BMW D-50

[Tommee]

Had a scare a couple of days ago with oil pressure drop. I had travelled under engine for about 2 hrs, all was well. Anchored up for about 4 hrs then started the engine & noticed the oil pressure gauge was showing lower than usual. Normally 65, was 45. I am religious about checking oil & water so knew that to be ok. The engine has done this a couple of times recently & I have stopped it, checked things, started again & all is normal again. So, this time I set off keeping an eye on the gauges. After about an hour the gauge dropped to nothing & slowly the temp rose to 95 so I shut her down & sailed to anchorage.
I changed the oil & filter even though it had not done the usual 100 hrs & now all is well again.
I had previously checked & cleaned the ground in case it was electrical.
I read that it may be the oil pump but have asked an engineer & he claims these pumps either work or they don't, they don't do intermittent.
This engine has an oil cooler. A sensor goes from the cooler to the gauge. Another sensor (recently replaced) goes from the block to the alarm & light. Does anyone with knowledge of this engine have any idea where the problem may lie please?
I have had the boat for 31 years, the engine is original & has done around 8000 hrs. Top end rebuilt at 5000 hrs. So yes, it's doing pretty well but I do maintain it.

[Engine man]

Install a QUALITY mechanical test gauge, this will eliminate any possible electrical issues, and reveal if your electrical gauge is reading incorrectly. You should also be using 15W40 diesel grade lube oil, not a single weight. Use a quality oil filter, such as NAPA sells. Fram filters are low quality, and cause problems.

You may also have a sticky oil pressure regulating valve, that is causing the problem, but first, determine if you actually HAVE low oil pressure. Joe

[Tommee]

Thank you Joe for your advice. I do use 15/40 mineral oil but I have been told that here in NZ even that contains a certain amount of synthetic because of "regulations!" It certainly seems thinner than it used to. I use Ryco filters but they may be cheap too. We do have a Nappa actually, so will buy those next time.
The engineer I spoke to on the phone also recommended fitting a manual gauge. Not quite so easily done as that oil cooler is in a ridiculously tight spot, but will see if it can be achieved. I guess that's where a new gauge would need to be fitted? That oil cooler is a menace actually as it is aluminium & prone to corrosion. To remove it I have to disconnect the prop shaft & lift the engine on its mounts to gain height & be able to access the dear object. Boats!!!

[tkeithlu]

While deferring completely to Engine Man's expertise, it occurs to me that given that your concern is stability of oil pressure rather than its absolute value, you should be able to install the mechanical gauge in a more accessible spot in the oil circuit.

Access to the engine is not a selling point at boat shows. The designer used every bit of available space for features that are.

[Oldwheels]

I agree.
Then engine will have several spots to tap into oil pressure.
My concern is"After about an hour the gauge dropped to nothing & slowly the temp rose to 95 so I shut her down & sailed to anchorage"
Though 95 C is not overly hot the reason for the temp rise is to which I assume is friction and lack of oil circulation not acting as a coolant.
I have seen bad oil pressure valves/regulators, smudge acting a check valve creating a temp blockage. Blockage in the return sections can happen.
I hate BMW engines. They can be great and awful. The support is marginal and parts are wishful.