Low oil pressure at full RPMs

[quicksandfarmer]

I have a Jinma 354 which I use mostly for mowing. I usually use a 60" rotary rough mower, and I like to set the two-speed PTO at 720 RPM and the engine at 1650 RPM, which gives me 540 RPM at the PTO. (I know you're supposed to run at full RPM's, but this way seems to work fine, and is quieter and smoother so more pleasant for the operator, me). Configured like this I can mow all day in 4L and the oil pressure needle stays right in the middle and the temperature stays steady.

I recently bought a 68" flail and I quickly realized it needs more power than the rough mower, at 1650 RPM the engine stalls if I hit patch of thick grass. So I pushed it up to 2200 RPM and shifted the PTO into 540 RPM, and dropped the gear down to 3L. This seems to work OK, but after a few minutes I noticed that the oil pressure had dropped down so that the needle was right on the border of the red. It's still in the green, but very close to red. Temperature is fine.

Switching back to the old mower the oil pressure is fine.

This worries me. I would expect the oil pressure to be higher as the engine RPMs increase. Is this a sign that I'm asking too much of the engine? Is there a risk in running at this low an oil pressure, even if the needle isn't quite in the red?

Thanks.

 

[5030]

The oil is getting hotter (because the engine is working harder) and it's getting thinner (loosing viscosity). Change to a heavier weight oil or get a larger tractor. Your choice.

 

[Redneck in training]

The thicker oil is not necessarily good thing. Every oil need certain viscosity to perform well and last. In other words if the oil is too thick or too cold the long hydrocarbon molecules get "shredded" in bearings and the oil loses lubricity. There is certain optimal temperature window for each particular oil to perform well. Large rotating machines have a temperature interlock and can't be started unless the oil has certain minimum temperature and trip if the oil gets too hot. There heaters and temperature control to maintain the temperature within the specified range installed on every lubrication system. Since it is hard to justify such control system for small engines the manufacturer designs the lube system so the oil has correct temperature at normal operating condition while using recommended weight of oil.

 

[greg_g]

Is it safe to assume you're still running 15W40 in the engine? And when was it last changed? Filter too? I ask, because - if the filter is bypassing - there's a chance that you're losing that modicum of oil cooling provided by the metal case.

Also, what do you see on the engine oil dipstick? Oil that has been diluted by fuel/coolant/hydraulic fluid gets hot too. Actually it's the contaminant that gets hot, but it still thins out the oil content. Depending upon the severity, this heat may or may not influence the coolant temp.

//greg//

 

[Bob Rooks]

I would check the accuracy of your temperature gauge. You may also have to add an oil cooler, which is not uncommon.
What type of oil are you using?
When was the last time you flushed the cooling system?

 

[quicksandfarmer]

I'm using 15w40. I changed oil and filter last week. I've never flushed the cooling system (330 hours). Oil on dipstick is still clear.

I'll look at the cooling system.

 

[greg_g]

Unless the temperature gauge indication of normal across the RPM range is inaccurate, I don't quite see a need to check the cooling system - for an oil pressure problem. But it's good preventative maintenance item anyway, so it likely won't cost more than time. Otherwise, there's still more to check on the pressurized lubrication system.

Whereas rarely reported, and occasional new oil filter may have a defective bypass valve. Only cost a few bucks to try another filter. And I believe there's also a pickup screen on the oil pump. Unfortunately that means draining your fresh oil and dropping the oil pan to inspect and clean. And you didn't mention it, but I assume when checking the dipstick you also determined that the level wasn't too high as well.

//greg//

 

[TTTTTT]

Do you have any readings at a normal idle with tractor warmed up tp operating temp. When I am at higher revs the oil pressure should be higher . I have an adjustment on the filter bracket that I had to adjust down as it loosened and was way too high at idle and high revs. check if you have it and try an adjustment. I did mine with the tractor on and watching the guage. As you loosened the nut and then adjust you may get a little leakage at the adjustment nut, just don't over loosen it as you don't want a face full oil when it pops off. I check mine regularly especially if I notice the oil pressure seems a little off, I check that adjustment nut.

Good luck Dave

 

[Rich Waugh]

If you have the TY-395 engine, you can forget about dropping the oil pan unless you're a glutton for punishment. On my 2007 TY395 it requires splitting the tractor to drop the oil sump.

You said that when you switched back to the rotary mower the oil pressure was once again fine - was this at the 2250 rpm or back down at 1600 rpm? If it is at 2250 rpm, then your oil is heating up and thinning out - a lot. If the pressure comes back up because you dropped the rpm back down to 1600 then I'd suspect a bypass valve opening, and opening too far, so that pressure is lost in the oil galleries.

I would verify the engine temperature reading anyway, then verify the oil pressure with a mechanical gauge. One or the other of the gauges might be reading incorrectly, as oil viscosity (and thus pressure) should decrease as temperature increases (rpm being constant), and should increase as rpm increases. Unless you have a bypass valve opening up at high rpm I don't see how the pressure can decrease while the temp remains constant. Again, the mechanical gauge should help diagnose the issue.

 

[Bob Rooks]

I concur with everything Rich posted.
The fact that the oil pressure comes back when you reduce the load, and subsequently the temperature, that validates the point of cooling issues.