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Old 31-05-2020, 17:23   #1
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Losing Oil Pressure?

Can't believe that the first sail of the year would be this late, but so it is.

So de-winterized, and both engines started right up. Yanmar 3GM30s

But after a while, the low oil pressure alarm sounded--intermittently, and only at low, maybe 900 rpm and below.

This has happened a couple of times in the past 20 years; faulty sending units each time. So I replaced the sending unit with the new one I had on hand, but the alarm still sounds at low rpm.

I've always thought oil pumps to be almost totally reliable, but maybe they do fail. As said, 20 years old, and with maybe 2500 hours.

I suppose I could get a mechanical gauge to install just for testing purposes and confirm what is going on.

Oil and filter were changed before winterizing.

Is there anything else that might cause this problem other than a bad oil pump?
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Old 31-05-2020, 17:35   #2
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Re: Losing Oil Pressure?

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.............

I suppose I could get a mechanical gauge to install just for testing purposes and confirm what is going on.

Oil and filter were changed before winterizing.

Is there anything else that might cause this problem other than a bad oil pump?
Yes, getting an independent gauge is essential to understanding what is going on.

Is there any oil leaking out of the external (usually rusty) steel oil pipe that takes oil from the block to the head - common fault with older Yanmars? The pipe is well hidden behind the engine

Did you use the correct grade of oil?

Has one or more of the rocker posts become loose on the top of the head?

Worn crankshaft bearings will allow oil pressure to drop when oil is hot and rpm is low.

If the oil pressure is only just high enough to extinguish the low oil pressure light when the oil is cold, then you may have some crud stuck in the oil pressure relief valve. It takes a good 30+ minutes at load to get the oil temperature stabilised.

The oil pump is probably the last item to check
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Old 31-05-2020, 17:41   #3
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Re: Losing Oil Pressure?

FWIW, I once had a low oil pressure light come on in the middle of an ocean. The engine had run OK for decades.

Discovered later all the bearings were shot and finally they allowed the oil pressure to drop to the point where the low oil pressure switch operated - the oil pressure must have been low for a long time but just high enough to prevent the alarm from operating.

I should have had a gauge!
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Old 31-05-2020, 17:43   #4
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Re: Losing Oil Pressure?

Do both engines have the same problem????????
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Old 31-05-2020, 18:11   #5
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Re: Losing Oil Pressure?

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Is there anything else that might cause this problem other than a bad oil pump?
It’s exceedingly unlikely the pump is worn to that extent, it’s possible, but I have never seen it myself.
Step one is to verify oil pressure with a gauge.
Once you determine you have low oil pressure then you start with most likely and go from there, starting with the cold oil bypass valve.
Unlikely but a filter if it has a defect could possibly restrict oil, then oil level of course, and grade of oil.
But first step is put a gauge on it, then with a cold motor crank it up, and write down oil pressure at idle, then advance RPM slowly in neutral before the motor is hot and write down max pressure it gets to, it should hit an RPM that above that there is no increase in pressure, that should be the oil pressure relief valve setting and checking it’s function. Also write down the RPM.
Then of course go out and run the boat normally, for say 30 min, then when it’s fully heated up, reduce to idle, let the pressure stabilize, and write down that number.

The senders, shouldn’t be senders, they should be switches, they are an on / off device, not a variable one, if they were senders then it’s possible you have a high resistance wire, but as it’s a switch, that’s unlikely.

What oil and weight are you using?

I’m hoping someone sold you the wrong switch. Not likely but it would be a good outcome
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Old 31-05-2020, 18:16   #6
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Re: Losing Oil Pressure?

Thanks for the quick replies, Wotname!

I've had that rusty oil pipe leak before, too. I will check closely to see if there is any oil leakage there tomorrow, but I didn't notice any oil near there today.

The oil is same as has been "forever", Rotella 10W40, and this problem is with only one engine. However, the other engine was rebuilt at about 1000 hours so is "newer". (A piece of a piston broke off; service manager thought it was defective from the factory.)

I don't know about the rocker posts, so will need to learn about that.

I would say the oil really hadn't heated up enough to cause low pressure, so perhaps your theory of something fouling the pressure relief valve could be on target. Do you know if this is accessible on the 3GM?

Again, thanks for the ideas!
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Old 31-05-2020, 18:34   #7
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Re: Losing Oil Pressure?

Thanks a64!

Yes, I guess you should call it just a switch, I wasn't thinking about a distinction between that and a sending unit!

You say the cold oil bypass...is this the same as the pressure relief valve? Is this difficult to clean/repair?

WRT the filters, interestingly I think this was one of a recent purchase of non-Yanmar filters so that's something to consider.

And the new switch is Yanmar but it could still be faulty. And I have always used Rotella 10W40.
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Old 31-05-2020, 18:37   #8
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Re: Losing Oil Pressure?

a64pilot pretty much said it all!
You should have 7 psi at idle on a very healthy engines and about 50-55 psi at hi idle. The oil pressure regulating valve is inside the protruding threaded part that the filter screws onto. Itís a simple ball and spring affair and is technically not serviceable so if the spring broke you need to get a new assembly. Spin off the filter and push the steel ball with a clean.... something. It should spring back
There is also a thin external oil pipe that goes around the back of the engine and it is known to rust through where it goes behind the starter motor but if this was the problem, there would be a swamp of oil in the bilge. Click image for larger version

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Old 31-05-2020, 18:41   #9
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Re: Losing Oil Pressure?

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Originally Posted by ggray View Post
.............

I would say the oil really hadn't heated up enough to cause low pressure, so perhaps your theory of something fouling the pressure relief valve could be on target. Do you know if this is accessible on the 3GM?

Again, thanks for the ideas!
There is a service manual for the 3GM30 in the library section of CF which is a good start.

https://www.cruisersforum.com/forums...?do=file&id=45

To be pedantic, it is worth noting that there are subtle differences between the 3GM and the 3GM30 although ATM, I don't recall what they are.
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