Oily substance in cooling system

[Mike Sibley]

I have a Perking M30 in a Lagoon 37 TPI and there appears to a black oily residu in the cooling water. My first thought is to drain the system, flush it out, fill it back up with water and antifreese, and then see if the oily substance returns.

Does anybody have any idea what might be causing this? The engine starts easily and runs smoothly, so I doubt it is a cracked head. Also, the engine oil remains black (rather than brown), so I doubt that water is getting into the engine oil. I recently purchased the boat, so I have no history, and the surveyor didn't note a problem.

Also, is there a good product to help flush out the cooling system?

All help greatly appreciated.

[gjordan]

It is very possible to have a crack that allows oil into the water system without water getting into the oil. I had it happen once a few years ago. Call a Perkins dealer and find out if there is any better method of diagnosing the problem other than just watching the antifreeze. In my case it was a cracked head, but it might be only a bad head gasket.____Just had another thought! Have your oil cooler tested, or bypass your oil cooler and run it for a few hours ( at moderate load) and see if it happens again. Oil pressure is always higher than water pressure in an engine so it is possible to have a one way leak._____Good Luck._____Grant.

[Mike Sibley]

After flushing the system a few times, I don't think that oil is in the water. It appears to be very fine black particles that turn the water black.

[sy_gilana]

Sounds like soot. Does the cooling water system build up a lot of pressure?
Could be a head gasket blowing combustion gasses into the cooling circuit.
A one way leak is common.
Good luck!