I finally launched my boat after winter lay up. Started motor and noticed oil leaking from flywheel housing joint. I know there is a seal and I assume that's the root of the leak.

But why? Normal wear or something darker? I don't see any evidence of prior overheating or funny knocks or noises. Exhaust seems clean.

Boat is new to me last year and no oil leaks noted through to lay up.

Seals do quit sometimes without help from any other issues.

By the way I assume you know about the 3YM30 undersized heat exchanger tubes on motors below serial number 05566. the issue does not become apparent until after about 300hrs.

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If you're not laughing, you're not doin' it right.

Greetings and welcome aboard the CF, dailymirror.

From Pages 110 & 114
Mounting the flywheel housing
1) Replace the used oil seal with new one. Press fit the oil seal in the flywheel housing, and coat the lip of the oil seal with engine oil.
2) Apply the liquid gasket on the mounting surface of the flywheel housing and mount the flywheel housing to the cylinder block, while matching up with the knock pins.
NOTE:
Be careful that the liquid gasket does not protrude onto the oil pan mounting surface.
➥ https://j109.org/docs/yanmar_3ym-2ym-service-manual.pdf

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Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

I note in some past posts that this seal can start leaking if the crankcase vent becomes plugged. Looks like that is accessed by lifting the rocker cover.

Don't know what type of drive you have but the Yanmar saildrives are prone to leaking at the input shaft seal. The drive oil leaks out of the bottom of the flywheel housing. Different color and smell than crankcase oil.

Greetings and welcome aboard the CF, Library.

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Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

No - I didn't know about the heat exchanger tube issue. Ill make a point of checking the serial number later this week when I can get back on board. As I say - I don't see anything which says this engine got way too hot sometime in the past. No weird discoloration, no other leaks, no alarms - and I gave it a good run before putting it to bed last fall.

No its not a sail drive; its a conventional straight shaft. Its clearly crank case oil, and I checked for a diesel smell and water. I'm really leaning toward a plugged "blowby pressure reductor" in the Yanmar manual...

We had this problem with our Yanmar 3GM30F. The problem is the front gasket is very thin and weak on the side by the oil filter. There's a good post Sam L. Morse Forums :: BCC Forum :: Yanmar 3GM30F gear case gasket - 5827. Mine seemed to get worse as we used the boat more.

It is a bit of work to repair. The injectors, fuel regulator and front cover all need to be removed. You/mechanic need to be careful setting the throttle/choke mechanism when reassembling. It is a day's worth of mechanic time.

Don

Thanks Don. I followed the link you provided and did some reading. Interesting. But I can't tell how closely related the YM and the GM series motors are... I assume they are essentially the same with a few tweaks from one to the other. So the YM would have the same weak gasket as the GM?