Hey Guys,
I was hoping someone can steer me in the right direction.
Last season I acquired a 1976 Southwind 18.5' with a built 468 with a jet. Please forgive me if I mess any of the terms up here (novice), but I believe it has a BDS 671 Blower, the guy I bought it from said it was tuned at 6lbs boost. Supposedly a Merlin block.
Anyways, took it out for its test run last year at Elsinore, and I noticed that the oil pressure gauge was not reading at all. (Stuck at 60lbs). So i replaced the oil sending unit on the block and tried it again. This time it was reading between 50-60 lbs and fluctuated with the blower surge. I figured this was normal and took it for a hot lap around the speed zone. Everything was fine, turned the corner at the end to come back, ran great and then i noticed the oil pressure falling. By the time I got back to the no wake zone, I was down to 15 lbs or so and I turned it off. I rowed back in in fear of causing permanent damage. Got to the dock, started it back up, same pressure, immediately shut it off. Nothing sounded strange during this, no knock, no chatter, no rocker noise. No elevated Temp. Let it set for 30 mins, pressure back up when started :hammerhea
I figured maybe a cause was that when I changed the oil before I ran it, I had put 30w instead of 50w. Put 50w back in, same thing happened next time at Elsinore. Ok, so I figure the gauge in dash is funky. Bought a mechanical Mac gauge to plug right into the oil galley hole and hold while i was driving. SAME THING!
So this is what a few peeople have suggested. Bad Lower Bearing, Oil Sender Hole wrong size, Blower scavenging the oil??? Any help would be greatly appreciated to point me in the right direction to pull it out and rebuild it or throw in the towel Thanks for the help guys!
A couple distinct possibilities come to mind since you got your pressure back when you fired it back up after sitting a half hour.
1) Your oil was overheated (reason undetermined). Oil thins out and you will see dramatic pressure drops if it gets too hot 260-300+ºF range, especially with big clearances and / or less than optimum oil quality and / or fuel contaminated oil. Fuel contaminated oil will even do this without the high temps.
2) You overfilled the oil pan to a level where the crankshaft was catching and aireating the oil into a foamy type of oily mess. This WILL cause pressure drop. Your oil level should be under the windage tray. With high volume pans and engines mounted in boats at an angle this can mean filling the pan to a substantially less level than the pan is rated to hold. For example 7-8 quarts in a pan rated for 10. This is different with every boat.
If I had to bet money I'd bet option #2 since one time around elsinore's speed zone isn't likely gonna overheat the oil but it is enough to get good and foamed up.
Those are two possibilities that immediately come to mind. Good luck.