Water in the oil
 

Ran my new to me boat for the first time and noticed that the oil pressure dropped from 75 down to about 35 after running for approx 10 minutes. Also, thought the batteries were weak b/c it had a hard time starting. Got back and noticed water in the oil. Changed the oil and ran again for few mitutes and same thing happened. I had it checked today and found a cracked header but we are also worried about other damage to the motor b/c of the oil pressure dropping. What do you think?

The motor is an older but "fresh" Merc 800 SC.

thanks!

I would change the oil a couple more times.I suggest using a can of motor flush the first oil change and just regular oil the second oil change.I would doubt that you hurt anything running for that short amount of time.

what about the oil pressure dropping?

What rpm's were you running when the oil pressure went to 35psi

I hope you're in fresh water rather than salt.

yes, fresh water and we were running between 3-4k rpms. I thought that I was still "breaking in" the motor so we weren't running hard.

I don't think you can get water in the oil with a cracked header, you can get it in the cylinder but not in the oil..most likely you have a seeping head gasket or intake gasket....you need to get that water out of there and in a hurry! Do not leave it set any longer than absolutely ness....I suggest to flush out with 1 gallon of diesel and 3 quarts of oil a few times..& you still will not get it all out..change oil filter repeatedly...This is critical..flush that motor !..you are most worried about the rod and main bearings...and you need to locate that leak fast!...I learned the hard way....good luck

Ok, let me back up a bit here...there is a chance your header leak is your problem..you could have a small leak & the water can be getting back through the exhaust port and through the valves down to your cylinders..after you shut the boat off the water in the manifolds could go into the cylinders and leak past the rings to your crankcase.....but if you have a bad leak it would normally lead to water in the cylinder and hydroloc...that is very bad because you will usually do some major damage at that point...again, get this fixed fast because otherwise you are looking at a teardown.

First of all, thank the Good Lord you're in fresh water! If it was the warm salt water we run in down here...

That "hard starting" could be partial hydraulic-ing. Pull the spark plugs and check the tips for evidence of water in the cylinders.

Crank the engine with the spark plugs out and see if water comes out the spark plug holes. It's not unusual for a leaky header to allow water back into the exhaust ports and eventually past the rings and into the crankcase.

I've even seen cooling system water get sucked into the intake manifold from not sealing the intake manifolds bolts properly.

A compression test will help find a bad head gasket.

Good Luck with it.

I am thinking the same thing it would have to get pushed by the rings if coming in by the headers.