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Question about 3QM30F mixing elbow

PostPosted: Sun Sep 30, 2007 12:47 am
by bruadair
Hey everyone,

meant to post this earlier but I guess I got erased from the system and had to re-register. I just cleaned my heat exchanger today and that went pretty good. It was about 20% blocked and now okay.

Then I took off my mixing elbow and was horrified to find that it was about 75% blocked with carbon buildup. It has taken all day and the best I can to is to get it cleaned so that it's about 10% blocked, but the inside walls are very rough. Wondering if this will be an issue or if 10% blocked is acceptable.

But here's the question. I actually had as a spare a new mixing elbow and installed it. The only problem is the hose from the nipple on the exhaust elbow comes straight out while my old one had a 90 degree nipple welded on.

The other hose on the manifold towards the rear also has a 90 degree nipple, but this hose is longer and would reach the new mixing elbow. So does anyone know if these two small hoses can be interchanged or where they go? I have been procrastinating emptying my lazarette to chase them down.


s/v Bruadair

Mixing Elbow

PostPosted: Mon Oct 01, 2007 2:00 pm
by mitchk
Hi Damon, I have just finished re-working my engine and am almost ready to put it back in the boat. I installed a new mixing elbow and had my heat exchanger cleaned at a radiator shop in their acid bath. I then had the sealing surfaces of the manifold milled smooth to insure a good seal. Acording to the Yanmar manual the nipple that comes out of the top rear of the exhaust manifold goes directly to the nipple on the mixing elbow. Mine is the same way, the new elbow has a straight hose nipple and the exhaust manifold has a 90 deg.

PostPosted: Tue Oct 02, 2007 3:14 pm
by bruadair

Thanks for the reply. The new mixing elbow came with a hose to connect these two ports together but mine is set up a little differently. I actually ended up emptying my cockpit locker and followed the two hoses from this port back up under the toe rail behind the helmsman seat. It seems that it's tied into the fiberglass box (muffler?) and acts as an antisiphon valve. That's great news as I can put the hoses on back wards where they fit better. Going to run up the engine today and make sure there are no leaks. When we get to Colombia we'll have the old mixing elbow cleaned better and keep it as a spare.


Mixing elbow

PostPosted: Tue Oct 02, 2007 5:54 pm
by mitchk
Well I didnt pay enough attention to mine when I removed it from the boat. Even though the manual shows it going from the exhaust manifold directly into the elbow I believe mine is set up the same way as your? Guess I better research it a little more I put the engine back in the boat!

PostPosted: Fri Oct 12, 2007 9:55 pm
by rick_bradshaw

We just found a gentleman here in San Diego who says he was envolved with the HCs when they were commissioned here in the U.S. back in the 80's. His name is Jim Wilson. Some of you who were envolved back then may be familiar with him.

Anyway, he was telling us the other day that it is better for the engine to run at higher RPM than lower. For instance, we were told by the previous (original) owner that we should cruise at 2,000 rpm. Jim told us that higher is better and that 3400 to 3700 is about tops and that there is a governor that will come into play. He suggested that we cruise at 2700 or greater rpm.

The result of the higher rpm is less buildup in the elbow. His experience is that those at lower rpm have buildup. Still someone else suggested that we get a replacement elbow as you have and be prepared to exchange it in a couple years. Don't know which is correct yet but time will tell.

I would be interested if this makes sense to you or anyone else.

PostPosted: Sun Oct 14, 2007 2:37 am
by bruadair
Hi Rick,

Thanks for the post. Yes, I agree very much that running at a higher RPM is much better for a diesel and we do so most of the time. But there is no way I could reach 2700 rpm as the max RPM on the Yanmar 3QM30F is only 2400 rpm. We typcially cruise at 2000 to 2100. However there are always exceptions, like here in Bocas del Toro, Panama. With all the shoals and coral patches everywhere we motor slowly so as to stay out of trouble, but when in the clear we rev her up to cruising RPMs.

We've had the boat only for five years, I suspect that with the amount of carbon build up in our exhaust elbow it had started well before we purchased the boat. Thankfully it was in our spares inventory that allowed us to replace it here.

What engine do you have? I suspect that if you are cruising at 2700 rpm or higher you might have a different engine...


PostPosted: Mon Oct 15, 2007 4:15 pm
by rick_bradshaw
Hi Damon,

We've had the boat for about a year and a half now. Most of that time has been spent in upgrading, fixing, fixing the "fix", and getting well after we got fairly ill for about 6 months in Mexico.

When we got her, we had to replace the exhaust elbow since it had a hole in it. Later we had to replace the new injection part because it was essentially clogged in manufacture and wasn't allowing enough water to get to the exhaust. Before we leave this time, we are seriously thinking of getting a couple more new parts for spares just in case that manufacturing process strikes again.

We appear to have the same engine that you do. We are a bit short in the paperwork department concerning the engine. Nothing I've found has told me about what the max rpm is. I'm going by what the previous (original) owner said and what I can glean from others such as Jim Wilson. I was thinking of easing the revs up a bit at a time until the governer cut in. Is 2400 where that happens for you? Current experience means a lot and I'd be interested in everything you can pass on.

We are madly working on the boat and getting ready to depart once again. Hopefully we will be back in Mexico by this time next month and this time wil continue on as we had "planned" last year as ammended by the needs of the day. We've really come to know what is meant by all plans and schedules are written in sand.