Possible explanations of 10 psi loss of oil pressure at cruise

Are you over full of oil?
Smell fuel in the oil?

Change oil and filter, Send in a oil sample to a lab.

That would be my first guess. The viscosity has thinned out due to fuel contamination.
As CR has said get a quick oil analysis done. It'll confirm or deny.

Do this for me first. When you check your oil, pull the dipstick out, wipe it and leave it out of the tube for a full 2 minutes, then slowly insert it into the tube, wait 10 seconds and pull it out and check the level. IS it different? I have to do this on a 10 cylinder -1100hp common rail boat I've managed since 2011, if you check it the normal way it will show low, you'll then add oil, and then be overfull. The other MAN's I manage don't exhibit this.


How long after getting up to cruise did it drop? I would first suspect fuel in the oil or overfull. What is the dipstick reading? How thin is it on the stick (does it drip right off?) How many hours and age on the oil? Start the motor, let it idle for 10-15 minutes, pull an oil sample and have it tested. I use CAT S.O.S. which the bottle can be purchased through a cat dealer. What brand of oil filters are in there? My next guess would be an oil cooler that is partially plugged or in poor shape. 70 PSI is still PLENTY of oil pressure. It's also possible that both gauges use the same oil pressure sender and it's the sender or loose connection.

Scott Marine Power has been a MAN dealer for a long time and a very good outfit. I prefer Marine Diesel Specialists over them, just because I've been using them for so long and they give me good service. (The other MAN dealer in Ft. Laud).

I was also thinking over full and oil foaming.

J, I do understand your dip reading requirement. On a C32, the owner was just pushing down the dip and rite back up. He kept telling me the oil was low and I told him it was full. Had to show him how to check his oil.
Not many realize the later dip tops are tight and push an air bubble down the dip tube.
That is why I tell all to check the oil cold in the morning, just pull the dip up and read it. No wipe, no mess, no guessing.

Yes there is.
Put a drop of oil on a paper towel.
If there's a lighter outer ring extending out then odds are you have fuel in the oil.
That's from my 67y/o DD mechanic.

That picture looks like a low pressure switch and not a pressure sender.

I'd just pull an oil sample and let it ride and wait on the results if you have that little hours on it. 70 psi is still way above spec for those motors. Could have been something as simple as the one motor running a higher load based on the way the boat was trimmed or not at the same rpm and running a higher oil temp. You're never going to have the exact same oil pressure between 2 motors. Halfway between full and add is fine, some motors will puke out the oil if it's on Full, depending on the running angle/motor angle.


Capt Ralph on that specific MAN 1100 boat I'm talking about, the dipsticks when you pull them cold will be on or near the add mark, even if the boat was sitting 2 weeks. The oil never comes up the stick, not sure if it has to do with the synthetic oil, or the dipstick seal, or what. All of the other man common rail boats (not 1100 hp or v10's) that I manage don't exhibit this.

There are by-pass valves in near most everything now days that allows for oil flow to bypass the filters (if clogged) and still flow to the motor.
My MAN go to guy that covers S FL is in Cocoa FL. Are you heading that far up?

Then it may not have fuel in the oil.
I did and it was obvious on the paper towel

That's a tight dip top seal.

Keep an eye on it, your temps and exhaust color. I would not do any drag racing but you have a family vacation on going. Ya got to run.
Get a SOS lab out as soon as you can. They can E-mail the results in a couple of days (in StPete).
I PM'd you my preferred shops info. Tap on Troy when you can.

I'm a bit confused, despite the fact this thread appears to be "closed" in terms of the need. Photo shows me an electrical sensor, and not what I know of as a "mechanical gauge". Shouldn't the "mechanical gauge" be a simple micro-tube feeding pressure to a mechanical gauge? And if in fact that is what is present, wouldn't the presence of debris at the entry point of the micro tube potentially interfere with pressure readings? Also, if there are multiple gauges fed from multiple senders on the same engine, are they all behaving in a similar manner?