Yanmar Oil Pressure Wrong Way Round!

robbieg

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We have a rebuilt Yanmar 3GM apparently running well.

I have hooked up a mechanical pressure gauge in place of the oil switch sender-not sure of accuracy but the issue here is relative rather than absolute pressures. Since it is a mechanical gauge sender issues shouldn't be relevant. New oil (15w-40w mineral) and filter etc and correctly filled so this shouldn't be cause of the problem.

When warm and at idle (850rpm) the pressure sits at 40psi. However, when I increase revs to 1500 rpm the oil pressure will either start to rise for a second or two and then drop suddenly to 30psi or so or drop straight to 30 psi. If I rev it further the pressure does not drop or rise but stays pretty static. Normally I would expect pressure to rise with revs and then fall back as revs decrease.

I thought it may be on the suction side of the pump-eg the oil pick up reinstalled too close to the sump bottom or having an air leak which would explain why when revs increase the pressure falls because the pump cannot get enough oil to pump round. However, if that was the cause I would have thought as I increase revs beyond 1500 rpm the pressure would fall further-which isn't what happens.

I am therefore tending to the view that is may be the oil pressure relief switch that on increasing revs (an increasing the oil flow) is opening and allowing pressure to drop and then closes when I go back to idle and oil flow decreases. However, these are a lot of dosh new and would welcome thoughts and views.
Thanks.
 

earlybird

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There is an oil pressure relief valve, a simple spring loaded ball, in the oil filter mounting near the timing cover. According to the manual, it operates at ~40 to 60 psi. Your 30 psi suggests it's setting might be a bit low once the ball has initially lifted, but it doesn't sound catastrophic IMHO.
Your gauge might not be that accurate either.
 

vyv_cox

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It's a new one to me but I thought before I read your final two paragraphs that a suction side air leak was the most likely explanation. I don't understand why pressure would decrease with increasing revs.

The fact that pressure is good at tickover suggests to me that everything downstream of the pump, and the pump itself, are in good condition. The pressure relief valve operates on pressure, not flow, it's usually just a spring-loaded ball, so I can't see how it could reduce the pressure under these circumstances.

Edit: If there is a blockage in the filter, or it's faulty or the wrong one, on some designs it is possible for the filter to be bypassed, which will cause a reduction in oil pressure. I don't know whether yours is such a design but it might be worth looking at.
 
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Its pressure that opens the pressure relief valve as the name suggests, and I very much doubt that its set to 30 psi. Even if it is, it will never allow 40 psi.

More likely you have a problem with the pick up. Might be worth having a chat with Yanmar technical - they were very helpful to me.

P.S. Having read the above posts which came in whilst I was typing, I would definitely have a word with Yanmar since the engine after a re-build should be getting to the pressure relief valve limit and it isnt
 
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rotrax

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Hi, I would suspect the mechanical gauge first. A lot of these are poor quality-as I know from bitter experience from the time my wife and I ran a classic motorcycle shop. I have a lab. quality one kicking around somewhere which can be relied on but it cost a mint about fifteen years ago. A pin hole in the diaphragm could cause your fault as the oil would go through and equalise on the other side causing whatever the diaphagm operated to mess about. The more expensive types I am familiar with have have a tiny rack and pinion. The relief valve in the filter housing is to allow the filter to by-pass oil in the event it becomes totaly blocked. The main pressure relief valve will be just after the oil pump.These can get sticky-early Minis and other BMC cars had problems in this area. All it takes is a tiny piece of grit or swarf and it can jam partialy open. If the engine was rebuilt professionaly talk to them-they may have a trusted gauge. Good Luck.
 
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steve28

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have you bled the tube upto the gauge, had problems with air in them years ago.
A good test for the gm engines is to time the the alarm from when the engine is stoped to when the alarm comes on when cold, it should be over 30 seconds with 15w40 oil.


I debated changing the pump and pressure valve on my rebuild.


steve
 

earlybird

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Further info. from the Yanmar manual:-
At 850 rpm a warm GM30 should show an oil pressure of ~8 psi. Your reading of 40 psi. is way out of spec. Assuming that the oil really is warm, a blocked oil-way might explain this.
At operating revs. the pressure should be controlled by the relief valve, which seems to be the case, albeit somewhat low at 30 psi.
I don't think that differing opening and closing pressures for the ball relief valve are necessarily unusual. This can be due to the valve passage geometry.
 

robbieg

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Yanmar Oil Pressure Specs

Thanks for the info. I have the "proper" Yanmar workshop manual and it talks about 'standard' pressure (measured at the oil sender outlet) being 42 to 56 psi whatever the discharge volume i.e however fast the engine is reving.

It also says 'secondary' oil pressure (whatever this means!) should be measured and is 7psi at idle and 42 to 56psi at 3600 revs). Not clear where you measure this-I have measured mine at the oil sender outlet and the rear main bearing the results being the same. If anyone knows what the terms standard and secondary refer to please shout-google hasn't come up with anything.

Add to this the Seloc manual says pressure should be 30/40 psi and I have seen some reference in a Yanmar opps manual can't find it now to checking pressure is circa 30/40psi.

All a bit confusing-basic principal is that the more volume of oil the pump sends round more pressure you would expect. The gauge may be inaccurate but the relative pressures should be right and it shouldn't go down as revs increase.

Plan of attack is try with another test gauge followed by swapping out the oil pressure valve. (appreciate this shouldn't be affected by volume but observing the gauge it is as if the pressure is rising when revs go on and then suddenly drops back as if the valve has opened).

For what it is worth the filter is a Yanmar orginal and it and the oil was swapped at an early stage of troubleshooting this so unlikely to be a faulty filter.

BTW when the engine is cold if I start it and stop it once pressure has risen it takes a while for the pressure switch to come on. All very strange!

Anyone else have a pressure gauge on a GM and want to share there experience?
 

mjf107

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A good test for the gm engines is to time the the alarm from when the engine is stoped to when the alarm comes on when cold, it should be over 30 seconds with 15w40 oil.

Hi Steve

I like the sound of your test, presumably it gives an indication of bearing wear, I.e. the 30 seconds of holding the pressure up indicates correct tolerance between bearings journals. Would you please confirm you test applies to a cold engine ie stopping soon after the engine has started so the oil has not had time to heat and thin out

Regards Michael
 

steve28

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you can just crank it over decompressed unit the alarm goes off then time it from there. you can use this as a gauge for the oils usage so to speak, as the season goes on you will find this time decreases slightly.

steve
 
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steve28

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robbieg,

the way i read the manual, they are saying you should be getting between 42.67 and 56.89 psi, and they quote 56.89 as a full open max pressure all at @3600rpm

It may be worth your while unscrewing the filter and removing the regulator, try removing the shim and see what your cranking pressure goes upto without starting. if its better then you may want to experiment with shim thicknesses.

How much oil are you seeing from the rockers, check it isnt changing with the oil pressure results seen on the gauge, indicating a possible problem there.

I have known faulty valves in the oil filters, its shocking to know they bypass the oil when they block by as little as 14.22 psi meaning all the UNFILTERED oil goes straight to the main bearings etc.


steve
 

mjf107

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Filter bypass

you can just crank it over decompressed unit the alarm goes off then time it from there. you can use this as a gauge for the oils usage so to speak, as the season goes on you will find this time decreases slightly.

steve

Thanks for reply Steve 'simples'

Its worrying about the bypass on the filters, do they all have them nowadays?

Michael
 

Lakesailor

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Put it the other way around. If the by-pass on the filter was set to a high pressure and the filter was very blocked there would be a limited flow of oil to the bearings. That would create a seizure risk on a long passage. If the by-pass operates and lets oil by-pass the filter then a full flow will reach the bearings. This may or may not damage them (how filthy is your oil? When did you last change the filter?) but with a full flow the damage which may occur will be scoring and wiping. This will at least allow you to complete your passage and replace the components in safety and at leisure.
It's flow that protects the bearings, not pressure. Very high pressure can indicate a blockage in the system.
I was once driving a Mini at 90mph in Dalby Forest on a rally and noticed the oil pressure at the top end of the gauge, about 90 psi. At the service we took the pressure relief valve apart and found the plunger stuck at the bottom due to a bit of swarf. I rubbed the plunger body smooth on a sandstone wall and put it back in. No probs. After that I always replaced the plunger with a ball bearing (from a CV joint).
Minis will run fairly happily at just 20 psi even though it's supposed to be 50 psi.

Diesels are a bit more fussy though.

I built a 1400cc Mini and even though the motor was assembled by a top Mini tuner I still got the erratic oil pressure like the OP. 30 psi at tickover. Climbed as the revs built and then dropped and started going backwards (easy to see on a rolling road) He suspected the pressure relief valve and packed it a bit more. His opinion was that air in the sensor pipe wasn't a big issue. It would just slow the ballistic nature of the reading. In this instance we fixed it properly by getting a new spring for the valve with a variable rate.
 
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