Yanmar 3JH3E - Oil pressure drops as RPM's increase

[boatbod]

Oil pressure should increase (slightly) as rpm increases. When this doesn't happen it's usually a wiring problem causing voltage drops in the harness (as alternator output increases). Manual oil pressure gauge is the best way to verify. Next best is disconnecting the sender and directly measuring resistance at various rpm.

[Compass790]

Originally Posted by sailorboy1

doesn't make sense to me that the oil pressure goes UP with reducing rpms

I've gone through 2 oil pressure and temp gages the last 8 years

Oil pressure can go up slowly as you reduce rpms ONCE OIL IS HOT as at say 2000 rpm your engine is producing ( for the sake of the argument only) twice as much heat as at 1000rpm. As the oil gets hotter it gets thinner & flows more easily thru the bearing clearances. Pressure is just resistance to flow. Then when you let it run at lower rpms the engine will slowly cool down & oil viscosity will increase which will make the pressure rise
In our engine oil pressure 40 psi @ 1000 rpm cold & 15 psi @ 1000 rpm hot but if we let it run after it's hot at 1000 rpm for 10 mins unloaded it will climb back to 30psi this is running 15w-40 diesel oil. Oil temp after running 2+ hrs under full load is only 59oC & pressure is about 20psi. Engine is raw water-cooled single.
But, I happen to know one of our main bearings has 4 thou clearance instead of 2 thou because some idiot took a little too much off when machining bearing i.d. Ask me how I know. That explains partially our below spec oil pressure
It is likely the OP's problem is related to oil temp but if his coolant temp is ok it's a little strange

[Compass790]

Originally Posted by 1boat2many

Thanks all - I will start by replacing the sender unit, inexpensive and within the range of something I can do (thanks Gord for those links, straight to the part number and an Amazon click away).

The work I had done at Indiantown involved pressure testing the cooling system. The pesky little leak was coming from a drain plug at the oil cooler of all places, on this engine it is just a tubular ring that runs coolant around the base of the oil filter. I wonder if that plug is socked in too tight and threads now obstruct coolant flow. It's a PIA to get to, need to remove the oil filter to get to the plug, so will do that as the next step. I'm also going to get an IR thermometer and start collecting data. Any suggestions on make and model or other fun diagnostic stuff would be great.

If the problem coincided with your pressure testing work I'm with Wotname, check that filter cooling loop & the plug.
But it could be coincidental if it got stinking hot after the work was done

[a64pilot]

Really the only way to be sure is to plumb in a direct pressure gauge, which can be an issue because Yanmar uses I believe British pipe thread and not the more common US NPT, they are very close, but there is enough difference so that it won’t quite work.
So getting the proper fitting isn’t easy.

It may be that the OP will have to buy a new sending unit and if that doesn’t fix it, buy a new gauge and if that doesn’t fix it, then it’s an oil PSI problem.
There are way to test both of course but it usually requires a dead weight tester for the sending unit.

[Compass790]

I suspect your oil pressure gauge & sender are fine as they are behaving as I'd expect a working system to, BUT I'm an electrical ignoramus. I THINK you are going to have to find the problem elsewhere. Don't have any argument with the idea of checking them first tho as that is easy.
I have successfully screwed a 1/8" NPT male tee into our Yanmars 1/8" female BSP fitting using thread tape without leaks over 200 hrs but you can get an adapter if you want to do it by the book.
Any way keep us posted what you find. My nxt port of call if ambient temps or oil cooler aren't causing yr problems would be pickup tube, as another poster suggested, or oil pump clearances. Dont be shy to shoot me down in flames if it turns out to be sender or gauge.I'm happy to learn from the hive. :-)

[Wotname]

To be pedantic, Yanmar uses a Japanese Industrial Standard (JIS) thread however it is a mute point as JIS is interchangeable with BSP.

A male NPT will fit into a female BSP without damage but you will need pipe sealant (as Compass790 posts). Note - the reverse is not true - a BSP male will not fit into a female NPT.

So yes, a common direct pressure gauge with a male NPT will fit the OP's Yanmar engine.

[1boat2many]

Thank you all again for a good discussion! I've shifted my thinking from just fixing the problem to really understanding the problem, taking some accurate measurements (as possible with manual pressure gauge) and posting back results. This may take me a few days, in the meantime, I will be closely watching the low pressure weather system expected to develop in the NE Gulf which could impact N. Florida (northwest coast predicted, but we'll have to see) We are in St. Augustine which is not very far away, so that may drive diagnostics to next week if I have to do something to get out of the way. All that to say that I have less worries about my oil pressure issue now that I understand it better, and that's a big help right now. Will post again once I do the prescribed diagnostics.