502 MAG Lost 4 Quarts of oil in a hour long run...any ideas??
 

I have a 1999 35' Fountain with twin 502 MAGS I went out yesterday for the day checked all the fluid levels before I left the dock. I ran the boat for 1 hour in the morning at normal cruise speed 3600 RPMS stopped and hung out for a while and then ran home for a hour or so at 3600 RPMS most of the time, opened it up to full throttle for a few minutes, got home and pulled the drain plug and a lot of oil cam out, and opened the engine hatch and there was oil covering the engine bay. I sprayed everything down with purple power and then sprayed off with water. Today I filled the engine back with 4 quarts of oil to bring it back to full and cleaned the engine compartment real good then ran it in the driveway for probably 10 mins changing engine RPMS and up to 3k rpms for 30 secs with NO OIL LEAKS, I am now stumped as to what the problem was, I figured it would be obvious what it was with that much oil. I checked the PCV valve and it seemed to work fine so crank case pressure should not have been a problem, I never lost oil pressure it stayed at 50 PSI while running and 30 PSI at idle.

Any ideas as to what went wrong?? Maybe a front main seal that only leaks when its hot? I have only owned the boat for a few months and this was the longest amount of time Ive run the boat since I bought it. The boat and engines only have 240 total hours.

Thanks!

Unfortunately running it 3000rpm in neutral is not a load on the engine. The engine should be pulling a healthy vacuum. 3800rpm under load is a different story.

Sounds like a crankcase was pressurized.

Agree with under pressure & it is...........doesnt sound like the engine is breathing, I've seen dipsticks blown out from this

Did it run normal speed at full throttle, or was it slower?

Normal speed

Where would the pressure come from the PCV valve seemed to be working properly and the other valve cover has a open breather.

Pressure comes from blow-by. Very common when you don't torque plate hone. Which GM does not.

Could be a cracked oil cooler, the oil goes out, then thru the engine cooling jackets, then with the exhaust water. A sheen can be seen on the water sometimes

tidewater, what speeds do you typically see? Also intake gasket could be pushing oil out. Lots of places to look.

have you scanned the engines, how many hours are on them, I see you say 240 is that scanned information or hour meter's