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Old 12-04-2009, 06:49   #1
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Exhaust Elbow Clogging Frequently

I have a Yanmar 3QM 30 with the U shaped exhaust elbow.

I just finished cleaning out the elbow because of clogging (I'm getting pretty good at telling when it's getting clogged, the engine temp starts going up just a little). This was a new elbow about 2 years ago.

It seems to me that this ought to last longer then a couple years. FWIW, we cruise full time, there are about 700 engine hours on this elbow (crap that's a lot of motoring :-)

Is there something else in the exhaust system that I should be looking at? Maybe the elbow needs to be higher? Maybe there's too much or not enough water flowing in the anti-syphon bypass? Maybe there's a restriction further down the exhaust system?

Or maybe getting 2 years out of an elbow is about right?


Any thoughts would be greatly appreciated.

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Old 12-04-2009, 07:15   #2
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Man is that nasty looking. What is it you're cleaning out, soot and carbon I am guessing?
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Old 12-04-2009, 08:34   #3
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Mixing Elbow, Yanmar 3GM30F

Apparently Yanmar recommends changing the exhaust elbow at around 2 years.
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Old 12-04-2009, 08:51   #4
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when you get your new elbow (assuming you are going to ) try to find a high quality hi-heat paint that will stand up to that environment and slow the corrosion
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Old 12-04-2009, 10:14   #5
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What comes out is soot, rust and corroded metal. There is still quite a bit of meat in the elbow. It's no where near needing replacement from being 'worn through'.

(a metal boat builder friend once told me that it doesn't take much steel to make _a_lot_ of rust)

The elbow before this one I cleaned once, then replaced after about 6 months and it was fairly clean when I replaced it. This leads me to believe that the bulk of the restriction happens quickly. This is the first time I've gone with the plan of cleaning it and then re-checking it again in a year. They are getting too expensive to just replace every year.

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Old 12-04-2009, 12:16   #6
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In two years I'd be pulling it. I pull mine at least every year and clean them out or replace them when need be. A lot depends on your type of motoring as to how soon they will clog up. At what RPM range do you run your motor under load? Also how clean is your system? Diesels are fairly simple beasts that require fuel and air in proper proportions. If either one is out of proportion it'll give you fits and buildup. As your elbow gets restricted you'll have a tendency to throttle more dumping more fuel than air into your engine accelerating the clogging problem and reducing energy efficiency. Do you change or clean your air filter often? Do you occasionally have your injectors checked?
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Old 13-04-2009, 07:13   #7
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500 Hour part

While we now have a Perkins, we did have a similar Yanmar on our prior boat, that proved a very reliable motor with the right maintenance. After having one problem with the mixing elbow, our Yanmar rep advised us that, in normal use, the elbow was a 500 hour (max) part. We followed the regime of replacing the part at or close to that interval, as has the young man that bought the boat, and there have been no problems since, despite the engine now approaching 22 years of service.

FWIW...
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Old 14-04-2009, 11:57   #8
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Thanks for all the input. Sound's like I'm in the ball park and should be checking it more often.

To answer a few questions:

The injectors are new, about 200 hours ago. Though they were never serviced before that (motor has 4300 hours). The injection pump was rebuilt about 700 hours ago. I would say the clogging has been about on the same interval regardless of the injection pump rebuild. We'll see if the injector rebuild does anything I guess.

I check the air filter often.

Probably the only thing I don't do properly is run it at full load. We used to run at 2200 RPM (max with our prop and hull is about 2800). That was when I had a job and didn't mind spending money on fuel. Now, that we're cruising I run the motor at 1800 or less. We don't go that much slower under power and the fuel consumption is much less.

I guess this problem could be partially attibuted to low load motoring, but I would say that the elbow would still clog if I ran at 2200 all the time, it would just take a little longer.

This motor really has been super. Besides being heavy and noisey I can't fault it at all.

Thanks again for all the help,

John
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Old 14-04-2009, 12:53   #9
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That is rust scale, plain and simple. When iron oxidizes it expands, reducing the inside diameter of your exhaust riser causing a restriction which causes overheating.

On my boat I bought stainless steel exhaust risers. Yes they were more expensive, but 12 years later, I have never had to remove them for a cleaning nor had to replace them. It was worth the money of buying stainless risers to not have to never have to go through that hassle again.

I would be willing to bet someone makes an aftermarket stainless steel version if it is not already offered by the manufacturer.

BTW, no paint is going to hold up in that kind of an environment.
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Old 14-04-2009, 14:41   #10
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The first pic shows more than rust. Secondly your savings on fuel consumption is going to be greatly offset by running your 3QM at 1800 RPM by the repair bills sure to follow. You're loving that engine to death. Let it run at faster RPMs if you want to see another 4300 hrs. Your 3QM should easily run above 3200 RPM. If max RPM is 2800 that is a sign of something not right. You could be over propped or your exhaust clogging was another. You should be able to reach 3600-3800 RPMs in neutural.
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Old 13-09-2009, 01:38   #11
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The stainless exhaust riser sounds like a good idea.
Also what about switching to Trident silicon 90 elbows? Unless this is the mixing circuit.
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Old 13-09-2009, 12:25   #12
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Old 13-09-2009, 15:11   #13
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We had this with our genset (clogging up with carbon) and upgrading to a larger diameter exhaust system solved it since. We had 600 hours on the genset when the problem was discovered, and put 5,000 hours on it since then without trouble. We average 3000 hours on an injection elbow (cast iron but outside painted).

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Old 18-09-2009, 19:27   #14
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I don't know about all this, sounds fishy to me. My Yanmar is 21 years old, and has 1900 hours, on the same elbow with no problems.
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Old 18-09-2009, 20:13   #15
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Sounds to me

Sounds to me like you're over propped. Your engine should be able to hit max rated RPM with a clean prop, a clean bottom and no current. Anything less than your boat hitting max rated RPM and you're producing more soot than is necessary. I would double check your RPM with a digital photo tach to be sure you are not hitting max rated RPM.

My neighbors Yanmar has 2200 to 2300 hours on it and has never had an exhaust elbow done in 18 years...
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