Is high engine oil pressure a bad thing?

[zaragozo]

Wow. Thanks guys.
I supplied the oil/filters/alt belt/impeller for the engineers to use. I bought the stuff from that Yanmar dealer in Lymington after giving him a complete rundown on the engine
I'm an hour away from my boat so can't get down till tuesday but I am thinking maybe I'm imagining the wrong readings. The boat manual says oil pressure should be between 1-1.5 kp/cm2. But in an added page in the manual there's some stuff about fault finding which suggests the pressure should be about 3.5 psi. I sort of assumed the gauge must be in kp thingies. Maybe it's in PSI? If so 4-5 psi ain't such a big deal? But on the other hand 10 kp is apparently 142 psi so 1.5 kp must 21 psi or so! Which is weird. I understand kp things are something to do with vacuum pressure rather than regular pressure....
Does 4 PSI seem like a reasonable pressure for a 75hp Yanmar? Or maybe I should take up golf..

 

[seabright]

If it has changed......

I can relate to waht you are going through......maybe especially this last bit.

If the pressure has changed substantially then something has happened. In the past, I have tried to convince myself problems aren't problems by refering to book / manuals / talking to the old geeza in the pub etc. Not the best ploy. If engines aren't your thing, I would try and find that 'good mechanic' local to you. There will probably be one but they can be hard to find as they tend not to have to advertise that much.

The good thing is the engine is still ok... but you would kick yourself if it did goose if you didn't sort this probelm.

Good Luck.

 

[earlybird]

[QUOTE
Does 4 PSI seem like a reasonable pressure for a 75hp Yanmar? Or maybe I should take up golf..[/QUOTE]

I wouldn't think so. It is very low. FWIIW, the Yanmar GM series run at about 40-50 psi. Since your OP said 1.5 kp/ cm2, I took this as OK, but it does seem rather low. As your present pressure is roughly the same as the GM series engines, perhaps there is no problem. Incidentally, ponds are rather old fashioned units.

 

[Evenspread]

!

Stu ...thanks for a useful response

O.P.
maybe I'm imagining the wrong readings.

maybe I should take up golf..

The sooner the better!

 

[Bilgediver]

No-one has remarked on the OP's comment which would suggest the motor is either very robust, or it is just the sender that's at fault.

No need to comment....I would not expect it to go down that quickly....However I would suck out the excess to reduce the chance of crank webs splashing and foaming the oil..

I saw a post from the OP before signing in questioning his own ability to read a pressure gauge.....Now it seems to have disappeared...Sounds like he doesn t know what the Oil pressure should be and asked if 4 PSI is OK

I would expect more like 40psi on one of these engines and certainly not less than 20....MAybe the OP can clarify what his manual says and what does the gauge actually indicate and in what units... I would certainly not consider 14PSI sufficient...

If the pressure is high then it is remotely possible the splashing around of excess oil loosened some hung up crud in the crankcase which then got into the relief valve and caused it to stick....If it is before the filter... But highly unlikely!

As others have said the main problem with overfilling is that the crank webs splash more oil into the lower cylinders below the pistons than the scrapers can cope with and so some of this excess finds its way into the combustion chamber and gets burnt....Makes smoke and plays hell with emissions..

 

[Avocet]

Just a couple of thoughts on this:

First of all, kp is a lousy unit, as the previous poster said. It also seems odd that it's per cm^2. I've never seen an oil pressure gauge that didn't read either PSI, Bar, kiloPascals (kPa) or kg/cm^2. Well worth getting a trusted mechanical gauge for a cross-check.

Secondly, too high an oil level can damage your engine. There are two sorts of oil pressure. There is the pressure made by the pump in the oil galleries. That's fine and too much of it is unlikely to be a problem, but in extreme cases could damage the oil pump or the drive to it because the pump is working so hard. Too high an oil level (as has been said) is unlikely to cause too much of THIS sort of oil pressure. The other sort is crank case pressure. This is just what happens when the oil and the air in the crank case warm up and the oil gets splashed around. Added to that you have whatever gets past the piston rings going in there too. A high oil level can increase crankcase pressure CONSIDERABLY and that's not a good thing. Eventually it can blow the oil seals and you'll start leaking oil.

Thirdly, (and I know NOTHING about Yanmars!) some oil filters have a pressure relief valve built into them. Thats so that if the filter gets clogged with muck, it opens this valve to let the oil bypass it. Clearly, it's then not doing it's job as a filter, but at least the unfiltered oil is getting to the bearings - which is better than none! Is there a chance that your old oil filter could have been choked and was running with it's bypass valve stuck open? If so, the pressure could have been LOWER than normal, and the new filter has simply restored it to what it should be.

 

[VicS]

THESE UNITS

kp stands for kiloponds and 1 kilopond = 1 kgf

A pressure of 1 kp/cm² is a pressure of 1 kgf/cm²

it is equal to 0.1 MPa (mega Pascals)

and to 14.22 psi.

A pressure of 4.5 kp/cm² is therefore 64 psi​

NOW it seems to me that a pressure of 64 psi is not excessively high.
The pressure before the oil change of 1 - 1.5 kp/cm² (14 - 21 psi) sounds distinctly on the low side.

Maybe there is no problem with oil pressure at all. Maybe before the oil and filter change it was too low!

However it will be interesting to know what the specified pressure is for the engine.

 

[tcm]

ooh, i've got yanmar engines and i did actually once overfill the oil and very shortly afterwards there was a global finacial meltdown! Also the motor a bit smoky. So everyone advising against is quite correct. I drained a bit of the oil, and the smokiness has gone, and the financial meltdown is not quite so bad, altho zillions of pounds lost and loads of companies gone bust etc, and all down to slightly overfilling the oil. Also i think yanmar slightly overquote how much oil you can get in there.

 

[zaragozo]

THESE UNITS

kp stands for kiloponds and 1 kilopond = 1 kgf

A pressure of 1 kp/cm² is a pressure of 1 kgf/cm²

it is equal to 0.1 MPa (mega Pascals)

and to 14.22 psi.

A pressure of 4.5 kp/cm² is therefore 64 psi​

NOW it seems to me that a pressure of 64 psi is not excessively high.
The pressure before the oil change of 1 - 1.5 kp/cm² (14 - 21 psi) sounds distinctly on the low side.

Maybe there is no problem with oil pressure at all. Maybe before the oil and filter change it was too low!

However it will be interesting to know what the specified pressure is for the engine.

The Yanmar manual is on the boat but you'd be surprised how difficult it is to find an internet opinion on this. Here's my new theory - apparently 36-42 psi is the normal reading for a Yanmar 3JH (I'm assuming that this ain't massively different from a 4JH). The added sheet in my boat manual says 3.5 psi is the normal. Maybe the author made an error and meant to write 3.5 bar (51 psi) - which isn't that far adrift from 64 psi, which you suggest doesn't seem too high. But it still leaves the issue of 1-1.5 kp thingy. Re-reading the boat manual it actually says:
"Immediately after starting the engine check that the idling oil pressure is at least 1-1.5 kp (14-21 psi) and that the red warning light then goes out"
So maybe this is the lowest pressure the engine will put up with before the warning light comes on? And regarding the gauge, as 1 bar is more or less 1 kp then it's probably a bar gauge (altho some US Yanmars are in kp's), and that I misread it before the service. We all make mistakes, but I will siphon off the excess.

 

[Avocet]

OK, All that makes sense, but still doesn't explain why you've got so much more pressure now than before the change.

I don't think the amount of pressure (above the minimum needed to maintain the film thickness in the bearings) is too important, but if it's only 1 bar immediately after startup, I'd have thought you'd be in trouble when it gets hot! Also, the Americans still use old Imperial units so I'd be surprised if an American gauge read in kp/cm^2 or Bar.

 

[VicS]

apparently 36-42 psi is the normal reading for a Yanmar 3JH (I'm assuming that this ain't massively different from a 4JH)

The manual for the 4JH3-TE series (which is one of the manuals I've located on line, gives the oil pump delivery pressure as 4.0 kgf/cm² (0.39 MPa), that's 57 psi. It gives the setting of the relief valve (which is built into the pump) as whopping 12 kgf/cm²! but it also describes a pressure regulating valve built into the oil filter bracket which is set (and can be adjusted with shims) to 4.0 +/- 0.5 kgf/cm². 1 mm of shim thickness alters the pressure by 0.5 kgf/cm² "or so".
On that basis your pressure of 4.5 kp/cm² is just on the top end of the spec.

(It also tells me that the relief valve built into the oil filter opens at a differential pressure of 1 kgf/cm².)

The manual I'm looking at is at http://www.motoren.ath.cx/download.php?filename=yanmar/4JH3.pdf

(There's a manual for the 3JH on the same site).


Remove the excess oil , quit worrying, go sailing

 

[gordmac]

Your oil pump pumps an ammount of oil which varies with speed, it doesn't create the oil pressure. The pressure comes from the resistances the oil has to pass through (think blowing through a big and a wee straw). The higher the volume of oil pumped the more pressure will be created (blow the straw harder) and at some point the relief valve will open to limit the pressure buildup. This is why your pressure is low at tickover and with increasing revs very soon reaches a maximum. Thin oil flows more easily so as the oil thins when hot the pressure goes down at tickover. Higher viscosity oil could be a reason for increasing pressure otherwise there has been a change in ressistance somewhere. The oil and the filter were changed so if the oil grade was correct maybe the filter is wrong or faulty. If the filter has a too high ressistance the bypass valve will open and bypass the filter. This will be followed quite soon by worn bearings etc. In my experience of this happening (different engine, faulty valve rather than filter) the oil pressure goes up and down with speed quite noticably. If the pressure relief valve has jammed shut the pressure is likely to go quite a lot higher and will likely go up with speed. I would expect debris in the oil to be more likely to jam the valve open.
If the pressure is within spec then the engine should be ok but it would be handy to know why the pressure was low before (if indeed it was?). Is the bypass valve in the filter?
Was the oil very old and lost viscosity, perhaps from diesel dilution? Is the engine running colder now?
The overfilling issue has been covered except that some engines will breathe in engine oil from a grossly overfilled sump, Diesels can run on the oil so it's speed will "run away".

 

[jason -and the arguenauts]

the oil pressure in all the engines that I have worked on for nigh on 50 years does not pressurise oils seals or piston rings!
...................... The overall pressure is governed solely by the oil pressure relief valve. The capacity of the pump is such that when the bearings get worn then the loose fit governs the oil pressure. That is basically how the vast majority of engines work.
........................ BUT is it the right oil? if he has put sae 90 oil in by mistake? that would give excessive pressure by not allowing the pressure relief valve to dump the exessive pressure quick enough.

Interesting post Stu - I'd never really thought it through before. But if it is as you descibe ( and I cant think of a seal that would be under pressure unless you count gaskets), then why do manufacturers bother with the extra cost of a pressure relief valve rather than just make the oil filter out of thicker metal? That has to be way cheaper.

You might be under a misaprehension about SAE 90 gear oil - its true viscosity in centistokes is little different to that of SAE 15/ 40 engine oil. They are on two different nominal SAE scales.

 

[Chris_Robb]

OK, All that makes sense, but still doesn't explain why you've got so much more pressure now than before the change.

.

May be the old oil had been there so long, and was consequently so thin, that the oil pressure was low.

 

[Heckler]

Interesting post Stu - I'd never really thought it through before. But if it is as you descibe ( and I cant think of a seal that would be under pressure unless you count gaskets), then why do manufacturers bother with the extra cost of a pressure relief valve rather than just make the oil filter out of thicker metal? That has to be way cheaper.

You might be under a misaprehension about SAE 90 gear oil - its true viscosity in centistokes is little different to that of SAE 15/ 40 engine oil. They are on two different nominal SAE scales.

Ref the oil pressure relief valve, dunno, never thought about it, they just do. Suppose it reduces load on the drive to the pump. I have seen a hex drive rod to a pump with the corners worn off and it only just working. Ref the gear oil, when its cold its a lot thicker than engine oil, think squeezing a liter bottle of hypoid oil in to a back axle hole and then think of doing it with 10/40!
Stu
Thinking on, tight new engine or unworn one, positive displacement pump, rev to 6000, the drive train to the pump would be under enormous stress.

 

[Avocet]

...Thinking on, tight new engine or unworn one, positive displacement pump, rev to 6000, the drive train to the pump would be under enormous stress.

I think the peak stresses on the pump and its drive system would be dictated by the opening of the pressure relief valve (regardless whether the engine was new or worn) until such time as the wear was so great that the pressure relief valve never opened!

 

[Avocet]

May be the old oil had been there so long, and was consequently so thin, that the oil pressure was low.

In my experience, old oil isn't appreciably thinner when cold - it thins out faster (and more) once it's hot though. I think the OP had this problem virtually from startup.

 

[cliff]

why do manufacturers bother with the extra cost of a pressure relief valve rather than just make the oil filter out of thicker metal? That has to be way cheaper.

OK I will try to make this simple, as already explained in other posts oil is pumped via a positive displacement pump (gear type usually) insofar as for eact revolution of the pump a relatively fixed volume of oil is pumped (we can ignore leakage, suction and bypass losses).
This oil is fed through a fliter, oil galleries etc until it comes to the bearings, either crankshaft, cam shaft, rocker shaft, gudgeon pin or sprayhole etc at which time it finds its way back into the sump.
As the engine speeds up more oil is pumped and the pressure goes up and hence more oil is fed thought to the varouus bearings. There is a balance point of flow rate (linked to RPM) and pressure in the oil galleries. Now the interesting point - too high a flow rate will erode the bearings rapidly and can cause nasty things to happen to the oil film keeping the shafts away from the bearing shells so to prevent the over pressurisation (and high flow rates) manufacturers fit a PRV somewhere in the oil galleries to limit the maximium pressure. Oh, yes, nearly forgot about the stresses on teh pump and drive mechanism when the pump becomes overloaded and basically suffers a hydraulic lock due to excessive pressure. So there you have it - run away high pressure (no PRV) will feck up the bearings and probably the pump - cheaper to fit a PRV.

Tribology is a facinating subject and frequently difficult for the lay person to understand, fortunately at least a couple of posters (Stu and GordMac) seem to have got their heads round at least the basics and have hit the nail on the head with their explanations.

 

[Heckler]

I think the peak stresses on the pump and its drive system would be dictated by the opening of the pressure relief valve (regardless whether the engine was new or worn) until such time as the wear was so great that the pressure relief valve never opened!

Avo
you missed the point, I was replying to jason when he asked why have a pressure release valve!

 

[zaragozo]

All is sorted. Checked out the boat today. Pressure gauge is in bar/psi and after siphoning off excess oil and starting engine it reads 3.5 bar at tickover. At 1000+ rpm and more it goes up a little to 3.7.
Maybe it was never 1.5 before. Perhaps I imagined it after reading the manual....
Anyway, thanks y'all. I've learnt quite a bit more about enginey things.