Jim,
It's true that the Yanmar manual calls for replacing the mixing elbow every 500 hours. The generic recommendation seems rather conservative to me, but I've seen some corroded elbows that were definitely ready to replace.
However, it's also true that in 2008, when Yanmar was making this engine for the R-25, there were two optional mixing elbows, one stainless steel and one cast bronze and painted gray. My boat is equipped with the bronze elbow, and I suspect yours is, too. There are line drawings of both in the parts book. The bronze part has four threaded plugs in various places.
I have 2875 hours on my boat, including the original engine and mixing elbow. I've had it off two or three times. Each time I cleaned it with a hose and a small brass wire brush. I've never found anything more that a light layer of soot. Other owners have reported similar results.
By all means, remove your elbow, clean and closely inspect it. Replace the gasket (129472-13520) and use some anti-seize on the stud threads before you reinstall. It's your decision, but after this, you'll likely be taking a new stainless mixing elbow off your Christmas wish list.
This would also be a good time to replace the exhaust hose between the elbow and the muffler and maybe the water hose that allows the seawater to enter the elbow. There's also a particularly devilish coolant hose (129672-49500 PIPE, OVER FLOW) that hides behind the mixing elbow and gets very hot. You'll need to at least partially drain the coolant to replace this hose. Mine was rather crispy when it came out.
While you're at it, I do recommend carefully inspecting all of your rubber hoses and replacing anything the least bit suspect. And so long as you are hanging upside-down with your head in the bilge, take a look at the pipe that connects the bottom of the turbo to the port side of the oil pan. You'll find another thread on that one from several years ago.
Good luck! Please send a private message if you want to talk directly.
Rich
