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Old 23-09-2018, 02:33   #1
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Yanmar 2QM20 oil pressure regulator

ok I have on of these beasts and it's a beaut, except for.....(always has to be some effing thing right)... yanmar, In their wisdom has decided a select few bits for the 2QM as well as the 3QM engines are now obsolete including the oil pressure regulator, the 2GM20 oil pressure regulator looks a very similar beast design wise however it does run at about 10psi higher oil pressure than the original, if on a lucky chance I could get the 2GM one to fit/ jammed in ok would the 10 psi oil pressure really make that much of a difference with regards to longevity of engine bearings etc ?
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Old 23-09-2018, 03:49   #2
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Re: Yanmar 2QM20 oil pressure regulator

I'm hesitant to comment as I'm not absolutely sure but I doubt the 10lb higher makes much of a difference. Not sure of the design of the pressure relief valve of your engine but if its a spring & ball type you could just shorten the spring a little to lower pressure. If it was 10lb lower I'd be more worried. Do you know what pressure yours runs at?
Our YSM8 says it can be 35-50lb so thats a 15lb range as standard
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Old 23-09-2018, 04:29   #3
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Re: Yanmar 2QM20 oil pressure regulator

OK, the QM oil relief v/v operates between 35 and 50 psi while the GM is 43 to 57 psi. The GM manual suggests the GM oil relief v/v should be replaced as a sealed unit.

The QM manuals gives advice on changing the shims to achieve the desired values. I note the v/v is slightly different between the 2 & 3 QM engines but the principles are the same.

As the QM v/v is a simple device, what is currently wrong with it that you can't fix without replacing it?

Personally, given the figures, I would be happy to use the GM one if it fits.
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Old 26-09-2018, 18:19   #4
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Re: Yanmar 2QM20 oil pressure regulator

I'm still having trouble trying to work out how the OP's pressure relief valve could fail to the degree it needed replacement. Clearly it isn't failing by not opening i.e. the oil pressure is too high so it must be failing by allowing the oil pressure to drop too low.

This might be because of some crud getting caught under the valve which would be rectified by cleaning.
It might fail (too low) due to weak spring - fixed by shimming; or broken spring, replace with aftermarket spring.
It might fail to erosion of the ball or ball seat - again fixed by replacing ball or machining the ball seat.


Or it was lost during a rebuild - then it does need complete replacement!

Or ?????
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Old 28-09-2018, 15:13   #5
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Re: Yanmar 2QM20 oil pressure regulator

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I'm still having trouble trying to work out how the OP's pressure relief valve could fail to the degree it needed replacement. Clearly it isn't failing by not opening i.e. the oil pressure is too high so it must be failing by allowing the oil pressure to drop too low.

This might be because of some crud getting caught under the valve which would be rectified by cleaning.
It might fail (too low) due to weak spring - fixed by shimming; or broken spring, replace with aftermarket spring.
It might fail to erosion of the ball or ball seat - again fixed by replacing ball or machining the ball seat.


Or it was lost during a rebuild - then it does need complete replacement!

Or ?????

sorry for taking a while to reply here.....life happens.., the oil pressure valve has been out and cleaned and it is the same style as a 3QM one with the screw in retainer, what happens is that initial startup all is good at around 50ish PSI oil pressure and as it warms this drops to 40 to 45 PSI (engine is out of boat and running from a 200 litre drum which gives 30 to 40 mins run time before the water temp gets to lukewarm. When the engine is up to operating temp (freshwater cooled with a heat exchanger and new thermostat in place) the pressure is 40 ish at idle but with a few revs added (1800 or close to) the oil pressure actually falls away to around the 36PSI mark and then rises again when the revs are dropped, I do have a mechanical oil pressure gauge fitted directly to the engine in place of the oil pressure switch so no sender issues, do wonder where aftermarket yanmar parts can be obtained as have yet to see any other than oil filters etc............
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Old 28-09-2018, 15:35   #6
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Re: Yanmar 2QM20 oil pressure regulator

Although it's a little strange that the oil pressure falls when you increase the revs the generally accepted minimum is 10psi per thousand revs & you are twice that at 1800rpm. Not something I'd worry about as there are countless examples of engines that go for decades with lower oil pressure than what you have. The oil pressure always drops as the temp rises to operating pressure as the oil viscosity decreases.
By all means fit a new pressure relief valve if you wish but it may not affect your oil pressure. Please let us know what happens if you do fit a new v/v.
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Old 28-09-2018, 17:55   #7
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Re: Yanmar 2QM20 oil pressure regulator

Yes it is unusual for oil pressure to decrease when moving from idle to higher RPM (with temps constant) but I have seen a couple of threads previously about the same issue. Sorry, can't find them ATM.

However it is extremely unlikely to be a problem with the oil pressure relief valve, they just don't work that way.

You might have a restriction in the supply side of the oil pump, this could cause such an issue or perhaps (but very very unlikely) a weird cavitation issue in the oil pump.

I agree with Compass790, your reported pressures are not concerning... but I would love to know why it does drop a bit with increasing RPM.
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Old 28-09-2018, 17:55   #8
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Re: Yanmar 2QM20 oil pressure regulator

Itís dropping most likely as the valve is cracking open at that point, then of course dropping from 40 to 36 really isnít much of a drop.
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Old 28-09-2018, 18:07   #9
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Re: Yanmar 2QM20 oil pressure regulator

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Itís dropping most likely as the valve is cracking open at that point, then of course dropping from 40 to 36 really isnít much of a drop.
But if it cracks open at 50(ish) psi when cold (as reported by the OP), why would it crack open at 40 psi when hot at idle and then 36 psi while still hot and RPM is increased?????
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Old 28-09-2018, 21:10   #10
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Yanmar 2QM20 oil pressure regulator

A theory is it cracks at 40 but cold oil it doesnít drop pressure much and itís open fully at 50.
Then it cracks open and once opening you may get some pressure drop.
I donít know, you explain how an increase in RPM results in a slight drop in pressure?

Of course if you wanted to chase this theory throw in a few shims and see if its behavior changes, if itís not the pressure relief valve, it wonít, if it is, it will.

I wouldnít concern myself with a 4 PSI drop to 36 PSI myself.

It could also be due to the cold oil bypass valve in the oil filter, if there is enough restriction there the valve opens and the filter is bypassed, this is going to effect flow rate, and flow rate affects pressure.
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Old 28-09-2018, 21:21   #11
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Re: Yanmar 2QM20 oil pressure regulator

^^ well that is a possible theory that would explain the reported symptoms. As for the decrease when increasing RPM, perhaps the rate of flow of oil at idle is so low, the relief valve is only just cracking and then reseating however once the rpm increases, the flow of oil is such that the relief valve is held properly open. Now I think my theory is far fetched but something is causing the changes the OP reports.

FWIW, IMO the oil pressure relief valve is probably not the issue.
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