Best Piece of Writing I've seen on This Issue-- logical, well written, and if nothing else, food for thought
(Thanks in advance to Kerno O. of Maui/So.Cal for contributing this to a Public Forum)
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Having owned two sets of Yamaha 225's, I've looked at the problem and wondered why it happens to some, but not all engines. All four of mine were about perfect, but I've seen others with far less hours that were very bad. The design of the engines is not a lot different from all other outboards, yet Yamaha has the worst problem. Here's what I think I know about the problem:
The exhaust on the engines passes from the powerhead down through the lower unit and out the center of the prop. The oil pan is right below the powerhead and is a casting which is kind of donut shaped and the exhaust passes through the center of it. That's why if the corrosion gets too bad, it eats into the oil pan. The exhaust passage is a lot like the exhaust pipe on a car - no cooling water passes through it, but since it ultimately exits through the prop, the exhaust passage is exposed to splashed salt water, especially if the engines are mounted low and the boat is sitting in choppy water. As the stern bobs up and down in the slop, salt water can splash up the exhaust. But as soon as the engine is started, the water is dried right out. As a result, you might expect the corrosion to be worse at the bottom of the exhaust system. But it is not. Also, the outside of the housing is exposed to salt water, yet seems to hold up just fine.
It is usually much worse at the top, especially around the pan. To me, the only thing that can trigger it is a combination of high heat and excess acidity in the exhaust which is eating the aluminum. High heat comes from running lean mixtures, something that improves mileage and mets emissions requirements. The repair kits come with parts that have completely different coatings than the original parts, so they are more corrosion resistant. But, why do some engines have the problem and others do not. Why does it make no difference if you muff flush them, hose flush them or not flush them at all?
Here's my guess. I think Yamaha used multiple suppliers for the castings or the casting material. Many alloys of aluminum contain zinc, which is very subject to corrosion. I sell a lot of aluminum scrap and in spite of my best efforts to have my guys keep the high zinc scrap separated from the low zinc, they do get mixed. If someone will send me a piece of a corroded exhaust housing, I'll be happy to give it to a lab and find out how much zinc is in the aluminum. The ideal amount in a marine application is zero. Another answer may be that the engines with a problem are running leaner than they should be either as a result of programming, restricted fuel or injectors which are not quite up to flow spec.
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