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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 12-06-2010
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Fouling of Mixing Elbow

I often run my Yanmar 2GM20F between 2500-3200 rpm. So I suppose that should be good for blowing off any carbon build-up. However to my dismay, I'm seeing fouling of the Mixing Elbow. Back pressure cause some black soot to come out of air intake and black soot in water when press hard on throttle. This was new elbow 1.5-2 years ago. Do I suppose my injectors need checking/servicing? It is possible its not atomising the fuel enough thus results in lots of unburnt fuel & soot produce? Should I send the injectors in for service/tuning?
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Old 12-06-2010
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2GM20F Mixing elbows are good for about 500 hours. Less if you typically run the engine at low RPM's. They can be cleaned but that does not extend their service life by much. FOrtunately, they are inexpensive and relatively easy to change.

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Old 12-06-2010
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Also, early Yanmar engines, GM's & HM's tend to run a little on the 'rich' side and this will contribute to the fouling. Treat elbows and risers as maintneance items and look to replace them every 2 to 5 years depending on the engines use.
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Old 12-06-2010
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Quote:
Originally Posted by svHyLyte View Post
2GM20F Mixing elbows are good for about 500 hours. Less if you typically run the engine at low RPM's.............
I'm puzzled. I have over 5,000 hours on my Yanmar 4JH3E with no changing of a mixing elbow and no maintenance of the piece. Is there some special difference in the 2GM2OF mixing elbow that would make a difference? It's still just a hunk of metal with raw coolant water in, exhaust in, and the mix of both out, right? If it's not corroding and it's cleaned out, why would it not function as good as new? Take care and joy, Aythya crew
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Old 12-06-2010
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Since I know nothing of your engine, I cannot comment. Having previously owned a 2GM20F for 15+ years, I can speak to my experience with that. The Yanmar tech's indicated 500 hours when we installed the engine and that's about what I found with mine. The parts are cheap and the fix is relatively easy.


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Old 12-06-2010
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Causes of injection elbow 'coking' are:
overly-rich mixtures or partly unburned fuel coming in contact with HOT metal surfaces.
1. engine lugging (not fully spinning at max./cruise rpm and is under extreme load and extreme combustion chamber pressure) due to being 'over-propped'
2. Fouled exhaust manifold (water side of ex. manifold) - exhaust gases not being properly cooled by fouled ex. manifold.
3. Contaminated/degraded fuel ... high content of 'alkenes' and fungus, etc. fragments - cause: 'stale' / degraded fuel in tank. Such 'particles' dont burn well in combustion chamber and 'blow through' to settle on hot downstream exhaust surfaces to form 'asphalt and coke'.
4. Combustion timing 'off' and needs adjustment - engine is 'over-fueling'.
5. faulty/failing injectors - "over-fueling" the engine

Last edited by RichH; 12-06-2010 at 12:11 PM.
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Old 12-07-2010
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The fouling I'm seeing is mostly rust with soot. Pretty hard stuff. I spoke to a Yanmar service engr and he says cooling water is the issue but we've no choice as we need it to cool the hot flue. There's another problem. In reviewing my maintenance records, it seems the failure/leaking of seawater cooling pump corellate with fouling of the mixing elbow. So the foul elbow not just created back pressure, shooting some carbon out via air intake filter, destroying the filter element and staining the engine compartment, it also back pressure the Johnson CW pump causing it to leak at the oil seal. Now failures seems to fall in place as results of the mixing elbow fouling.
The fouled up mixing elbow exhaust hole is not even a quarter the size of a new one. That must have created alot of back pressure.
From reading above replies, I gathers I'm resigned to taking out the elbow for cleaning every 6months to a year. My records also shows I've this fouling problem every 2 years. Hopefully preventive maintenance will extend the replacement period. Just wonder how much of the Injectors contributes to thsi fouling problem.
Thanks for replies.
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Last edited by trantor12020; 12-07-2010 at 09:18 AM.
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Old 12-07-2010
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If you are visualizing a lot rust in the injection elbow coking .... I'd be highly suspicious that the exhaust manifold is starting to also fail. The exh. manifold is cast iron which is good in sea water when it internally develops 'ferrous oxide' or 'black rust'. Black ferrous rust is protective vs. sea water corrosion but can quickly change to destructive ferric/red rust which develops a LOT of pressure between the 'stratifications' inherent in iron castings and once the 'stratifications' (like the structure of an onion) begin to 'push apart', will develop large 'flakes' or platelets of rust. Once 'slab rust' develops in a iron casting it will begin to 'self propagate' ... and in an exhaust manifold is the principal cause of the manifold to develop 'pin-holes' between the gas side and the water side. Running the engine hard/often for long periods to heat soak the iron will re-convert the ferric rust back to ferrous rust - the reason that most navies, etc. dont shut down ship engines with cast iron water cooled components. (Slab rust develops 'exponentially' fast when the cast iron seawater cooling passages are drained and exposed to air. So, to long term 'store' such an engine, you really need to fill it with anti-freeze (mixt.) with rust inhibitors.)

So if you have rust + coke at the inj. nozzle, that would merit that you should remove and inspect also the exhaust manifold - most times you can invert a manifold, go inside with a stiff wire to break up most the dislodged 'platelets', and then pressure-test the water side to be sure that it hasnt developed 'pin holes' - the danger is that a pin hole will 'back fill' a cylinder with water when the engine is shut down.

;-)
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Quote:
Originally Posted by svHyLyte View Post
.......... The parts are cheap and the fix is relatively easy...
Thanks, - With the added info in the post following yours, I understand the nature of the "coked" deposit. I was imagining the chalky carbon deposit that can be easily scraped off with a knife. I now see the choice for the simple replacement.
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Old 12-08-2010
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Quote:
Originally Posted by RichH View Post
If you are visualizing a lot rust in the injection elbow coking .... I'd be highly suspicious that the exhaust manifold is starting to (Slab rust develops 'exponentially' fast when the cast iron seawater cooling passages are drained and exposed to air. So, to long term 'store' such an engine, you really need to fill it with anti-freeze (mixt.) with rust inhibitors.)
;-)
I have a Yanmar 3GMD and was thinking after each use to flush and leave the engine filled with fresh water to eliminate corrosion the sea water might cause. Usually my boat will sit 1 to 2 weeks between use. Would you advise this? My engine is 30 years old and was considering a rydlyme flush. Would you advise this? I am afraid I might clean out some of the good corrosion and leave the engine in worse shape than before. Engine does not overheat but I might be getting some steam (looks like white smoke) out of the exhaust. Thanks for your informative info on how cast iron corrodes.
Casey
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