Possible explanations of 10 psi loss of oil pressure at cruise

rtrafford · Aug 19, 2020

mapism said: ↑

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.
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Thank you so much for the explanation. I was trying to wrap my head around his issue and understand the culprit, but I was stuck on that sender versus what I know of, well, Murphy Switches. That micro tube or capillary set up is my experience, a lengthy one on a wide variety of engines.

motoryachtlover · Aug 19, 2020

mapism said: ↑

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.
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Sorry I am late to get back to you all. I have photos of the engine room gauges at 1000 RPMs and temp of 170 and the oil pressure on the port is a little over 50 and the starboard a little under 50 (mechanical gauges) I don’t have pictures of idle. The viscosity is 15W40.

mapism · Aug 20, 2020

Viscosity is fine, and also 50 psi at 1000 rpm is plenty.
For your future reference, at idle even a bit less than 30 psi would still be fine.
You do have a surprisingly high pressure at 2k rpm, but better than the opposite, anyway!

rtrafford · Aug 20, 2020

mapism said: ↑

Viscosity is fine, and also 50 psi at 1000 rpm is plenty.
For your future reference, at idle even a bit less than 30 psi would still be fine.
You do have a surprisingly high pressure at 2k rpm, but better than the opposite, anyway!
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Consider swapping gauges between engines, see if the issue follows the gauge? Repeat with senders?

Capt J · Aug 23, 2020

rtrafford said: ↑

Consider swapping gauges between engines, see if the issue follows the gauge? Repeat with senders?
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In reality, 70 psi at cruise is a NON issue or really anything over 40 psi at 2000 rpms.......I just delivered a yacht with the same engines and at cruise it only made 50-60psi.....

Soulstice · Aug 28, 2020

motoryachtlover said: ↑

Sorry I am late to get back to you all. I have photos of the engine room gauges at 1000 RPMs and temp of 170 and the oil pressure on the port is a little over 50 and the starboard a little under 50 (mechanical gauges) I don’t have pictures of idle. The viscosity is 15W40.
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I have similar pressures on my MAN’s with about 45 psi at cruise and running 15W40. I wouldn’t worry too much about the pressure you are running at as it sounds pretty normal.

motoryachtlover · Aug 28, 2020

Thanks to all that commented. Much appreciated

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Possible explanations of 10 psi loss of oil pressure at cruise

rtrafford Senior Member

motoryachtlover Senior Member

mapism Senior Member

rtrafford Senior Member

Capt J Senior Member

Soulstice Member

motoryachtlover Senior Member