Oil in heat exchanger

"Hi everyone. New to this foru

"Hi everyone. New to this forum. I have a 33 Egg Harbor with 1989 model Marine Power 454's. The port engine has been running fine (and still does) with no major mechanical work lately.It has closed cooling. The transmission and engine oil coolers are raw water cooled.I now have some oil in the coolant expansion tank but no water in the oil. The crankcase oil is clean and clear.I've searched this forum as well as talking to my usual mechanic. Have not really started troubleshooting yet since I am hoping someone has come across this before and may point me in the right direction.
I will be grateful for any input.
Thanks, Wayne"

do a leak down test
and coola

do a leak down test
and coolant test

"Agreed. You might have a wate

"Agreed. You might have a water leak in your heat exchanger, though usually--since there's a lot more coolant pressure than raw water--the coolant leaks OUT.

Jeff"

Thanks for the replies. It tur

Thanks for the replies. It turns out that the oily stuff in the heat exchanger is not engine oil but some rusty/black slightly oily stuff like you might see in an old manifold. The manifolds and risers on this engine are not 2 years old. Has been run fairly hard and in heavy seas lately so maybe something has cleared itself? Also no color to coolant.I drained about a gallon out and added fresh antifreeze. Going to run it for a while and see what happens.