Marine engines all have exhaust mixing elbows where the excess cooling water is injected and mixed with the exhaust gases exiting the engines exhaust manifold, this cools the exhaust and also attenuates noise levels.
However this also creates a very hostile corrosive environment, especially in seawater where the hot acidic gases and salt mixture will seek out any possible weakness in the components.
These elbows will also often become clogged over time with carbon and salt deposits. As these deposits build up they may restrict the exhaust flow and gradually choke the engine leading to poor running.
For these reasons they require regular inspection, on Yanmar's for example every two years. Also you should note that any leak as well as being annoying can cause additional problems, for example water dripping onto nearby wiring looms and connectors (see very end of this article).
Elbows can be made from a varierty of materials including cast iron, stainless steel and or dur-aluminium (which is reputed to be very long lasting). They may be of fabricted and welded construction or made as castings.
There are many aftermarket replacements available, but not all of these aftermarket versions are as complex as the OEM option, for example they may not have an integral water jacket.
There are several threads on the forum discssing muffler waterlock issues but in particular see:
Volvo Penta (some images no longer available)
The valuable thread above by Trevor Bird is reproduced below:
When doing general checks after purchasing our SO 42DS I noticed some corrosion around the exhaust elbow and the evidence of a water leak that had caused corrosion below that point on the gearbox housing and engine mount. The rest of the engine is perfect and this caused me some pain as I try to keep things as close to new as possible. The exhaust mixing elbow takes the cooling salt water and mixes it with the exhaust gases from the exhaust manifold. From there it goes to the muffler (housed in the port side engine mount housing) and off to the exhaust outlet on the port quarter. It is made out of stainless steel (316 I think).
I thought I would check out the cause of the problem which grew into quite a saga. Firstly the join between the exhaust mixing elbow and the manifold had signs of salt water leakage. When running the engine I noticed a VERY slight bubbling of water on the outside of the pipe to flange join. This was the reason for the corrosion. When the elbow was removed and cleaned up a very fine crack could be seen. You can see the crack in this photo.
I thought I would try to weld it and resolve the problem. The result is below. Not perfect but good enough and better then the original as I don't think this will develop any cracks.
The elbow was put back in place and tested. After the job was finished I wondered why the manifold to elbow gasket had shown signs of corrosion and it occurred to me (as I was driving home believe it or not!!) that the inside of the elbow may have the same cracking as the outside of the elbow. Too late now as I had put the whole thing back together but it was worrying me.
In the meantime, the exhaust pipe to the water muffler had developed the classic Jeanneau 42DS fault as outlined in this forum thread. I had obviously disturbed it when doing the work on the mixing elbow.
As it was leaking quite badly, I had to remove the exhaust elbow again so I had another chance to look at the elbow and sure enough, when I looked it was cracked on the inside as well !!
Once again I took to it with my welder. Not that tidy but once again functional.
I tightened the flange into the muffler and reinstalled the Exhaust elbow.
These exhaust mixing elbows are supposed to be replaced alarmingly regularly. If they fail they can leak salt water on the outside but more importantly they can leak salt water to the inside of the exhaust manifold.
I am surprised that an exhaust manifold that is made out of stainless steel can fail with less than 500 hours of use. This is a trap we should all take pretty seriously. Apart from those hairline cracks the exhaust elbow looked almost brand new. No carbon build up and certainly no sign it needed replacing. I have heard you can purchase these as an after market part at a competitive price that is more robust than the original. I must confess I am not very impressed with the Yanmar original. I have not yet been able to locate a more robust replacement. Perhaps someone may know of them?
I should have replaced this elbow rather than muck around and fix it but it has bought me some time to locate a replacement and I learnt something by playing around with it!
I hope this helps someone who may experience a similar problem.
Trevor Bird 2013
Also carefully inspect any electric loom and connectors which may be located directly below the exhaust mixing elbow. Any leaks can lead to corrosion, but they can be relocated as shown in the before and after images below.