What is this stuff in my closed loop cooling system??

It's a combination of several different chemicals and minerals. Almost looks like some salt is in there also. Without a lab test, it's impossible to say exactly what it is, but it looks just like the corrosion you would find in a car radiator when the anti-freeze in it wasn't treated with anti-corrosion chemicals and lubricant.

The ONLY cure I can think of would be to flush out your system. Add a radiator flush into the water, run the engine for a while, drain, then do it over and over again until you dissolve as much of it as you can, then put in fresh water, anti-freeze and anti-corrosion chemicals to prevent it from happening again.
 
thats what shows up when your riser gasket is leaking anti freeze into your exhaust system. Clean the surface of both the riser and manifold, use a Mercruiser gasket(should be silver in color) and put it back together. If you want to go further, check the compression, pay attention to number 4
 
Devil, pay attention to what Spareparts says first. He's the expert in these things. I'm just a backyard wrench turner. He does it for a living. :party: So if he says that's what to do, by all means do it first.
 
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