Strange Oil Pressure Problem

[gqchris]

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!

[Rexone]

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.

[DEL51]

Check the oil pump pickup inside the pan, it could be blocked or fell off of the pump.Try a different filter.

[Liberator TJ1984]

Forget about the Blower , that is not a worry........it has nothing to do with the oiling system or pressures. :wink:

[gqchris]

Thanks guys for all the replys. Rexone, I am thinking you might be right with #2, the dipstick is also "homemade" and it has notches that someone etched in for the full level. It is a 10 quart pan and that is what I always put in. I wonder if this is what is causing it:)
Chris

[Rexone]

Thanks guys for all the replys. Rexone, I am thinking you might be right with #2, the dipstick is also "homemade" and it has notches that someone etched in for the full level. It is a 10 quart pan and that is what I always put in. I wonder if this is what is causing it:)
Chris
Seen it happen more than once, back it off a quart or two and run it. Just watch the oil pressure closely (I know you will be) :smile:
Also as mentioned, if you have a Fram filter, some have had very bad luck with them. I don't think that's it here though because your pressure came back after the down time. Foamed up oil seems to fit this scenario.

[gqchris]

Hey Guys,
Just wanted to follow up that I ran the boat up in Kings River this weekend, and took everyones advice. I changed the oil and kept it to 8 quarts. I also changed to a Fram Racing filter from Rex, and added a little Lucas Oil Treatment.
Ran great all weekend. Got down to 10lbs PSI or so after warming up again after 15 mins which again freaked me out, and after idling down river a bit, would build itself up to 25-30 lbs again. I ran it and didnt stress, figured if it blew than oh well! :)
I think maybe the bottom end was bult loose just to compensate for the blower? Never got hot, never made a noise, ran like a raped ape!
http://www.sybianrodeo.com/images/chrisjet.bmp
Thats me letting my friend drive his first blower boat. :) FYI- Kings was a great place to camp and have fun. Water moves pretty fast, but all GLASS.

[Liberator TJ1984]

Right On,
just curious...what wt. oil did you go back with and when you changed the oil what did it look like ???
if you still have the old filter ? cut it open and ck whats inside ,looking for any signs of bearing materials...this is a good practice to do every time you change the oil :cool:

[gqchris]

Hey Liberator!
Yea I went back with VR1 Racing 50W, thats what the previous owner said GS Marine always put in it, so I stuck with that.
The old oil looked great. Not dark, no burnt smell. I did open the filter and no signs of metal, used a magnet also. Nadda. I was even tempted to send the oil out for analysis just because:)
Thanks again for the advice!

[TPI]

I really would not stress about idling oil pressure too much(unless it was less than your 10-15 lbs) Are you getting 10 lbs per 1000 rpm? Any idea what the operating oil temps are? What kind of oil cooler do you have? As stated above, if the motor has loose bearing tolerences, hot oil is going to make a big difference to your pressure ESPECIALLY at idle. Oil thickens right back up if it has a little bit of time to cool off, especially if you run a big cooler. I am in agreement with this resolution, and I would put my money here for the solution.