Oil Pressure Decreases as RPMs Increase

231S · Jan 28, 2017

Hello,

I posted this on another forum as well, so some of you may have already seen it.

I just got around to installing a mechanical oil pressure gauge on my little YM2000, after putting a temp gauge on it last summer. Install went smoothly. I used Bosch brand gauges and a "street T" at the block so that I could keep the warning light along with the mechanical oil pressure gauge. No leaks, I like the way it turned out.

I changed the oil and put a new filter on it while I was at it. Filter is Wix 51064; oil is Rotella 15-40. Cranked it and let it warm up, it idles at 40psi at 800-900rpm. The thing is that when I run it at 1800 to 2000 rpm then the oil pressure drops to 30-35psi. This is while mowing some patches of winter grass around here. As soon as I bring it back down to idle, oil pressure pops back up to 40 psi. This was after it was thoroughly warmed up, and it did that very consistently over about 2 hours of run time.

Coolant temp was 160-170 deg. I put some cardboard in front of the radiator so the tractor would get up to a good operating temp. Ambient temp here today was in the 40s.

Anyone ever seen this before?

Any thoughts or insight would be much appreciated.

RNeumann · Jan 28, 2017

Never heard of it but I was so curious I googled it! Seems the first step is to play with oil level. Try running at or just above the "add" or "low" mark. If you are still having issues try running it a bit above the full mark. The research I saw found less oil solved the problem.
The other option is a sump pickup that is either too close to the pan or blocked. As the RPMs increase the oil pump is starved of oil.

Good luck

CADplans · Jan 28, 2017

Sounds like a defective oil pump.
As the pump speeds up, the pump cavitates. Cavitation - Wikipedia
The exact same thing can happen to a ship/boat propeller.
New propellers work much better than old, worn ones,,,

Neat 1500 · Jan 28, 2017

Mine does the exact opposite, low when cold and higher when warm. Doesn't change much with revs either.

normde2001 · Jan 28, 2017

Why worry about it? The oil pressure light turns on/off at about 4psi. My Ym2000B has been running that way for 16 years.

RalphVa · Jan 28, 2017

I'd inquire to someone at Yanmar or a good service person. This is the kind of thing that is the reason why we no longer get oil pressure gauges nor amp meters because they often gave us weird information. However, I had an amp meter on a 1965 Valiant that told me something was going on. Found out what it was a fixed it easily.

For this, it may have something to do with the way the bearings, etc. are loaded at speed vs. idle. Could be completely normal. One thing that could cause a problem is if the oil level is so high that the crankshaft is hitting it. This would cause foam, and the oil pump would be pumping a combo of air and oil. Once, at our service station, my dad forgot to drain the old oil out of the school superintendent's Olds. He came and picked up the car but quickly returned because the hydraulic lifters were all banging away. The crankshaft was, of course, churning away at the oil and turning it into foam.

231S · Jan 28, 2017

Thanks for the responses.

I really don't think it is hurting anything since the oil pressure is fine even when it is going down with the increased RPMs.

It is curious behavior though for the pressure, you would expect it to come up or stay steady as RPMs increase.

I have done some looking and found two posts where two cylinder Yanmars with the exact same oil pressure gauge set up that I have (street T to keep the light and gauge) were acting the same way. One poster with a YM1700 replaced the oil relief valve and pulled the pan looking for blockages on the pickup and nothing changed. The other was a YM2000 like mine.

Makes you wonder if it is something peculiar to the T fitting set up that may cause it? I doubt it but stranger things have happened. Or maybe it is normal for the 2 cylinder engines like RalphVA said?

As far as when the oil pressure warning light comes on, the Hoye website says "around 10 pounds" in the
Knowledge Base. However, one of the guys at Hoye told me 14 psi yesterday. Just FYI, I don't know that it matters all that much.

RNeumann · Jan 28, 2017

With two posts about high oil levels it would be interesting to hear back if we're so inclined to test that.

231S · Jan 28, 2017

I have taken note of the oil level comments, sorry I didn't mention that.

With 5 quarts, my YM2000 is not quite full per the dip stick. At 4 quarts it is barely on the stick.

Now, to add a twist, if you look at the Nebraska tractor test for the YM240 (the American market twin of the YM2000), Yanmar engineers ran 4 quarts of oil during that test.

But the manuals state 5 quarts per Hoye and 5.3 quarts per the I&T Manual.

It would be an easy test to drain a quart and see what it does, you never know. I dont think it would hurt it. But I don't have any reason to think it is over full either, except for the weird behavior of the oil pressure.

Thanks for the comments, I will keep you posted.