Possible explanations of 10 psi loss of oil pressure at cruise

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'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.

I'm late to this thread, but I fully agree with this previous statement.
In fact, I'd rather be concerned that the pressure relief valve on the OTHER engine that runs at 80psi might not be working as it should.
What oil viscosity are you using?
Also, what pressure are you seeing at idle on both engines, when fully warmed up?
I never came across any MANs showing more than 60/65 psi at 2k rpm.

Good on ya' MYL. Follow up much appreciated.
Shout out to @Capt J @MBevins and @Capt Ralph as well. @mapism nice added experience and thoughts.

That's a possibility, but the VDO instruments of old MAN mechanical engines work differently.
There is a sender on the engine, which depending on the pressure changes its electrical resistance.
According to VDO testing instructions, its resistance should be 10 Ohms with the engine turned off (i.e. no pressure), and should increase to 105 and 152 Ohms respectively for 40 and 60 psi.
The values shown by the gauge must change accordingly, of course.
Not the most accurate system on earth, but it works.

Consequently, the potential issues with debris in the micro tube that you mention do not exist.
And a single sender can feed more than one gauge.

If you (or anyone else) are interested, I can upload a scanned copy of the old VDO instructions I was referring to.