We are currently having work done on our Onan MDKAV and the workshop have told me that the exhaust manifold and elbow are both beyond salvage.

After spending a few hours yesterday afternoon we have managed to clean both of them up a lot, removing quite a lot of carbon and rust from the air passages of both parts, and while the passages look like they are still slightly constricted from their original diameter they are a big improvement over what was there before. 

 

My main concern at the moment is that there is quite a lot of lost material from rust damage at the point where the two parts join, particularly on the manifold side where the mating surface in places is eroded by a good 5mm or more into the metal casting. On the elbow side the loss of material is less, but there are a couple of spots where I can see what looks like a breach between the inner pipe where the exhaust air goes, and the outer jacket where the cooling water goes. I've tried to add a couple of photos, not sure if that will work or not. 

 

Has anyone had damage similar to this repaired successfully? Or more generally does this seem like something a decent machine shop should be able to fix? I’m guessing they would need to replace the lost material by welding, then grind back to produce a clean mating surface on both parts. 

 

If the parts do seem salvageable does anyone have any tips on how to clean the interior passages further? I wonder about using either an acid or a caustic oven cleaner to remove the stubborn carbon but don’t want to use something too aggressive and end up creating more problems.

Any suggestions would be apprecaited

Neil Meyrick
SM390 Revelation
Valencia, Spain

Thanks Bill, very helpful - will look for a machine shop who can assist with the manifold. 

Do you mind if I ask why you say to replace the elbow, as that looks to be less damaged than the manifold. Is it due to the outer jacket being breached?

--
Bob Thomson
SY Memo
2001 Amel SM2000
Ser No 326

As has already been pointed out, an INTERNAL leak of the elbow is worse, in some ways.  You can't see it, and the consequence can very well be a flooded engine. I strongly suspect that the corrosion of your mainfold is due to water spray from the exhaust elbow. The do not corrode significantly from dry diesel exhaust. Disassembly and inspection of any diesel engine wet exhaust elbow should be a routine maintenance procedure.  They all have a finite life, be it from clogging or corrosion. The alternative to regular inspection is to run it until the engine doesn't work any more and that doesn't seem like a good plan for a cruising yacht.

We had an older model generator than you (an MDKD), and our experience with the exhaust elbow was not good. About 4 years ago it developed a small leak that sprayed water out into the engine room, fortunately missing anything important.  When we contacted the local Onan dealer in Annapolis, they quoted us $1600 for one. WHAT???? We tried other dealers across the country, and they all wanted MORE!

It turns out the part had been obsoleted by Onan, and supply was limited to the stock on hand.  Onan's entire dealer network in the USA had exactly ONE left, so supply was short, and I guess they were hoping demand was high.  We had our leaking one patched by a welder, and went looking. The welder's patch held, but not surprisingly, the elbow soon started leaking in other places.

We never found an aftermarket supplier who had them for our model, although at least one was willing to make them.  But we had to send them the part, and it would be 6 to 8 weeks turn around.  Well, we can't sit in one place at a dock and wait that long. We eventually found a replacement in good nick at a marine engine salvage yard.

This was an important lesson for us, even though in a pinch one of these elbows could be easily cobbled together by any machine shop who can cut and weld stainless tubing.  Onan parts supply for units older than 25 years is spotty, or non-existant.  While we could still easily source parts for the engine side of things from any number of Kubota dealers, the marinization and electrical parts were gone--unless they are in common with more recent models, or were gathering dust in backstock.  Many of the parts of course are not Onan unique, but there are enough (like the main circuit board) that the situation was a serious concern for us.

Having an uncertain parts supply left us in a quandary.  We were very uncomfortable having such a critical part of our boat's systems basically being unmaintainable and unrepairable, even though it was working fine.  The last thing we wanted to have happen was be in some remote place and NEED to get a new generator because that was the only alternative.  Hence our decision to replace the old Onan after almost 9000 hours. If our cruising plans had been restricted to major yachting areas we might have nursed it along, but we tend to go to more remote places, and a reliable generator is important.

Bill Kinney
SM160, Harmonie
Great Inagua, Bahamas (Heading to the Plana Cays in a few hours)
http://www.cruisingconsulting.com

Thanks all, really appreciate.  I’m going to take both parts to a machine shop to see what they can do. Given the stories about leaking elbows I’m going to be pretty cautious on that one