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Old 12-08-2015, 04:10   #1
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Why low oil pressure?

My Yanmar 4JH3-DTE 125HP engine has seen a drop in oil pressure over the last month from 0.4MPa to 0.2MPa. Quite honestly I don't know if it happened all at once or if it has been a slight decline. (I have foolishly tended to focus on the water temperature too much) All I know is that I noticed it was not at its typical 12 o'clock location yesterday morning as we were motoring along. I watched it like a hawk the last 20 miles of the motor and it did not appear to fall anymore over that time and the water temp stayed perfectly normal.

I changed the filters a month ago and there is no leak. The oil stayed topped off from the mornings check before the 25 mile motor (total) day yesterday.

What could be causing this? Is 0.2MPa a safe operating pressure?

Mark
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Old 12-08-2015, 04:59   #2
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Re: Why low oil pressure?

I believe that the specified operating pressure is 0.43 MPa Mpa @ 3600 RPM, and 0.049 MPa @ 800 RPM.
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Old 12-08-2015, 05:10   #3
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Re: Why low oil pressure?

when it happened to me it was corrosion in the wire harness
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Old 12-08-2015, 05:13   #4
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Re: Why low oil pressure?

It's either the wiring or the sending unit.
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Old 12-08-2015, 11:11   #5
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Re: Why low oil pressure?

First you need to determine if the oil is the correct viscosity and is not contaminated with fuel or coolant and the filter is not stopped up and the coolant is the correct temperature.
Next you need to determine if you actually have low oil pressure or a sender is not functioning properly. It has been my experience that the sensing system is less reliable than the engine oiling system and therefor is prone to producing inaccurate readings.

The simplest and most reliable test is to install a known good mechanical test gauge in the engine oil gallery [where the sender is attached is probably the best place] and run the engine through a normal load and temperature cycle while observing the accurate gauge. You do not require a special gauge a commercial 1 or 2% gauge with correct pressure range and face marking from McMaster.com is acceptable. I personally us a 3.5" 0-100 psi / 0-700 KPA gauge with a 270 degree swing with 1% accuracy marked in 10 psi increments.
You need to determine the correct fitting prior to purchasing the gauge to get the proper gauge. A short section of automotive power steering pressure hose and some adapters and hose bibs will make the connections
After determining accurate oil pressure you can continue with your troubleshooting.
Never ignore low oil pressure readings!!
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Old 12-08-2015, 11:39   #6
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Re: Why low oil pressure?

Quote:
Originally Posted by Kenomac View Post
It's either the wiring or the sending unit.
Or the rod bearings or the main bearings or the cam bearings or the oil pump etc. etc.
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Old 12-08-2015, 13:27   #7
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Re: Why low oil pressure?

Quote:
Originally Posted by tulsag View Post
My Yanmar 4JH3-DTE 125HP engine has seen a drop in oil pressure over the last month from 0.4MPa to 0.2MPa. Quite honestly I don't know if it happened all at once or if it has been a slight decline. (I have foolishly tended to focus on the water temperature too much) All I know is that I noticed it was not at its typical 12 o'clock location yesterday morning as we were motoring along. I watched it like a hawk the last 20 miles of the motor and it did not appear to fall anymore over that time and the water temp stayed perfectly normal.

I changed the filters a month ago and there is no leak. The oil stayed topped off from the mornings check before the 25 mile motor (total) day yesterday.

What could be causing this? Is 0.2MPa a safe operating pressure?

Mark
Take a large wrench and beat on the sending unit. If your diligent enough to notice it like that, I wouldn't get ulcers about it being catastrophic.
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Old 12-08-2015, 14:11   #8
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Re: Why low oil pressure?

Have you got/can you borrow an infrared temperature gun?

Measure the temperature of the exhaust coming out of each cylinder head. If any trail behind the others then it may have a fuel problem. Ideally you would have measured the temperature at different times when the engine was running well and the difference would be obvious but if one cylinder head/exhaust is cooler that may well be the issue.

If the fuel is not being burnt properly, as shown by a cooler temperature, then fuel can slide past the rings into the engine oil where it reduces its viscosity and thus ability to achieve pressure.

You can sometimes feel the difference by rubbing some of engine oil between finger and thumb.
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Old 12-08-2015, 15:44   #9
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Re: Why low oil pressure?

If you're getting blow by I believe you would see a rise in the oil level.
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Old 12-08-2015, 16:13   #10
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Re: Why low oil pressure?

I wouldn't worry about all the disaster causes posts, if you really had a pressure problem it would be all over by now.
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Old 12-08-2015, 16:32   #11
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Re: Why low oil pressure?

[QUOTE=Cadence;1889195]Take a large wrench and beat on the sending unit.

That should do it.
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Old 12-08-2015, 16:44   #12
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Re: Why low oil pressure?

Quote:
Originally Posted by tulsag View Post
My Yanmar 4JH3-DTE 125HP engine has seen a drop in oil pressure over the last month from 0.4MPa to 0.2MPa. Quite honestly I don't know if it happened all at once or if it has been a slight decline. (I have foolishly tended to focus on the water temperature too much) All I know is that I noticed it was not at its typical 12 o'clock location yesterday morning as we were motoring along. I watched it like a hawk the last 20 miles of the motor and it did not appear to fall anymore over that time and the water temp stayed perfectly normal.

I changed the filters a month ago and there is no leak. The oil stayed topped off from the mornings check before the 25 mile motor (total) day yesterday.

What could be causing this? Is 0.2MPa a safe operating pressure?

Mark
I like to confirm the actual pressure with another gauge first. I keep a calibrated test gauge in my toolkit.

Then you can work through, sender, guage, voltage drop, pressure relief valve, oil cooler pin hole and so on.

You mentioned an oil change. I assume you didn't try a radically different viscosity.

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Old 12-08-2015, 16:46   #13
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Re: Why low oil pressure?

[QUOTE=Guy;1889313]
Quote:
Originally Posted by Cadence View Post
Take a large wrench and beat on the sending unit.

That should do it.
Guy, sounds you have also had that. I think we all tend to assume the worst case. I hope a new sending unit solves the problem. Or should I say a nonexistent problem.
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Old 12-08-2015, 22:43   #14
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Re: Why low oil pressure?

Hi Mark. After you checked your wire connection, gauge and oil pressure switch, contact Yanmar technical support for advice/help.Here in Australia I found them very helpful and honest.
Good luck
Dieter
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Old 13-08-2015, 01:14   #15
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Re: Why low oil pressure?

I can't tell everyone how much I appreciate the info. We are on the SE coast of Spain right now and getting ready to jump offshore to the Balearic Islands. You saved the trip. I'll keep watching the temp's carefully to make sure nothing is more serious.

Thanks again!!!!!

Mark
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