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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 07-19-2011
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Smoking Yanmar

The Yanmar 2GM20F in my 1990 Hunter 30 is driving me crazy.

In 2007 the mixing elbow coked up, I pulled it, cleaned it, and reinstalled it. Since then, the engine starts lightly smoking (gray) after about an hour at cruising RMP (2850-2900, 80% of max rating), and leaving an ever present soot stain on the hull after of the exhaust port.

At idle there is always a slight soot discharge visible on the water surface (on return to port).

I've run tests to make sure the 3 bladed prop is the right size. I get 4100 rpm in neutral, and 3750 in trials, achieve hull speed of 6.7 knots.

At the end of last season, I had a mechanic pull and rebuild the injectors, and adjust the valves. At the same time I pulled the mixing elbow again, it was half blocked, and replaced it. Also replaced the air intake filter.

The engine runs at normal temperature--I can always touch the mixing elbow even after hours of hours running, our cool Northwest waters keep it just warm to the touch.

The engine runs well and consumes very little oil between annual changes. I had the oil lab tested this spring and all is normal. The engine has 700 hours on it. I never idle for prolonged periods or run at low RPMs; always at 75% to 80% of max rating.

Its not the elbow, not overheating, not injectors, not burning oil, not overpropped, but still emitting unburned fuel.

I think about just running it, but I know the elbow is getting clogged, and slow but steady damage may be occuring to the pistons and cylinders.

Diesel experts, please tell me where to go next with this.
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Old 07-19-2011
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I'm stumped, seems like you've covered all the bases. Based on your observations I would suspect injectors, sometimes re-built isn't "like new"?
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Old 07-19-2011
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If your elbow is that coked up, I bet your valves look pretty crusty as well, leading to incomplete burn. When was the last time you adjusted the valves?
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Old 07-19-2011
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To begin with I am a mechanical incompetent. I just want to make that quite clear. However we had a similar problem right down to the lowish water temp. Always the smoke, always small amount of soot. Never need to top up oil between changes(yearly).

Then our water trap developed a leak . Replaced it. Still get a bit of smoke but the soot problem went away. I couldn't believe the crap that came out of the old trap.

(We don't have an elbow in our system as such. Pipe rises from water trap to underside of cockpit seat then drops back down at the stern before outlet. I guess you could call that a somewhat distorted elbow ?). Does the elbow perform same task as water trap ?
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Old 07-20-2011
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1. Injectors were replaced, not rebuilt. They gave me the old ones.
2. Mechanic adjusted the valves when installing the new injectors.

3. tdw - this is interesting. I'm not sure what the "water trap" is to which you refer. I'm guessing it is an anti-siphon valve that has to be in place somewhere between where the sea water exits the engine and the exhaust port. This is a safety feature, so a siphon can't be created and sink the boat. The mixing elbow is a "U" shaped pipe on the exhaust manifold where the exhaust gas mixes with the sea water and exits through a muffler and to the exhaust port. Because there is a rear stateroom in this boat, there is no place for a anti-siphon valve between the engine and the exhaust port. So there is one right before the seawater line attaches to the mixing elbow. It has been leaking, like yours (if we are talking about the same thing), and dripping sea water on the transmission, causing it to corrode. If it is clogged, it would reduce the amount of water flow through the eldbow. Because our waters in the Pacific Northwest remain at 50 degrees F, the engine may run cool enough, but the water flow might be too low to carry the gases (soot) away from the hull. This would cure the soot, but not the slight smoking. Thanks for the idea.
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Old 07-20-2011
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My 2GM20 acts somewhat similarly, but a bit worse. I always assumed that it was due to the low operating temp of the raw water cooled engine. Could it be that your engine's thermostat is faulty and the cooling system never lets it get quite up to temp?
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Old 07-20-2011
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Dean,
As I vaguely suspected your "elbow" equates to my "water trap". Water Trap is container located at the point wher, as with the Elbow, the exhaust gases mix into the outgoing cooling water. It doesn't so much act to stop water siphoning back into the boat as much as it stops water siphoning back into the engine's workings.

Slow,
Thats what i think. Our engine never gets anywhere near proper operating temp.
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