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Old 05-10-2013, 10:15 PM   #1
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Repetitive Onan Genset Exhaust Manifold Failure

Hi All,

I've experienced repeated failures of the exhaust manifold on my Onan MDKAU genset (approx 5KW) and I'm wondering if anyone else has had a similar experience.

The genset is in a 2002 Island Gypsy Eurosedan. I've experienced 3 identical failures of the exhaust manifold at the point where the cooling water-exhaust mixer bolts on. The aluminum exhaust manifold and the steel hex-head bolts attaching the steel water-exhaust mixer to the manifold corrode away until the mixer separates from the manifold. In the 8 years I've owned the boat (bought as a dealer demo in 2005), the exact same failure has recurred three times. I've had two failures when the genset was running, dumping cooling water into the bilge and releasing exhaust into the engine room. When winterizing the boat last fall, I noticed the manifold had failed again. The first two times I replaced the manifold (see the photo below); this time I have a service call scheduled with a Cummins service tech to see what can be done.

Anyone have a similar experience? I assume the problem is the result of the use of an aluminum exhaust manifold. Any knowledge of a source for a cast iron or SS manifold?
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Old 05-11-2013, 07:55 AM   #2
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Try , Marine Exhaust Systems of Alabama, Inc. they may have a higher quality than dealer stock.
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Old 05-14-2013, 11:41 AM   #3
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Hi All,

I've experienced repeated failures of the exhaust manifold on my Onan MDKAU genset (approx 5KW) and I'm wondering if anyone else has had a similar experience.

The genset is in a 2002 Island Gypsy Eurosedan. I've experienced 3 identical failures of the exhaust manifold at the point where the cooling water-exhaust mixer bolts on. The aluminum exhaust manifold and the steel hex-head bolts attaching the steel water-exhaust mixer to the manifold corrode away until the mixer separates from the manifold. In the 8 years I've owned the boat (bought as a dealer demo in 2005), the exact same failure has recurred three times. I've had two failures when the genset was running, dumping cooling water into the bilge and releasing exhaust into the engine room. When winterizing the boat last fall, I noticed the manifold had failed again. The first two times I replaced the manifold (see the photo below); this time I have a service call scheduled with a Cummins service tech to see what can be done.

Anyone have a similar experience? I assume the problem is the result of the use of an aluminum exhaust manifold. Any knowledge of a source for a cast iron or SS manifold?
Might want to reface the surface and use it again. Since you are using SS bolts and the elbow is cast they have a higher galvanic rating then aluminum. Aluminum is use as a sacrificial metal in some applications, so It may be protecting the rest of the engine, and/or boat. Aluminum should to be isolated as much as possible. I would defiantly zinc protect the manifold with a grouper zinc that you hang over the size.
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Old 05-14-2013, 12:26 PM   #4
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The problem started because the outlet elbow gasket was leaking. Water and combustion products corroded the mating surfaces and the (non stainless) fasteners until they failed.

You obviously have some excessive loading on the elbow from something otherwise the elbow would not have moved enough to allow the initial leak. Or, the bolts weren't torqued properly - maybe because someone worried about stipping the aluminum manifold?

If you replace with the same type parts, use a better gasket, seal the gasket, torque the bolts to the factory specs and check them again after a period of operation. Find out where the stress is coming from that breaks the gasket seal. Maybe you need a longer length of exhaust hose between the elbow and wherever the exhaust goes from there, or a hump hose to provide more flexibility.

Inspect the joint frequently and as soon as you see a sign of a leak, fix it. You might have to coat the mating surfaces with something like Belzona but as long as there is no saltwater and exhaust leak you shouldn't have that problem any more.

Unless you store the boat by sinking it at the dock, hanging a zinc shaped like a fish or even one shaped like a pelican isn't going to make one ion of difference to the problem you are having.
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Old 05-16-2013, 07:29 AM   #5
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Thanks, guys.

A few more details of the design of the exhaust manifold. The manifold is also a coolant reservoir, and is mostly filled with coolant. The coolant flows out of the manifold/reservoir, through the coolant pump, through the engine block, through a heat exchanger mounted inside the coolant reservoir and then back into the reservoir. The raw water flows from the seacock & strainer through a raw water pump, through the heat exchanger and then into a side port on the water-exhaust mixer. The mixer has baffles inside to disperse the raw water into the exhaust flow, which then flows down to the muffler. The exhaust passes from the engine block through a passage in the wall of the manifold to the water-exhaust mixer, where it mixes with the raw water. These passages are surrounded by the coolant in the reservoir.

The raw water really doesn't flow through the manifold, except that portion that burbles/splashes up from the mixer into the manifold exhaust outlet. For this reason, I don't believe the corrosion is galvanic in nature and it's not clear to me how an anode could be mounted to address that issue. BTW, this genset was designed without an anode. The heat exchanger has shown no sign of corrosion (perhaps it's been protected by the aluminum manifold). I suspect the issue may be corrosive combustion by-products that are condensed out of the exhaust stream by the cooling effect of the coolant bathing the exhaust passages and then mix with the salty raw water splash-up. That would explain why the corrosion is seen at the exhaust outlet, which is the lowest part of the manifold and where such a corrosive mixture would pool. No other part of the manifold is corroded.

I took FF up on his suggestion and contacted Marine Exhaust Systems of Alabama. They agreed to provide me with a quote for a cast iron or SS version, which will include a zinc.

BTW Rick, I've never noticed a leak (until the entire thing fails).
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Old 05-16-2013, 07:36 AM   #6
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BTW Rick, I've never noticed a leak (until the entire thing fails).
If there was no leak there would be no corrosion. It is obvious that moisture has penetrated between the gasket and the manifold.

Those mild steel fasteners did not corrode and fail because they were clean and dry.

You didn't use a nickel or copper based anti-sieze between the gasket and the manifold by any chance?
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Old 05-16-2013, 10:05 PM   #7
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You didn't use a nickel or copper based anti-sieze between the gasket and the manifold by any chance?
No. If I did, it would have been a graphite-based anti-seize compound and then only on the bolt threads.
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Old 05-21-2013, 12:31 PM   #8
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No. If I did, it would have been a graphite-based anti-seize compound and then only on the bolt threads.

I hate to tell you this but Graphite as one of the highest galvanic rating of all metals. Aluminum is like 20 and Graphite is 99, so your bolts would tend to loosen up. Very important to when dealing combinine metal that a galvanic table be referred to.

RickB has a point but I still think some galvanic action as attributing.
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Old 05-21-2013, 03:28 PM   #9
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I hate to tell you this but ...
No you don't, you love to trot out that silly table as a cause or a solution for everyting that happens on a boat.

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RickB has a point but I still think some galvanic action as attributing.
Yes Phil, it is galvanic and electrolytic corrosion that ate the metal away, that is why I asked about the type of anti sieze used. If a metallic anti sieze is used on the gasket, it can hasten corrosion if the gasket leaks. The gasket leaked because the surfaces were not clean and parallel and/or the clamping force was inadequate. The leak caused the corrosion.

That doesn't mean sticking a zinc fish or even a zinc zebra in the water is going to reduce a local corrosion cell. Not every problem has its roots submerged in seawater and your passion for zincs and bonding is not a universal solution for the existence of dissimilar metals.

The galvanic table is not the holy grail of machinery failure and repair either.
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Old 05-31-2013, 01:47 PM   #10
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Try , Marine Exhaust Systems of Alabama, Inc. they may have a higher quality than dealer stock.
Ditto on MESA.....talk to "Mark"!
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