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Old 08-07-2017, 05:55   #1
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black in exhaust

i just bought a beneteau with the yanmar 3gm 30
when we got it running the was a lot of black in exhaust the mechanic said it was fuel, he wanted me to order a exhaust manifold mixing elbow and all the gaskets, the boat hadn't run in 2 months or more
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Old 08-07-2017, 07:22   #2
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Re: black in exhaust

Seems like it might be worth trying to clean it before buying new?
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Old 08-07-2017, 08:23   #3
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Re: black in exhaust

Also clean the air intake filter. Too much fuel to air mixture will run black. I understand that the Yanmar mixing elbows are rather pricy, so you might want to try less expensive things first. Pull the injectors and have them pop tested. A stuck or damaged injector tip will give you raw fuel rather than a metered spray and it wont burn properly. Two months without running wont normally make any difference unless a mouse nest or something else clogs the air intake. If the easy things dont solve it then I agree that cleaning the elbow and examining it, is better (on your pocket book) than just replacing. Does your mechanic work directly for a Yanmar dealer that wants to sell parts? Just some thoughts. ____Grant.
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Old 22-07-2017, 13:10   #4
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Re: black in exhaust

i bought just the mixing elbow because of cost

we couldn't get elbow off exhaust,so took them both off engine ,when we separated them the prior owner had used jb weld on it the surface on the manifold was so worn ,there wasn't enough surface to machine flat so we replaced with a new one
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Old 22-07-2017, 13:23   #5
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Re: black in exhaust

i bought just the mixing elbow because of cost

we couldn't get elbow off exhaust,so took them both off engine ,when we separated them the prior owner had used jb weld on it the surface on the manifold was so worn ,there wasn't enough surface to machine flat so we replaced with a new one
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Old 22-07-2017, 14:32   #6
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Re: black in exhaust

The engine will run fine with the mixing elbow plugged to a point. The mixing elbow problems on the Yanmar's are usually with the water injection port not the actual exhaust passage clogging up. Supposedly you can't clean the mixing port out on the elbow. At least that's what's been the advice that I've heard. The elbow runs less than $200. Replacing it is relatively straight forward if you can get it off. I had to call in a mechanic to remove the 3GM elbow on mine because of the excessive force required to unscrew it. Took a humongous vice, large pipe wrench with cheater bar and liberal use of the 'blue wrench', (acetylene torch) to get the threads to break loose. It was a PO install so don't know whether they used pipe dope or Never Seize on the threads but the the combination of exhaust and salt water had corroded it in place. You'll need a new gasket if you take the exhaust pipe off so order or buy one before disassembling.

Apparently the mixing elbows clog up quickly on the Yanmars if they are run at idle for regularly or for long periods of time. The elbow on the water injection on my 2GM, looks the same as the one on my 3GM, plugged up with a little over 200 hours on the engine. Owners were elderly and didn't use the boat much and then had title issues when they died. Sat for several years with the engine being run at the dock occasionally to keep it from seizing.

The air cleaner filter in Yanmar's is foam and it breaks down over time. GJordon's advice to check the air filter would be a good idea.

Having said the above, I'd look at taking the boat out and running the engine at cruising rpm for at least an hour and see if the exhaust smoke still is happening. Might be just caused by lack of use and a good run will clear out what is causing the too rich fuel mixture. A plugged mixing elbow could cause a problem but a too rich mixture is usually on the intake side of the equation. Assume you have the right prop on the boat as too steep a pitch or diameter will cause the engine to smoke at higher rpm's and not allow the engine to get to maximum rpm.
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Old 22-07-2017, 17:16   #7
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Re: black in exhaust

Ive never had much luck cleaning old exhaust elbows. In my experience, cleaning them usually reveals more serious problems and you end up replacing them anyway.

Last time around I had new ones fabricated. Like them much better than the originals.
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Old 22-07-2017, 21:08   #8
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Re: black in exhaust

Talking of these risers/mixers needs an input: Dont forget if they are in such bad condition there is a major concern they will leak sea water into your manifold and flood your engine with sea water, the worst part of the mixer/riser is usually the interior which is not visible!!

Back to to black smoke in order of probability

1) Air cleaner blocked/damaged
2) Propeller needs cleaning or perhaps the hull it self
3) Damaged/corroded injector tips or fuel pump issues
4) Turbo charger issues(if fitted)
4) Exhaust issues (last)

Black Smoke is really just an imbalance in the air to fuel ratio - too much fuel to not enough air. This means either too much fuel is being added to the mix or there's not enough oxygen being supplied to burn the fuel. The black smoke is full of particulates that are basically large diesel particles that normally would be burned as fuel.

Cheers Steve
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