Click for Ocean Alexander Click for Westport Click for Llebroc Click for Cross Click for Nordhavn

Possible explanations of 10 psi loss of oil pressure at cruise

Discussion in 'Technical Discussion' started by motoryachtlover, Jun 27, 2020.

You need to be registered and signed in to view this content.
  1. motoryachtlover

    motoryachtlover Senior Member

    Joined:
    Jan 29, 2007
    Messages:
    336
    Location:
    smithfield, VA
    On my family vacation and on a 5 hour run yesterday I noticed my oil pressure on my stbd main engine had dropped apprx 10 psi at cruise. This was on a VDO gauge at the helm. Wasn’t too concerned thought maybe gauge was messing up but went down in ER to confirm and sure enough the engine room gauge reflected about a 10 psi loss. Now I have gotten concerned.

    Boat is a 1995 Viking with 820 hp 10 cylinder Mans. Cruise Rpm 2050 and oil pressure is normally 80 psi and has dropped to 70 psi. These are mechanical engines with no computer. All else appears to be normal. I have visually looked at the oil but that reveals nothing to my eyes. Would rather not deal with a repower right now. But hopefully getting ahead of myself.

    Also headed North from Key West. Does anyone have any thoughts on Scott Marine Power in FLL.

    Thanks for your time.
  2. Capt Ralph

    Capt Ralph Senior Member

    Joined:
    Sep 8, 2004
    Messages:
    9,204
    Location:
    Satsuma, FL
    Are you over full of oil?
    Smell fuel in the oil?

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

    MBevins Senior Member

    Joined:
    May 23, 2008
    Messages:
    658
    Location:
    Windsor On. Canada
    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.
  4. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    13,400
    Location:
    Fort Lauderdale
    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).
    Last edited: Jun 27, 2020
  5. Capt Ralph

    Capt Ralph Senior Member

    Joined:
    Sep 8, 2004
    Messages:
    9,204
    Location:
    Satsuma, FL
    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.
  6. motoryachtlover

    motoryachtlover Senior Member

    Joined:
    Jan 29, 2007
    Messages:
    336
    Location:
    smithfield, VA
    Thanks to all who responded.

    Used Capt J’s method for checking the oil level and it is midway between the add and full mark. Capt J you were right to suggest this method as I have been down the check, then add, then over full route with these engines.

    I would say about 4 hours in the trip is when I noticed the pressure being down. I am a gauge hawk so I feel good about that answer.

    Smell test doesn’t smell any different when compared to port engine. Drip test off of dipstick appears the same from one engine to the next.


    Don’t have access to my invoices but oil was changed back in the winter with maybe 30 hours on it. Covid prevented use of the boat till now from January. Oil was sampled at that time and nothing abnormal showed up.

    Thought I had found the problem. See picture attached of oil pressure sender, but confirmed that ER gauge is mechanical.

    Trying to line up an oil change and sample for Monday but Key West is not that easy to find someone on short notice.

    Is there a redneck way of seeing if you have fuel in the oil?

    Thanks Again

    Attached Files:

  7. MBevins

    MBevins Senior Member

    Joined:
    May 23, 2008
    Messages:
    658
    Location:
    Windsor On. Canada
    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.
  8. motoryachtlover

    motoryachtlover Senior Member

    Joined:
    Jan 29, 2007
    Messages:
    336
    Location:
    smithfield, VA
    Just tried that and don’t see what you describe and both engines oil looks the same on the napkin. Thank you for the tip.
  9. Capt Ralph

    Capt Ralph Senior Member

    Joined:
    Sep 8, 2004
    Messages:
    9,204
    Location:
    Satsuma, FL
    That picture looks like a low pressure switch and not a pressure sender.
  10. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    13,400
    Location:
    Fort Lauderdale
    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.
  11. motoryachtlover

    motoryachtlover Senior Member

    Joined:
    Jan 29, 2007
    Messages:
    336
    Location:
    smithfield, VA
    Capt Ralph and J thanks. You mean go ahead and run the engine as normal if nothing else changes further? Atleast till I get to Miami and see if Scott or that other MAN dealer can get me. Trying to get mechanic down here lined up on short notice is not bearing much fruit. Sure would like a sample and change the oil to see if that changes things. Supposedly there are oil pressure relief valves (I think I am correct on that) on these engines. So hoping nothing catastrophic is brewing.
  12. Capt Ralph

    Capt Ralph Senior Member

    Joined:
    Sep 8, 2004
    Messages:
    9,204
    Location:
    Satsuma, FL
    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?
  13. MBevins

    MBevins Senior Member

    Joined:
    May 23, 2008
    Messages:
    658
    Location:
    Windsor On. Canada
    Then it may not have fuel in the oil.
    I did and it was obvious on the paper towel
  14. Capt Ralph

    Capt Ralph Senior Member

    Joined:
    Sep 8, 2004
    Messages:
    9,204
    Location:
    Satsuma, FL
    That's a tight dip top seal.
  15. Capt Ralph

    Capt Ralph Senior Member

    Joined:
    Sep 8, 2004
    Messages:
    9,204
    Location:
    Satsuma, FL
    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.
  16. motoryachtlover

    motoryachtlover Senior Member

    Joined:
    Jan 29, 2007
    Messages:
    336
    Location:
    smithfield, VA
    I realize it has been awhile but I waited and waited for the invoice and oil sample results from the Key West mechanic. Finally got the invoice but no sample. He said lab messed it up. So had another sample done and the oil checked out fine. Negative on fuel in the oil. I have 2 well thought Of mechanics say let it be. Most likely pressure relief valve.

    the real point 0f this was twofold. One, I wanted to let the forum know the outcome and two wanted to thank Capt J, Ralph, & M Bevins for their help on this. I sincerely appreciate you alls time and expertise and thank you.
  17. mapism

    mapism Senior Member

    Joined:
    Aug 9, 2008
    Messages:
    977
    Location:
    Sardinia
    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.
  18. gr8trn

    gr8trn Senior Member

    Joined:
    Dec 3, 2012
    Messages:
    414
    Location:
    Portland, OR
    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.
  19. rtrafford

    rtrafford Senior Member

    Joined:
    Jul 13, 2019
    Messages:
    843
    Location:
    Vero Beach
    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?
  20. mapism

    mapism Senior Member

    Joined:
    Aug 9, 2008
    Messages:
    977
    Location:
    Sardinia
    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.