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Old 08-03-2015, 11:47   #1
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Low oil pressure reading

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On our Yanmar 4JHE the oil pressure gauge on the engine panel has, in the last 6 months, begun registering low kg/cm. In the past the gauge needle pointed straight up at 4 kg/cm in a normal operating rpm range. Now it registers 4 until it's warmed up then it slowly drops to 1.5 to 2 kg/cm. I did change the sending unit with no change in the reading.

My diesel mechanic checked the oil pressure with his gauge and found it to be okay despite the gauge in the panel. He said it is not uncommon for the older Yanmar engine panel gauges to read low oil pressure. His suggestion is either upgrade the panel or add a mechanical gauge near the engine I can check when the panel gauge reads low.

Any thoughts ?


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Old 08-03-2015, 13:19   #2
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Re: Low oil pressure reading

Follow his advice, it's exactly what I would recommend


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Old 08-03-2015, 22:58   #3
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Re: Low oil pressure reading

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Originally Posted by a64pilot View Post
Follow his advice, it's exactly what I would recommend


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sounds like you don't trust him. you should have two anyhow.
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Old 08-03-2015, 23:09   #4
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Re: Low oil pressure reading

Have you used a different engine oil recently? Sometimes oils may be too thin for the diesel.

Pressure gauges and sensors do go bad, though if its electric, it might just be corrosion on the joint between the ring terminal and wire. Yanmar seems to have learned electrical wiring from lucus electrics.

Could also be a bad sender. My preference is a mechanical oil gauge.
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Old 08-03-2015, 23:40   #5
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Re: Low oil pressure reading

1.5 KG/cm = 15 PSI. That's too low for any engine at normal operating rpm, especially as it used to be higher. You need to figure out what the cause is. The engine is at or near the end of its life if this really is the oil pressure, and you don't do something about it.

Reasons could be :

Oil is too thin
The oil filter is blocked
The guage or sender is faulty
The oil pump is worn
Broken spring in the oil pressure relief valve
The pickup is partially blocked
The engine's internals are badly worn

But I'd say one situation where I would be alarmed by an oil pressure reading is the one you have : fairly rapid drop in oil pressure in an older engine.
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Old 09-03-2015, 08:49   #6
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Re: Low oil pressure reading

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Originally Posted by Tayana42 View Post
Attachment 98498

On our Yanmar 4JHE the oil pressure gauge on the engine panel has, in the last 6 months, begun registering low kg/cm. In the past the gauge needle pointed straight up at 4 kg/cm in a normal operating rpm range. Now it registers 4 until it's warmed up then it slowly drops to 1.5 to 2 kg/cm. I did change the sending unit with no change in the reading.

My diesel mechanic checked the oil pressure with his gauge and found it to be okay despite the gauge in the panel. He said it is not uncommon for the older Yanmar engine panel gauges to read low oil pressure. His suggestion is either upgrade the panel or add a mechanical gauge near the engine I can check when the panel gauge reads low.

Any thoughts ?


S/V B'Shert
Take the mechanic's advise. If he is getting a good reading and you replaced the sending unit, check the wiring, put in a new gauge put in the second mechanical gauge. Stop getting ulcers, I know low oil pressure readings will cause that.
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Old 09-03-2015, 10:01   #7
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Re: Low oil pressure reading

It is always best to confirm yourself or get a second opinion. Would you trust a doctor who checks your blood pressure and gets a bad reading on the electronic one and does not pull out the old cuff, stop watch and stethoscope to confirm or deny the reading? In your case, I would trust the mechanic, but fix the issue starting with a new sender. If you want to check yourself first:

First, check what the safe range is for your particular engine, found normally in the specs on the manufacturer.

Pressure is only an indicator and high oil pressure with low flow can actually be worse than low oil pressure with good flow. The import thing is the pump is getting the oil to ALL of the engine parts. Oil pressure, either too high or too low tells us we need to look further into the health of the engine.

Second, determine what your pressure really is. This is pretty simple. Purchase an inexpensive manual gauge with a plastic hose that goes between the sender and the gauge. Be very mindful when attaching the hoses, the little copper cones are a one shot deal. About $20 US for a decent sunpro gauge and it does not take a lot of space. Some people will T off for both electronic and manual gauges, I prefer to just keep the manual one in the tool box and can easily be loaned to other cruisers as well.

Unscrew the current sender. It is normally located close to the oil filter, has a single wire attached to the top of it and is the one that is fat on the top. Be very careful when unscrewing it, make sure to use the threads under the fat part and do not use a wrench on anything but the threads under the fat part of the sender.

After you installed the temporary gauge, start the engine and let it run for a little bit until the gauge is reading and make sure no air or oil is leaking from the hose. Compare the readings to what you were getting on your electronic gauge. If the manual gauge is in spec with the engine, you can replace the sending unit that you removed and see if that solves the electronic problem.


---- Hopefully this is the stopping point -----


If you still have low oil pressure, the first thing to always do is change the oil and make sure you are using oil that is recommended and approved for your engine and the correct viscosity, don't use a heavier or lighter oil. Careful of some of the newer and synthetic blends, they are formulated for engines that run hot and run long and actually worse for a lightly used recreational sailboat engine.

I like to send out a sample of the oil to Blackstone Labs . You need to contact them a few days before changinng the oil and they will send you the sampling kit and return envelope. They don't charge for the kit, so order the kit first and have it ready for the next oil change. They charge when you return the kit for analysis.

Log the pressure readings at idle cold < 140
At idle and cruising speed warm (140 - 170)
At idle and cruising speed hot (170-185)

Do this before and after changing the oil.


So, ~$25 to verify the pressure and about 1 hour of your time.

An extra $25 to get a basic oil analysis done. On top of standard oil change costs. And just a little more observant and logging when on the water.

Example: If the analysis comes back with a high flashpoint or dillution, that is easily backed up by an increased pressure reading after an oil change. Leaky injector, worn rings, etc. Testing would now go to injectors, compression and maybe leak down test.

If the analysis comes back with metals present and the oil pressure is low and drops the hotter the engine, can easily indicate worn main or cam bearings, etc. The worst possible issues. Clean of metals and dillution, but smaller changes in oil pressure, look to seals/gaskets.

It helps narrow where to look and possible scenarios.

Gather that information, then can go from there.
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Old 09-03-2015, 10:09   #8
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Re: Low oil pressure reading

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Originally Posted by gah964 View Post
sounds like you don't trust him. you should have two anyhow.

No, I do trust him. He is a well trained and experienced diesel mechanic.


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Old 09-03-2015, 10:12   #9
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Re: Low oil pressure reading

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Originally Posted by sailorchic34 View Post
Have you used a different engine oil recently? Sometimes oils may be too thin for the diesel.

Pressure gauges and sensors do go bad, though if its electric, it might just be corrosion on the joint between the ring terminal and wire. Yanmar seems to have learned electrical wiring from lucus electrics.

Could also be a bad sender. My preference is a mechanical oil gauge.

Thanks for the thoughts. No change in oil. I'll check connections. The wiring harness connections are just below the mixing elbow which we needed to replace a couple of years ago. I put on a new sender with no change in the reading.


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Old 09-03-2015, 10:13   #10
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Re: Low oil pressure reading

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Originally Posted by Cadence View Post
Take the mechanic's advise. If he is getting a good reading and you replaced the sending unit, check the wiring, put in a new gauge put in the second mechanical gauge. Stop getting ulcers, I know low oil pressure readings will cause that.

Thanks, don't need ulcers.


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Old 09-03-2015, 10:20   #11
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Low oil pressure reading

Tdoster, thank you for that clear and helpful reply. He did put in a temporary gauge and got good readings. With the mechanic I replaced the sending unit with no change in the readings at the panel.

Thanks all for the feedback and confirmation of the mechanic's advice.


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Old 09-03-2015, 12:05   #12
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Re: Low oil pressure reading

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Originally Posted by a64pilot View Post
Follow his advice, it's exactly what I would recommend
x2 assuming the mechanic's gauge is accurate. Installing your own mechanical gauge will verify that.
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Old 09-04-2015, 13:10   #13
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Re: Low oil pressure reading

A few of weeks have gone by and we finally got the mechanical oil pressure gauge installed. It is a Murphy model 20P-150. The engine is a Yanmar 4JHE naturally aspirated.

At idle the gauge reads 30 psi. Under load at 2000 rpm it goes to 60 psi. All good. However, after about an hour of running at 2000 rpm the pressure drops to 40 psi. Increased engine rpm to 25000 and pressure reading was 50 psi. I didn't keep it at 2500 long. This is the same pattern as the electronic gauge readings that caused me to install the mechanical gauge.

My mechanic, who I've used for years, says it okay. He says 2000 rpm is the bottom of the cruising range for this engine and 40 psi is good. I'm happy to take him word for this...but I wouldn't mind some input from the forum. Is 40 psi okay for the Yanmar 4JHE at 2000 rpm?


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Old 09-04-2015, 13:53   #14
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Re: Low oil pressure reading

This is from the manual for a newer 4JH series engine. No verification that it is accurate for your exact model, but I suspect it is close:

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The 4JHE manual itself is largely silent (maybe someone has a detailed service manual, although I didn't see it there either), it just has the warning level published:

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Old 09-04-2015, 14:28   #15
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Re: Low oil pressure reading

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Originally Posted by Dsanduril View Post
This is from the manual for a newer 4JH series engine. No verification that it is accurate for your exact model, but I suspect it is close:



Attachment 100179



The 4JHE manual itself is largely silent (maybe someone has a detailed service manual, although I didn't see it there either), it just has the warning level published:



Attachment 100180

Thanks for the reply. Seems to confirm, 3000 rpm @ 41-78 psi but I'm still uncertain about running at 2000 rpm at 40 psi for long hours. Or should I interpret this to mean I should only cruise at higher rpm?


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