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Old 31-12-2012, 15:42   #1
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Low Oil Pressure - advice required please

Hi all and Happy New Year to everyone - This forum has helped me heaps and you have fixed all my problems on my beautiful boat and my lovely Perkins 4.108 - now a new question - Why does the oil pressure fall to around 20 psi when operating for an hour or more - it starts off at around 60 psi fall to 40 psi sits there for 45 minutes and then drops to 20 - I did not take any noticed of the oil pressure before, it is only after fixing the overheating problem (thanks to you all) that I started taking a closer look now that I have learned what stats are normal - can anyone help ?? - Oh new oil and new oil filter using 15W/40 - Fusch Titan Diesel engine oil - operating in a mild climate - SE OZ - MVR
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Old 31-12-2012, 15:46   #2
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Re: Low Oil Pressure - advice required please

At all RPM? Or only low RPM?
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Old 31-12-2012, 16:23   #3
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Re: Low Oil Pressure - advice required please

I would place my money on the problem being electrical in nature. Bad connections, frayed wires, bad gage, bad sender.

Both of my boats have had oil pressure indication problems (one was high and one was low) and both times it was electrical.

If you really don't believe this you need to put a real pressure gage on the sender location and watch it under the same conditions.
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Old 31-12-2012, 16:35   #4
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Re: Low Oil Pressure - advice required please

It's normal for oil pressure to start off high when the oil is cold and gradually go down, but 20 is too low at normal operating speeds but it would be in the ballpark at idle.
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Old 31-12-2012, 16:36   #5
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Re: Low Oil Pressure - advice required please

My 4-108 ran pretty low once warm at idle. But should be 40 or so at higher rpm....
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Old 31-12-2012, 17:02   #6
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Re: Low Oil Pressure - advice required please

Oil pressure is an indication of resistance NOT Volume flow,,any motor with wear in the main bearings will show high pressure (oil thick/engine.cold) at start up. and then drop pressure when engine warms up
(Expansion of parts/gaps). Oil still flows as it should doing the job it should.
unless you have confirmed oil leaking or burning,or confirmed pump wear/failure, this is never a reason for worry.
( I was a Fleet Mechanic for many years,i know from where i speak )

The only problem i have with your scenario is your use of Graded oil instead of Single grade.
I myself would always use a straight 40 weight Oil. Not 15W nonsense.

Just remember....Clean Air Filters, and Clean Oil Filters are your best life extender, you cannot change filters too often.

Fair Winds.
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Old 31-12-2012, 17:13   #7
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Re: Low Oil Pressure - advice required please

I don't think that engine warming can explain the problem gven how long it takes.
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Old 31-12-2012, 17:24   #8
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Re: Low Oil Pressure - advice required please

I once replaced an engine in a early '80s Blazer due to low oil pressure. The problem persisted with the new engine. After the oil pressure sending unit was replaced, the problem went away. It was an expensive lesson.

With an oil pressure problem, always start with the cheapest possible reason and work towards the most expensive possible reason.
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Old 31-12-2012, 17:29   #9
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Re: Low Oil Pressure - advice required please

When engine is cold,,all bearing tolerances are tight.Oil is Thick...Ergo..Small gaps,Cold Oil and higher resistance to Oil passage..Result: High Oil Pressure.

As Engine warms up, bearing tolerances expand..Oil thins out. Ergo...Wider gaps,Hot Thinner Oil and less resistance to Oil flow..
Result: Low Oil Pressure.

The Pressure gauge takes measurement reading on resistance to Flow,,,not Flow it'self.

Engines wear and as they do their oil gaps increase,this may only be a few thousands of an inch on some surfaces, and way less than this on others.
As long as the Pump is functioning properly there is no reason to worry.

On some engines an Oil bypass valve may get stuck in the open,or closed position, this situation has been known to show drastic drops in oil pressure,,or the reverse with inordinate high Oil pressures. But is Rare.

Quote:
Originally Posted by Don Lucas View Post
I don't think that engine warming can explain the problem gven how long it takes.
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Old 31-12-2012, 17:35   #10
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Re: Low Oil Pressure - advice required please

As you say, it could be an Electrical sender problem, but i was just pointing out that our Oil pressure Gauges give up False security in that they don't indicate what they should..."Oil Flow"...
Lack of Pressure should make one Wary if it happens suddenly after years constant readings.
But after checking all other factors,,lack of pressure has never been any cause for worry.

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I don't think that engine warming can explain the problem given how long it takes.
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Old 31-12-2012, 18:14   #11
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Re: Low Oil Pressure - advice required please

Quote:
Originally Posted by Nemo55 View Post
But after checking all other factors,,lack of pressure has never been any cause for worry.
You have to be kidding!

You must have gone to a lot different machine mechanical design course than I did.

Flow is just to cool, pressure is what is keeping metal parts apart.

And if your bearings have so much clearance that after 45 minutes of run time they open up so much that the oil pressure pump can not keep; you aren't going to have to worry abut it very long.
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Old 31-12-2012, 18:24   #12
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Re: Low Oil Pressure - advice required please

Hydrostatic pressure, and Hydraulic bearing support is what you are talking about and yes i agree, it is critically important.
My experience has been real world applications in the Truck and Bus industry.
Sounds like you just want to pick...and get into a pissing contest.
Go ahead if it makes you feel better.


Quote:
Originally Posted by Don Lucas View Post
You have to be kidding!

You must have gone to a lot different machine mechanical design course than I did.

Flow is just to cool, pressure is what is keeping metal parts apart.

And if your bearings have so much clearance that after 45 minutes of run time they open up so much that the oil pressure pump can not keep; you aren't going to have to worry abut it very long.
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Old 31-12-2012, 18:27   #13
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Re: Low Oil Pressure - advice required please

You have never worked around Diesels which have clocked Two Million Miles Have You ?????

Quote:
Originally Posted by Don Lucas View Post
You have to be kidding!

You must have gone to a lot different machine mechanical design course than I did.

Flow is just to cool, pressure is what is keeping metal parts apart.

And if your bearings have so much clearance that after 45 minutes of run time they open up so much that the oil pressure pump can not keep; you aren't going to have to worry abut it very long.
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Old 31-12-2012, 18:37   #14
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Re: Low Oil Pressure - advice required please

60 psi would never keep rod bearings from grinding. It's the viscosity not the pressure. But we digress.
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Old 31-12-2012, 18:47   #15
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Re: Low Oil Pressure - advice required please

Thank You...


Quote:
Originally Posted by daddle View Post
60 psi would never keep rod bearings from grinding. It's the viscosity not the pressure. But we digress.
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