets done?
Compression check?
Usually blowing black smoke is overloading, too much back pressure, insufficient compression, air intake restriction or faulty injectors.
thats a few things for you to start on.

[Wotname]

G'day Dave, welcome aboard CF.
First up, your problem is probably simple - it is hard to kill a 2GM20 although it would be nice to know what was done in the supposed rebuild eight years ago.

I suggest you sort out the problem yourself unless you get a genuine recommendation of a good mechanic in your area.

Have you removed, checked and cleaned the exhaust mixing elbow - these clog up and cause exactly the problems you are describing!

Then clean your prop, it doesn't take much fouling to overload the engine, especially if it over propped to begin with.


What hull is the engine fitted to?
Can you redline the engine (3600 rpm) in neutral?

Previously -
At what revs do you reach hull speed in calm water, no wind and with a clean bottom and prop?
What revs can be reached at WOT in the same conditions?

[Wotname]

Compass is faster and gives some good ideas!

[bobnlesley]

As above, this sounds like a classic case of a blocked exhaust elbow.If you current 'mechanic' who did your servicing hasn't already suggested this, then be sure to never let him touch your engine again.

[Jimmy_G]

Thanks Compass790 and Wotname,
The tappets were not serviced last time so I'll keep that and a compression check up my sleeve.
I will go and check the exhaust elbow for blockage. Hopefully that will do the trick!
Otherwise, into the next stage: Injectors check....

She is a Hartley sister twin keel, plywood boat; and I cannot easily redline the engine in neutral due to a press-in button that switches between neutral and not in the way of the accelerator lever. I think we reached about 2800 revs when testing after a clean and prop replacement.

Will repost what happens next.
Thanks again.
(any updated views still welcome

[Jimmy_G]

Thanks very much bobnlesley,
sounds like the next step is locked in.
D

[alctel]

It could also be a fouled prop. Does it blow the smoke in neutral?

[Jimmy_G]

No smoke in neutral of low revs. The prop was anti-fouled in December 2018, but has some limited encrusted fouling.

[NYSail]

It’s a blocked exhaust elbow. Remove and replace..... easy job.

Greg

[Boston Steve]

Originally Posted by Jimmy_G

I had my Yanmar 2GM20 serviced a few months ago and then found it blowing black smoke and carbon spots on the water (not oil) when over 1800 rpm. It seemed to run fine below those revs. The hull is quite clean but the prop it accumulating a few crusty critters.
I noted that the mechanic overfilled the sump so I got him back to bring it back to full. that made no difference to the smoking above 1800.
My mechanic suggested removing the air filter in case it was air blockage via the intake, but that didn't change it.'My next move was to look at the air exit for a narrow or mostly blocked exit, but I haven''t done that yet.
Yesterday we went out, but after about 4 h running of the engine it lost revs then died after a few minutes.
Blowing out the carbon accumulation was an early suggestion, but there was too much black smoke for that scenario and running it harder didnt make things any better. So I have all but ruled that out.
I have had injectors suggested, but not much else to go by.
I am not aware of the engines history other than it was installed in 1990 and underwent some sort of rebuild about 8 years ago.
I am about to try and find a good quality marine diesel mechanic in the Brisbane area (SE Queensland) (any suggestions?), but was looking for any expert advice that may be out there.
I recently read a long blog (~27 pp) on this channel about a guy on holidays
with a similar problem and don't want to go through the semi-blind, 'lets try this' style of approach to finding the problem, which eventually ended in him having to buy ad install a new engine, plus costs of all the experimentational, but unsuccessful attempts at diagnosis and work ($25K).
Ideas welcome.

regards,
Dave

Most likely faulty injectors or fuel problem but it sounds like your injectors are full of carbon its a good place to start . relatively easy to remove your injectors on the Yan Mars make sure you clean around everything before you start and be careful not to lose the washers .

[bouton]

I had similar symptoms on my Farymann. Black smoke on revs 2000. If I put throttle for more revs. With load. Without load I reach 3500 revs and no black smoke. I found out that My sailboat was overpropped. When I put smaller prop engine works ok with max 3200 rev and without black smoke. Can you confirm, that without load there is no black smoke?

[Jimmy_G]

Hi again folks,
I've just been down to the boat, took the exhaust elbow off and found that the mixing elbow was about 20% blocked (so 80% open), but the cooling water pipe that enters it at the top just as it leaves the engine block had its entrance to the elbow pipe blocked over with carbon.
So my questions now are: (i) I assume the cooling water would normally enter the mixing elbow with an open and clear passage; its hard to be sure given I'm a novice (ii) would a blocked cooling water exit pipe cause the symptoms I described? (iii) if this is the case should I simply be able to punch through the blocked section into the mixing elbow to resolve the issue, and (iv) when the engine died while I was underway, I assume it meant that water was not exiting the engine. Do I now need to be aware of other damage??
I'm hoping that I can now simply unblock the cooling water pipe entrance into the mixing elbow, put it all back together and she'll go well? Am I dreaming?
I noted carbon build up inside the engine block at the mixing elbow point. Probably about 50% blocked. Is this an issue in the short term?

Thanks again for your thoughts.
Dave

[Compass790]

I can't give you a hard & fast answer but I would want to clear all the carbon out myself.
You could try just clearing the elbow & the water inlet but if your manifold is 50% blocked it is unlikely to improve & could be the cause of your problems.
Of you have a plastic waterlock muffler i'm surprised you haven't burnt a hole in it. You need to check on startup that your engine is pumping water out the exhaust as a habit.
This is a dirty trick you can try. Get a spray bottle of water ready. Warm up the engine & set it to run at say 1500 rpm & spray water in the air intake until it starts bogging ( dropping revs down to stalling point) then let it rev up again & repeat.
The water flashes off to steam & blows a lot of carbon out. just be careful that it's just a mist you are spraying in there, you are not trying to drown it.
If you are not confident you understand what this idea means better not to do it.
Is your overheating alarm working?

[Compass790]

Whoops, meant to say only try the spray bottle trick AFTER you have cleaned elbow & water injection point.