Stock HP 500 Carb. Popped the hose off the water pump to begin to change the impeller and oil poured out. I was told its due to a bad oil cooler. Could it be anthing else?? There is no water in the oil, dipstick is clean, no water in valve covers.

More then likley it is the oil cooler, If oil press is higher the water press then very little if any water will get in the mtr

I am having the same issue with my 525 (closed loop cooling). Oil was in the coolant tank when I got the boat, I drained and cleaned it to the best of my ability, but here we go again, it's back... I was thinking about the oil cooler as well. Engine runs great and there's no water in the oil, but the level on the dipstick does reflect what's going into the coolant tank. Anybody else have this problem? Does this warrant a replacement? How much do these things cost and how much of a pain are they to replace?
Think I'll call Merc this week about this. Boat was like this before I bought it (dealer pictures show an almost black coolant tank and I thought it was just bad lighting at the time), and see if there's any solution that won't cost too much money. I know that these things have very short warranties but you never know...

unsane, normally the oil cooler is not part of the closed cooling system, Your problem sounds more like a head gasket problem

hmmm.......but the engine runs great, is there any other way to visually inspect this issue?

Another thing to check is the water passage in aluminum manifolds, that one sneaks up on guys. I have seen Merc Merc EFI's go bad on 4 years old engine in salt, it is rare in fresh water but does happen.

Phil

A leakdown test should either pinpoint the problem or eliminate the head/compression possibility. I'm with goodt on this one---probably a failed oil cooler. --- Jer

So if everything has run perfect to this time and there is no water in the oil, low oil level, oil in the water pump, oil cooler is bad??? Is there any other possibilty??

Get the oil cooler pressure tested to eliminate doubt. Usually a radiator shop can do it.

Going to do a leakdown test this week and see what's up. She did feel a little sluggish this weekend, but it was hot and the boat was heavy.