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Discussion Starter #1
We have a 3 GM 30 Yanmar in our boat. The engine looks and runs like new, but is about 10 or so years old and has been properly maintained since new.

The other day I climbed into the lazarette to check the Racor, which is behind the engine. At the rear of the engine is a 2" X 2" flat plate held on with 4 12MM bolts which also secure the engine lifting eye, and a small clamp securing a hose. On the top and bottom of the plate where the edge touches the engine, there is a white powder substance which looks like shaving foam emerging from the crack between the two surfaces. It is dry and I cannot find any trace of water leaking from the area.

This is the second time it has appeared in a month. The first time I washed the area down with distilled water and assumed that something had spilled there.

I suspect it could be corrosion as I have had experience with a similar appearing substance on aircraft. I cannot be certain but do not believe that there should be any salt water contact at that location. I am wondering if the heat exchanger has been compromised, allowing salt water into the fresh water side. Yanmar uses no anodes in this particular engine. I detect no salt contamination in the cooling water, which appears normal in color. I have no idea what is behind the plate and need to determine that before removing it.

Any ideas or suggestions?
 

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Is there gasket behind the plate that is failing? I'd clean it off and climb down there with the engine running. bring some soapy water. put it around the plate and look for bubbles.
 

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Discussion Starter #3
Great idea, will try it in the morning. There could be a gasket but if so it is not visible.

It seems that if the exchanger was compromised the water level in the reserve tank / overflow, would not be at or stay at the proper mark as salt water entry would allow fresh water exit also. Hope it is something simple as heat exchangers are costly items.
 

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Kynntana (Freedom 38)
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That sounds like an easy fix. After having scraped quite a few old gaskets off the same type of Yanmar, it does sound like the gasket has failed and you'll probably find it practically welded to the back side of the engine when you take off that plate. You'll want to get a pretty smooth surface without any of the old pieces.
 

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Discussion Starter #5
Is there gasket behind the plate that is failing? I'd clean it off and climb down there with the engine running. bring some soapy water. put it around the plate and look for bubbles.
Thanks jsaronson, tried the bubble test which was negative.

gamayun, you are probably right, and that would be a good thing. Did not want to remove the plate until I determined what was behind it and if coolant would be at that high level with the engine off. I wanted to be prepared in advance if possible. Could find no reference to it in the service manual. Must just be corrosion, not salt, caused by dissimilar metals, as there is no good reason to have salt at that location.

Thank you both
 

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I was at my boat today and looked at my 3GM30F and the service manual that's on the boat also. That plate gives access to the cooling passages in the head. In the raw water cooled engines, there is a zinc installed in a similar plate (with threaded hole for the zinc). In our engines, it's just a flat plate with a gasket. Sounds like yours is leaking a little. Doesn't sound like a big deal. Your can either ignore it until (or if) it gets worse, or change the gasket now.
 

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sure sounds like some sort of galvanic corrosion. When whas the last time the coolant was changed. There are no anodes on that engine. Diesel coolant contains comopounds that act to inhibit galvanic corrosion. This is one reason coolant should be regularly changed. Look in your owner's manual for the interval. Usually the interval is given in engine hours. If it has been more than 2 or 3 years, it's time to change the coolant.
 
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