I doubt it is the oil cooler. The oil pressure is much higher than the water pressure, so it would just empty the oil pan, not fill it with water. If I had to guess, I would say a head gasket is leaking. It is probably not leaking into a cylinder, but leaking into the lifter valley. This could especially be true after you installed thermostats and the pressure went up. You can either pressurize it with a water hose or you can use a cylinder leak down tester if you don't want to put any more water in it
Just block off all of the water lines, hook up the leakdown tester and see if it holds. I would not put more than 25 psi in it for pressure or it will lift the head gaskets. I have seen Fel Pro head gaskets lift at higher water pressures and settle back in place at lower pressures. They would not leak until the pressure got high enough. At lower pressures, they were fine. It was tough to pin down the problem. The water would just push past the gasket and into the lifter valley at higher pressures. It left just the slightest sign on the gasket. Obviously, we had to pull the heads to find it. Good luck. Let me know if I can help.
By the way, I'm with Bill. Be careful running it with milk in it. Water is a terrible lubricant.
Eddie

Jeff this may be a dumb ? but did you install the bypass back on when you put the T-stats in? If not then that is your problem.

Hey drop it off on Dereberry Drive


Jeff is this the boat you bought this spring off ebay fresh and water ready ??? NO way coolers & heads on both motors ...gunna be the head gaskets BIG Daddy from the last build .


more YMI $$

its the one you are thinking of, but it was not even close to being water ready. 1 engine was cooked. and the other was really rouhgh. Guy I bought it rom neglected it outside for 3 years before he decided to sell it.

It needed everything.

I am with Eddie on it probably being the head gaskets, but I will check the coolers first. I an actualy leaking towards it being the head bolts leaking. I have only had it happen one time. but I have had water leak up throught the head bolts from the water passage.

Either way, It will ge fixed before spring. I wanted an excuse to take them back apart and upgrade cams anyway.

THese flat tappet kindergarden cams are wearing me out.

Gotcha : drink:

you figure out the problem yet?

Worry about it after I get home from Key West. I was only about 74% happy with those engines anyway.

Going to upgrade the cam, springs, etc.

There could be head bolts leaking if they're Mark 4 blocks. Gen 5 and 6 have blind head bolt holes. Sounds like head gaskets could be it also.

wish i was in key west...........instead of the wet west!!!!!!!!1

I had same problem 2 years ago, ended up droping a valve and replacing both motors. Alluminum pro comp heads had serious corrosion problems in the water passengways