Dropping Engine Oil Pressure Yanmar 4JH2E

[EKW]

We have "baby'd" this engine over the years, running it 2200-2600 RPM when motoring (mostly 2200 RPM), but recently I decided to do a higher power run at 3000 RPM for about 20 minutes. After going back down to 2200 RPM, the oil pressure steadily dropped to well below the "green band" on the pressure meter and continued to drop. We did not get a low pressure alarm, but I cooled the engine down, shut down, and checked oil level, which was middle of band. Upon startup, the engine oil pressure came up fine and remained in the green band. (Our Yanmar meter does not show pressure in numbers, which is frustrating.)

The engine has less than 2000 hours and was installed in 1994.

I am wondering if what happened was the oil cooler did not keep up with cooling down the oil, or if some other phenomena occurred.

Open to input / suggestions.

[boatbod]

The only way to know exactly what's going on is to connect a mechanical pressure gauge in place of the sender. If pressure really is low then you likely have worn main bearings. More probably, the gauge is giving you bad readings due to sender our wiring issues.

[RaymondR]

My engine does the same thing (Kubuto V2002) and I think it's related to the oil I use. If I change the oil it stops doing it. I think it might be related to the use of the newer synthetic 15-40 oils which are sometimes all that is available in the places I wander to.

[jamhass]

I would suspect that the oil heats up at higher revs, maintains pressure because the pump is working faster. Then when you reduce revs, the oil is still hot but the oil pump is working slower, thus lower pressure. When you cool and restart, the oil is cool again so the pressure is higher.

You could consider going to a higher viscosity range if you plan to run at higher RPM in the future.

[EKW]

Appreciate the feedback. Will take all under advisement.

[a64pilot]

Originally Posted by jamhass

I would suspect that the oil heats up at higher revs, maintains pressure because the pump is working faster. Then when you reduce revs, the oil is still hot but the oil pump is working slower, thus lower pressure. When you cool and restart, the oil is cool again so the pressure is higher.

You could consider going to a higher viscosity range if you plan to run at higher RPM in the future.

This, the oil got hot and lost viscosity, this can be for many reasons, like fuel dilution for instance or your engine running hotter than ideal, or as has been noted too low an oil viscosity to begin with.
It could be an indicator of worn bearing as well, but I don't think so, that is usually low oil pressure at normal temps

[a64pilot]

Also if for some reason your oil is breaking down and is past the time when it should have been changed it may act in this manner

[Caliber40]

Also for what it's worth "babying" at 2200 RPM's not really helping you. Yanmar's like to be run harder than that especially if you are running over long periods of time.

Every few hours run it up to 28-3000 to help clean it out!

SV Ti Amo

[01kiwijohn]

As has already been stated; before anything else, get a good gauge and check the pressure.
You should be using a 15W-40 oil. At those hours I doubt you have worn bearings to the point where that would cause such a pressure drop.

[DeepFrz]

If you suspect the oil is overheating to the point of losing viscosity then I would recommend you get an oil temp. gauge as well as an oil temp. gauge.

[cherikita]

I run my Yanmar at 2400 rpm all the time is it true that it is good to run it higher sometimes and how much

[GordMay]

Greetings and welcome aboard the CF, EKW.

Prolonged no-load operation of the engine will cause it to blue/gray smoke at low rpm with an associated increase in oil consumption. For every 3-5 hours low load operation (trolling) or no load operation (running the fridge or charging batteries) should be countered by 20 minutes near maximum continuous rpm.
Do not operate for extended periods below 80% maximum rpm. The cylinder bores will glaze, causing an increase in oil consumption and blue smoke. Don't 'baby' your diesel engine!
CRUISING RPM (Yanmar): GM, JH, JH2 series about 2900 rpm.

[SailorHarry]

I agree with the several posts about installing a mechanical pressure gauge to verify your electronic gauge. I installed a new 4JH2E engine in my Corbin 39 in 1997. While in Australia in 2007 I had a similar problem with "loss of oil pressure". It turned out to be a faulty oil pressure sensor. I replaced it with a very expensive Yanmar sensor, and again it failed two years later in Singapore.
I cut the two of them open and saw that the wiper had pounded a break in the resistance wire. To make a long story a bit shorter, I finally decided it was time to replace the motor mounts (I had noticed that screws were backing out in the cabin trim due to the increased vibration). Replaced motor mounts with a well known replacement brand, and had only worse than tolerable vibration. The dealer allowed me to return them for full credit and I then purchased genuine Yanmar motor mounts. Vibration problem solved, oil pressure sensor problem solved.

Lesson learned: if your motor mounts are eight years old or older change them before they cause other things to fail.

[boatbod]

Very true. With Yanmar mounts, a sure sign of degradation is when the top metal piece sags down and touches the bottom piece. Also, separation of the rubber from the metal is common, but you usually have to lift the engine to find that out.

[MarkSF]

Originally Posted by RaymondR

My engine does the same thing (Kubuto V2002) and I think it's related to the oil I use. If I change the oil it stops doing it. I think it might be related to the use of the newer synthetic 15-40 oils which are sometimes all that is available in the places I wander to.

I would expect synthetic oil to be better then standard in this respect, for 2 reasons :

1) It stays in viscosity grade for longer, so should have better properties when you are near to oil change time.

2) The change in viscosity with regard to temperature is less.

However, what could be the issue is variance in viscosity as designed. Oil viscosity numbers are rounded off. So two 15w/40s could actually be a 13/35 and a 17/44, for example. Most manufacturers quote actually viscosity in Cst at 20C and 100C, in their spec sheets, which are a better guide to real behaviour.

Maybe you used to get a "thick" 15w/40, and now you're getting a "thin" one?