I have a 350 with vortec heads and an rv cam(dont know the specs on the cam ). Its the standard marine engine set up and dynoed at 300 hp. I had the engine rebuilt at the end of 2006 and have about 80 hours on the rebuild. rebuild consisted of new rings, bearings, gaskets and heads. I have 40 lbs of oil pressure at idle. When I take the boat out and open it up at full rpm (4200 -4400 rpm) I have 60 -70 lbs of pressure) After about an hour or less of use my oil pressure will bottom out at about 2-3 lbs. I have an electric sender and have confirmed the pressure reading removing the sender and putting a mechanical gauge in the shunt. Both readings are within a couple of pounds. When I change the oil filter I get pressure back to normal. I have gone through the use the boat for an hour and lose presure cycle a couple of times and each time changing the filter seems to be the key. I have tried several different brands of filters with each giving the same results. I changed oil and filter this past weekend got an hour or less of run time before the pressure bottomed out. I turned off the engine and let it sit for 10 minutes on resart I had about 20 lbs of pressure.

Each time I have the problem the engine is running fine. So I dont think the problem is the distributor grear because I not hearing or noticing any missing or other engine problem besides the loss of pressure. I dont have any oil being blown out anywhere so I dont think I have overpressure causing the oil to not return to the pan.

I know enough to be dangerous but not much more. I cant figure out why the oil filter would cause the problem. I cut a couple of the filters open and found the pleats/media to be okay, some were leaning on the other pleats but on the whole the filter did not look totally collapsed.

Could a filter cause a total loss of pressure? Could my filter bypass somehow cause this problem and if so how? How can i have this intermittent pressure problem? Is there somewhere with a good discussion of the 350's oiling system? Any diagnosing tricks that I can use short of pulling the engine and droping the pan?

Arnold,
Which filters are you using ? Is it a remote filter ? Oil cooler ? What weight oil ?

I would use a Mercruiser filter and see what happens. The relief valve will not affect oil pressure. When you have the problem, if you shut it off and let it sit for 30 minutes or so, does the pressure come back to normal ? Did you change the pump when you did the engine, and if so did you use the correct screen and get it spaced properly from the bottom of the pan ? Do the lifters start to get noisy when the pressure drops ? If not, you might want to try another mechanical gauge just to be sure you really have a problem.

Cant think of anything else right off hand. You might also want to log in to the Offshore Only tech forum and post the question there, someone else might have seen the same problem.

http://www.offshoreonly.com/forums/forumdisplay.php?f=166

If it didn't do this before the rebuild, I'd be suspect of the clearance you have between the pickup screen and bottom of pan. Could be too close and doing weird things when level in pan gets lower. Or possible leak at pump where pickup mounts.

I have had the same problem after installing an RV cam or a HP camshaft.The oil would break down to the point of being as thin as water and no oil pressure after warm up.A high volum oil pump seemed to make the problem worse.10w30,10w40 and 20w50 would all break down and I almost lost a few motors.The engines all had good main and rod bearing clearances.I starting using straight 40 weight oil and the oil pressure was fine and no oil break down.I use straight 40 weight oil in all HP motors that I build.Hope this helps

Thanks. It does help. I took out my sender, put in a nipple and ran the engine to try and pump out an dirt or obstruction. I took out about two quarts of relatively dirty oil. I had only run the engine for about 8-10 hours since the previous change so I was surprised to see the oil look so dirty. I added back in two new quarts and ran the engine, all oil is currently 10W-30 Valvoline, and had the same problem. Lots of pressure, 40+ lbs, during warm up and idle and during full throttle use good pressure, 50 lbs. On the way back in after about 10 minutes of full throttle use the pressure started to drop and went as low as about 18 lbs, engine temp was 160, from there the oil presure went to zero(from 1 lbs) in less than a second like someone had turned off the oil pump. I turned the boat off and let it sit for about 5 minutes. On restart I had about 20 pounds of pressure. It dropped again from about 18 pounds to zero(once again instantaneously) on the way back in. After about 5 minutes I restarted and had 20 lbs again. After I got back to the dock I started the boat intermittenly over the course of several hours. The longer I waited the more pressure I had. After sitting overnight I went out and restarted the boat I once again had 40 lbs.

I talked to my engine builder and he thinks I have a sticky oil pump relief valve. Would and open valve cause me to lose all pressure in an instant? Even if it did how and why is the oil getting so dirty so fast?

What is the oil level when the pressure drops? How much oil are you adding when you change filters? Have you tried to add a quart of oil when the pressure drops?
If it is at all possible drain your oil and insert a thin screwdriver into the pan and feel around on the bottom. Possible your pickup screen fell off of the pump.
The "dirty" oil could be metal from the bearings due to loss of lubrication.
Verify that you have a marine oil pan and that your pickup screen is the proper one for that oil pan.
Adding a quart of oil as soon as the pressure starts to drop can possibly narrow it down to an oil level problem. Add the oil while it is running if at all possible.

Squido

Thanks. It does help. I took out my sender, put in a nipple and ran the engine to try and pump out an dirt or obstruction. I took out about two quarts of relatively dirty oil. I had only run the engine for about 8-10 hours since the previous change so I was surprised to see the oil look so dirty. I added back in two new quarts and ran the engine, all oil is currently 10W-30 Valvoline, and had the same problem. Lots of pressure, 40+ lbs, during warm up and idle and during full throttle use good pressure, 50 lbs. On the way back in after about 10 minutes of full throttle use the pressure started to drop and went as low as about 18 lbs, engine temp was 160, from there the oil presure went to zero(from 1 lbs) in less than a second like someone had turned off the oil pump. I turned the boat off and let it sit for about 5 minutes. On restart I had about 20 pounds of pressure. It dropped again from about 18 pounds to zero(once again instantaneously) on the way back in. After about 5 minutes I restarted and had 20 lbs again. After I got back to the dock I started the boat intermittenly over the course of several hours. The longer I waited the more pressure I had. After sitting overnight I went out and restarted the boat I once again had 40 lbs.

I talked to my engine builder and he thinks I have a sticky oil pump relief valve. Would and open valve cause me to lose all pressure in an instant? Even if it did how and why is the oil getting so dirty so fast?

The most likely source of your grief, in my humble opinion, is clogged, or partially clogged, oil return passage(s) from the rocker arm area. Oil pumped to the rocker arms might not be returning to the pan quickly enough - after the engine runs a while, most of your oil is in your rocker covers and not in the pan where it belongs. Oil pump starts sucking air, pressure drops, shut the engine off, the oil trickles slowly back to the pan, start it back up ten minutes later and we start the process all over again.

Pop the rocker covers off, you'll probably find quite a bit of sludge up there. Clean it out as best as you can and check the holes where the oil drains back to the pan - you may need to chase a wire brush through them and do a couple more oil changes to correct the issue.

I read your post regarding oil pressure in your boat. I have a simular problem.

I have a 1992 Chaparrel with a Mercruiser 5.7 Brovo 2 motor. It has a fresh water cooling system and a remote oil filter. Last year we were in the San Juan Islands when I over heated. We never found the specific problem, but suspect sea weed plugged the water intake. On the way home, the oil pressure dropped and the motor would heat if I exceeded 1,500 RPM.

That was the last trip of the season, and I changed the oil and filter for the winter. This year, the motor over heats and the oil pressure acts very differently. The oil pressure has always stayed at 40 pounds (hot or cold, fast or slow). I also checked the guage accuracy and found them OK. I replaced the thermostat, fresh water pump (which is on the motor) and oil pressure sensor. I back flushed the outdrive, and checked the heat exchanger and exhaust risers. All OK.

While in the sound this year, the temperature would go up to 200 degrees and the oil pressure woud drop as low as 10. I found that Boatland had sold me the wrong thermostat. I replaced it again, this time with a 'full flow' 160 thermostat to allow more water to flow. It still over heats, but just to 170, and the oil pressure still drops to 10.

I discovered that NAPA had sold me the wrong oil filter. I replaced it with a Mercrusier unit which seemed to change the oil pressure condition. So now I suspect that the oil cooler is some how in line with the filter, and a more restrictive filter doesn't allow adequate cooling of the oil. I'm going to change filters again if I can find out which one to try.

Does any of this make sense?

Chuck

Tore the engine down over the spring and found we had a couple of different problems.

1.) Our rebuilt motor had glazed cylinder walls causing rings not to seat/blow by

2.) We had a water leak problem with the crapy mercuiser exhaust and lost a piston

3.) A little excess bearing wear but probably not a source of immediate problem.

4.) Shop reused a cam from engine that was pretty well toasted. All in all just not one problem but many.

5.) really did not see an indciation of a cloged galley or a failed by-pass spring in oil pump.

Suggest that you do a leak down test to get a sense for whats going on as it will give you best indication of state of pistons, rings and cylinders.

Good luck