corrosion in exhaust manifold on yanmar 3gm30f

[tikiavenger]

Just purchased a new boat, and the engine survey called out some rust on the exhaust elbow and noted that it should be replaced. I ordered a new elbow and when I went to replace it, I was surprised to see the exhaust mainfold completely rusted. The mixing elbow didn't actually look that bad.

I'm planning to replace the manifold but I'm concerned that I haven't identified the root cause. Is it just leaking through the exhaust elbow? I've checked the anti siphon loop and it seems fine. I'm planning to pull off the muffler and check to see if salt water is backing up there.

Has this happened to anyone else? Any other things I should troubleshoot?

exhaust manifold



exhaust elbow

[Emmalina]

Looks like it coming from above . Is that your heat exchanger :? Very poor gasket made for that ..

[tikiavenger]

Originally Posted by Emmalina

Looks like it coming from above . Is that your heat exchanger :? Very poor gasket made for that ..

The first two pictures show a corroded exhaust manifold port, where the exhaust comes out of the cylinders and then is funnel through the mixing elbow. The heat exchanger is right above the corroded area, and one concern I have is coolant from the heat exchanger leaking into the manifold and hydrolocking the enine.

[skipperpete]

If you chip away the built up crust in pic 4 you’ll probably find the weld on the inner tube is corroded through to the larger raw water injection tube. It’s fairly common and is the reason why Yanmar gives this poorly designed elbow such a short service life. It can be rewelded but not easily due to the corrosion and salt. This is a cast 4LHA mixer but it gives you a clue to where the raw water gets through.

[Emmalina]

still looks like the heat exchanger is leaking but the elbow is so thin i would make another one out of SS but depends on how your welding is LOL
Is it mild steel or cast iron ?

[Oldwheels]

The elbow was spraying water into the manifold. The Yanmar is crap. There is a company making a cast manifold that is much better. The Yanmar is not repairable. The inner tube is pitted and you'll find pitting everywhere.
Cause is heat+ exhaust gases + sea water make material choices tough and corrosion imminent. lovely mixture of acids, chlorine.
I had the same problem on mine.

[tikiavenger]

Originally Posted by Oldwheels

The elbow was spraying water into the manifold. The Yanmar is crap. There is a company making a cast manifold that is much better. The Yanmar is not repairable. The inner tube is pitted and you'll find pitting everywhere.
Cause is heat+ exhaust gases + sea water make material choices tough and corrosion imminent. lovely mixture of acids, chlorine.
I had the same problem on mine.

Oof Ok. I guess I'll plan on checking the elbow yearly, and planning to replace at the soonest signs of rust/corrosion inside, so I can save my new manifold. Anything else I can do? A friend suggested I flush out the raw water system with fresh water after each time I use it, but that seems like such a hassle.

[skipperpete]

If you lay up the engine for storage, by all means run it on Fresh water to displace the raw water in the heat exchanger and aqualock, some add coolant conditioner when doing the flush, but for normal usage it’s probably of little value to flush out the system on a regular basis.
If you see corrosion on that mixer elbow (as in pic 3) then you have a problem or at best, a warning. There is no way that salt water corrosion should be present on the gasket seam or the weld on the mixer flange, yours was actually dripping water onto the lift pump directly below.

[Moontide]

The manifold doesn't look that bad to me. Little wire brush and clean it up. The mixing elbow is not terrible and to me it is repairable. Had one that the inside was completely separated. Took it to a muffler shop and they welded it for $10. Have a few hundred hours on that elbow since then, although i do have a spare onboard in case it fails.



Given that the manifold just looks like surface rust to me I would just clean and inspect it before replacing.



As other have mentioned, it also looks to me line the water is coming from on top or the manifold. I would defintely check/replace the gasket on the heat exchanger.

[tikiavenger]

Originally Posted by skipperpete

There is no way that salt water corrosion should be present on the gasket seam or the weld on the mixer flange, yours was actually dripping water onto the lift pump directly below.

Hmm, I guess I'll keep a close eye on the lift pump and perhaps get a spare ready for when that goes.

[tikiavenger]

Originally Posted by Moontide

The manifold doesn't look that bad to me. Little wire brush and clean it up. The mixing elbow is not terrible and to me it is repairable. Had one that the inside was completely separated. Took it to a muffler shop and they welded it for $10. Have a few hundred hours on that elbow since then, although i do have a spare onboard in case it fails.



Given that the manifold just looks like surface rust to me I would just clean and inspect it before replacing.



As other have mentioned, it also looks to me line the water is coming from on top or the manifold. I would defintely check/replace the gasket on the heat exchanger.

I ended up replacing the entire manifold just to be safe, but maybe I'll wire brush it and see if it improves. I was mainly worried about the surface not being flat/clean, to make a seal with the exhaust elbow gasket.

[skipperpete]

Wow, that’s the heat exchanger and all,I like your style. Usually it’s enough to just file the manifold face flat but your one had more corrosion than usual so ... good call.

[wannacat]

It looks just like mine did every few years, normal for Yanmar and good advice from Oldwheels and Moontide. Yours definitely had a pinhole leak as I can see by looking at the 3rd and 5th pic down. I did silver solder one of mine from the outside and tried it out, then kept as a spare. The rust on the manifold looks no where as bad as mine was. I just used a good grade of Permatex hi temp gasket repair and it sealed no problem. Did that for many years. Have fun!

[tikiavenger]

Originally Posted by wannacat

It looks just like mine did every few years, normal for Yanmar and good advice from Oldwheels and Moontide. Yours definitely had a pinhole leak as I can see by looking at the 3rd and 5th pic down. I did silver solder one of mine from the outside and tried it out, then kept as a spare. The rust on the manifold looks no where as bad as mine was. I just used a good grade of Permatex hi temp gasket repair and it sealed no problem. Did that for many years. Have fun!

Interesting about the manifold. My mechanic warned me that there is corrosion between the heat exchanger area and the exhaust manifold, and that if i didn't swap out the manifold, I could risk having coolant drain into the engine and hyrdrolock it. Is there any validity to that?

[skipperpete]

Your mechanic is absolutely correct. Not as common as cylinder flooding with salt water but definitely a source of hydrolock damage.