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Old 06-06-2020, 18:06   #1
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Another Low Oil Pressure Query

I know you all think that I think I already know it all, but I don't (obviously).

Here is another low oil pressure query.

In 27 years we've put 6658 hours on this engine. It has had (until now) no major problems. It does not smoke, always starts instantly, loses no oil or water or leaks any, runs at 80c based on the thermostat we have in, and rarely gives us any concern or problems. We have regularly serviced and inspected it.

But we have had oil pressure issues for at least six years and have not solved them, (Gauge has showed low or inconsistent numbers) but we always blamed it on the gauge or wiring. We have replaced the oil pressure sending unit, with no change.

Since R&R on the wiring harness the gauge has at least been consistent; It reads high at start up (50lb/in2) then slowly goes down as the engine operates, to about 15-20lb/in2).

(Interestingly, the pressure increases when we idle the engine down. Example: at 2000rpm, after 1-2 hours operation, oil pressure is 15-20lb/in2. Slowing to idle it goes up to 25-35lb/in2. That baffles and concerns me.)

Until now the oil pressure alarm has never gone off.

But now the oil pressure alarm has started to go off (sound, loudly) after about 45 minutes (so far, two days). Today it went on as soon as I started the engine. I do not have a spare. The gauge, on the other hand reads normally (as above) 20-30lb/in2.

So far I have opened the oil gallery while running the engine and heaps of oil comes out (engine cold and hot). I cannot hold it with my finger. But I have also inspected the rocker arms and tappets, they are barely wet when previously they were running in oil. All very confusing, so we are heading back to port where we will continue diagnosing the problem.

First I will replace the oil pressure switch (which runs the alarm) and install a mechanical oil pressure gauge to try to understand what exactly is going on.

Engine History:
27 years since installation;
Total hours 6658
Hours powering 523
Hours charging (at anchor or sailing) 6135

While charging we run it at 2000rpm and produce approx 80 amps of 12v DC, so it has some load.

So, thoughts, anyone?
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Old 06-06-2020, 18:12   #2
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Re: Another Low Oil Pressure Query

Quote:
Originally Posted by wingssail View Post
I know you all think that I think I already know it all, but I don't (obviously).

Here is another low oil pressure query.
........
First I will replace the oil pressure switch (which runs the alarm) and install a mechanical oil pressure gauge to try to understand what exactly is going on.

..........
So, thoughts, anyone?
Sounds like a very good plan.
There is one thing more you can do in the meantime should the LOP alarm sound again. Immediately disconnect the wire off the LOP switch on the engine. If the alarm continues to sound, you have a wiring / alarm fault rather than an engine or LOP switch fault.
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Old 06-06-2020, 18:16   #3
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Re: Another Low Oil Pressure Query

Increasing pressure when coming back to idle makes me think cold oil bypass valve, or bad wiring to the gauge / bad gauge. Voltage changes at idle and that gauge is electric.
Bypass valves are usually nothing but a valve held by a spring, the spring determines pressure at which they open.
They are very often very easy to remove, I’d just replace it with a new one as they are usually not expensive.

However I would most certainly install a direct reading gauge just for trouble shooting because a loose or corroded or high resistance wire will affect readings. In fact I wouldn’t do anything until I put a direct reading gauge on it.
What is your oil change interval? Running at anchor to charge if anything is less wear on a bottom end as the loads are usually low, but you may want to consider a generator to off load all those hours spent charging.
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Old 06-06-2020, 18:19   #4
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Re: Another Low Oil Pressure Query

Unless I haven’t been paying attention, you don’t mention what engine you have. Nevertheless, in broad brush strokes I would be leaning towards a pressure relief valve going “soft”.

A calibrated spring in the valve determines the pressure at which the system will operate. These springs sometimes break or go soft and diminish the system pressure accordingly.

The volume of oil you see under “no-load” conditions are not an indication of pressure. A lot of oil can flow without there being any pressure behind it. The buzzer is a clear indication that your pressure is indeed low. The gauge and the buzzer generally work on different switches and it’s really unlikely they’re both dodgy.

Edit - a64Pilot and I posted at the same time, sorry to duplicate info.
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Old 06-06-2020, 21:07   #5
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Re: Another Low Oil Pressure Query

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Originally Posted by Wotname View Post
Sounds like a very good plan.
There is one thing more you can do in the meantime should the LOP alarm sound again. Immediately disconnect the wire off the LOP switch on the engine. If the alarm continues to sound, you have a wiring / alarm fault rather than an engine or LOP switch fault.
Thanks,

Been there, done that. The alarm stops.
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Old 06-06-2020, 21:09   #6
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Re: Another Low Oil Pressure Query

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Originally Posted by CassidyNZ View Post
Unless I haven’t been paying attention, you don’t mention what engine you have. Nevertheless, in broad brush strokes I would be leaning towards a pressure relief valve going “soft”.

A calibrated spring in the valve determines the pressure at which the system will operate. These springs sometimes break or go soft and diminish the system pressure accordingly.

The volume of oil you see under “no-load” conditions are not an indication of pressure. A lot of oil can flow without there being any pressure behind it. The buzzer is a clear indication that your pressure is indeed low. The gauge and the buzzer generally work on different switches and it’s really unlikely they’re both dodgy.

Edit - a64Pilot and I posted at the same time, sorry to duplicate info.
Thanks to both of you. it's a Yanmar 3JH

Quote:
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A calibrated spring in the valve determines the pressure at which the system will operate. These springs sometimes break or go soft and diminish the system pressure accordingly.
Yes, I know about the pressure relief valve. It will be great if that's what it is. I'm worrying about crank journal bearings or something.
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Old 06-06-2020, 21:46   #7
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Re: Another Low Oil Pressure Query

If a test with a mechanical pressure gauge returns the same results as the electric gauge It might be time to do the traditional engine health check, take off the lube oil filter , cut it open and inspect the paper folds for signs of shiny metal flakes.
Sadly on the 3JH 3 the oil pressure control valve is integrated with the oil pump in the sump so if you have to lift the engine and open the sump it would be reasonable to inspect all the crankshaft bearings and suction screen at that time and remove doubt.
Wow, 6,135 hours of light running at anchor certainly blows the...” light running glazes cylinders” theory completely out of the water.
Where do you get the oil pressure sensor reading from, the gallery at the rear of the block or the point at the oil filter housing?
I’m hoping for a good reading from the mastergauge
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Old 06-06-2020, 22:05   #8
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Re: Another Low Oil Pressure Query

Are you sure the oil pressure is the problem? My boat, and many others, have a single sound alarm that does double duty. It goes off if the engine overheats as well as indicating low oil pressure.
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Old 06-06-2020, 22:18   #9
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Re: Another Low Oil Pressure Query

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Where do you get the oil pressure sensor reading from, the gallery at the rear of the block or the point at the oil filter housing?
I’m hoping for a good reading from the mastergauge
You’ll need to remove the pressure sender unit (there will be 2, either one will work) and screw an appropriate fitting in for the master gauge.
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Old 06-06-2020, 22:19   #10
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Re: Another Low Oil Pressure Query

Question for O.P.
When you say the oil pressure rises at idle does it do that instantly or over time? I can see how it would drop after some running time under load but the rise at idle is unusual. Bit baffling to me too if it isn't the PRV
However sounds like oil is not getting to the end of the chain if the rocker arms are dryish. Not something you can ignore if it has changed.Makes me think the gauge is working but surprised oil doesnt get to the rocker arms at 20psi
Following with interest after you fit mechanical gauge
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Old 06-06-2020, 22:21   #11
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Re: Another Low Oil Pressure Query

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Are you sure the oil pressure is the problem? My boat, and many others, have a single sound alarm that does double duty. It goes off if the engine overheats as well as indicating low oil pressure.
Yes but low pressure is also being indicated on the gauge and they run off different senders so the problem is most likely not cooling.
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Old 06-06-2020, 22:36   #12
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Re: Another Low Oil Pressure Query

3JH what?

For instance, this manual, for the 3JH2, shows the pressure relief valve located on the pump mounted on the front of the engine under the front gear cover, with the pressure regulating valve located in the oil filter housing on the side of the engine.

https://www.manualslib.com/manual/73...age=122#manual

Either valve could cause the problem you're seeing, but it's much easier to get at the regulating valve than the relief valve (at least if you have the 3jh2)
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Old 06-06-2020, 23:06   #13
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Re: Another Low Oil Pressure Query

With the drop of oil pressure what temps are you seeing on the engine?

Also, when you do a oil change are you cutting the filters and inspecting the elements, or doing any oil trends?

Any odd engine sounds?


Like the others said, I’d get a hand held gauge and get a direct reading.
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Old 07-06-2020, 02:12   #14
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Re: Another Low Oil Pressure Query

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Originally Posted by jimbunyard View Post
3JH what?

For instance, this manual, for the 3JH2, shows the pressure relief valve located on the pump mounted on the front of the engine under the front gear cover, with the pressure regulating valve located in the oil filter housing on the side of the engine.

https://www.manualslib.com/manual/73...age=122#manual

Either valve could cause the problem you're seeing, but it's much easier to get at the regulating valve than the relief valve (at least if you have the 3jh2)


You’re right, the pump is under the timing cover but the sump gasket rarely survives if the timing cover is removed so it needs to come off .....might as well check the suction strainer too and put in new gaskets
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Old 07-06-2020, 02:42   #15
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Re: Another Low Oil Pressure Query

Following.

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