Oil Pressure Decreases as RPMs Increase
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Shaneard
Veteran Member
- Joined
- Nov 18, 2016
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- 1,345
- Location
- Southern Virginia
- Tractor
- Ford 4630, Two IH 140's, John Deere A
You may have gotten a bad gauge.
flyerdan
Veteran Member
Have you done a no load, static test? Do a stationary run up and note what the pressure does at 1000, 1200, 1400 etc. It shouldn't make any difference if it's under load or not. Logic tells us that oil system has a fixed volume, and speeding up the pump should increase the pressure since it can't increase the volume. Since the pressure drops, that might indicate that the bypass is letting too much through due to a weak spring or such. Possibly the pickup is partially blocked due to gasket chunks or sealer from assembly. I can't imagine operation at the proper level would cause foaming issues unless a big design flaw occurred, but experimenting with oil level is the easiest place to start.
I will do the static test and report back. If I remember correctly, it didn't take much off idle for the pressure to drop. And it never went below 30 psi on the gauge.
Could it be that this tractor just has really good oil pressure? It is a Fredricks rebuild from 2006 and got new pistons, rings, rods, bearings, etc only 250 hours ago. I lucked out and found it on CL with 200 hours last summer. So it should have pretty good oil pressure.
So if the oil pressure is 40 at idle, and the relief valve is set just above that, could it be that the relief valve is opening and dropping the pressure as RPMs come up to keep the oil pressure from going too high? It just has an idle pressure close to the relief valve setting.
The more I think about it the more I think that may be what is happening.
Is my logic flawed here?
Could it be that this tractor just has really good oil pressure? It is a Fredricks rebuild from 2006 and got new pistons, rings, rods, bearings, etc only 250 hours ago. I lucked out and found it on CL with 200 hours last summer. So it should have pretty good oil pressure.
So if the oil pressure is 40 at idle, and the relief valve is set just above that, could it be that the relief valve is opening and dropping the pressure as RPMs come up to keep the oil pressure from going too high? It just has an idle pressure close to the relief valve setting.
The more I think about it the more I think that may be what is happening.
Is my logic flawed here?
California
Super Star Member
- Joined
- Jan 22, 2004
- Messages
- 14,988
- Location
- An hour north of San Francisco
- Tractor
- Yanmar YM240 Yanmar YM186D
FWIW, hope this is relevant.
I use the YM240 to run my backhoe, no sustained hard use. So I put in 4 quarts at the annual oil change. (No noticeable consumption by the time of the next change).
The oil dipstick has 12 crosshatches, this 4 quarts fills it above the seventh cross hatch, so say about half way on the stick is a quart down and near the bottom of the stick is two quarts down. I hammered a tiny dimple into the dipstick at the 4-quart level so I can monitor for oil consumption. You can barely see the dimple in this photo so I put a wire next to it for the photo.
That was in a thread where someone had mixed up the transmission and crankcase dipsticks so the photo shows the other one too.
I see I also posted the oil capacity (YM240 Operation Manual) in that thread:
I use the YM240 to run my backhoe, no sustained hard use. So I put in 4 quarts at the annual oil change. (No noticeable consumption by the time of the next change).
The oil dipstick has 12 crosshatches, this 4 quarts fills it above the seventh cross hatch, so say about half way on the stick is a quart down and near the bottom of the stick is two quarts down. I hammered a tiny dimple into the dipstick at the 4-quart level so I can monitor for oil consumption. You can barely see the dimple in this photo so I put a wire next to it for the photo.
That was in a thread where someone had mixed up the transmission and crankcase dipsticks so the photo shows the other one too.
I see I also posted the oil capacity (YM240 Operation Manual) in that thread:
clemsonfor
Super Member
California runs 4 quarts in this ym240. I run 5 quarts in mine as I want the oil to run cooler in the 95F southern heat I bush hog in usually.
I will look in my Yanmar shop manual for the 240 and see when the light turns on.
Thought, put the gauge directly in and take out the sensor and just let that be on while running just the gauge, that takes the T theory out of the equation.
I will look in my Yanmar shop manual for the 240 and see when the light turns on.
Thought, put the gauge directly in and take out the sensor and just let that be on while running just the gauge, that takes the T theory out of the equation.
clemsonfor
Super Member
Well I responded before I read all posts I see that California already stepped in here.
If really curious pull the oil pan, its just a flat piece of steel and you can have it off and back on in no time. BUt if it was just rebuilt, 250 hours ago they would have to have a really dirty funnel they always used or have all kinds of crap fall in the filler hole one time when it was open??
If really curious pull the oil pan, its just a flat piece of steel and you can have it off and back on in no time. BUt if it was just rebuilt, 250 hours ago they would have to have a really dirty funnel they always used or have all kinds of crap fall in the filler hole one time when it was open??
Well I did a little more mowing today and paid more attention to what the oil pressure is doing.
It idles right at 40 psi when warm. Then when the rpms start rising the needle will also start to rise to say 42-43 psi, then fall quickly down to 35 or lower. When up over 1500 rpms the pressure stays down between 30 and 35 psi. Bring it back down to idle and the pressure comes right back up to 40 psi. It acts pretty much the same way loaded or unloaded.
However, when cold the idle pressure is lower, and acts like you would think it should. By that I mean that the pressure comes up with RPMs or at least doesn't go down. This is what makes me think it isn't a wear or bearing clearance issue, because in that case the hot idle pressure would be lower than the cold idle pressure.
It almost seems like the oil pressure relief valve is sticking closed and then when it opens, it has a weak spring and opens all the way. This may be flawed logic, I'm not sure.
At any rate, it always has acceptable oil pressure. It just seems to me like it is something to do with the pressure relief valve, along the lines of flyerdan's post.
It idles right at 40 psi when warm. Then when the rpms start rising the needle will also start to rise to say 42-43 psi, then fall quickly down to 35 or lower. When up over 1500 rpms the pressure stays down between 30 and 35 psi. Bring it back down to idle and the pressure comes right back up to 40 psi. It acts pretty much the same way loaded or unloaded.
However, when cold the idle pressure is lower, and acts like you would think it should. By that I mean that the pressure comes up with RPMs or at least doesn't go down. This is what makes me think it isn't a wear or bearing clearance issue, because in that case the hot idle pressure would be lower than the cold idle pressure.
It almost seems like the oil pressure relief valve is sticking closed and then when it opens, it has a weak spring and opens all the way. This may be flawed logic, I'm not sure.
At any rate, it always has acceptable oil pressure. It just seems to me like it is something to do with the pressure relief valve, along the lines of flyerdan's post.
Last edited:
normde2001
Elite Member
- Joined
- Mar 10, 2002
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- 4,163
Get rid of that gauge and just use the light.
PILOON
Super Star Member
The old basic rules of thumb were ;
Cold oil is thicker therefor pressures were higher.
As oil warms up it gets thinner and pressures usually drop some.
Loose bearings will show better pressures with thicker oils and pressure will drop off as engine gets warmer.
Now worn oil pump can also cause lower pressures with warm oil and dirty or clogged filters can also cause oil to warm up faster.
Lots of things to consider!
Best to not over think things and simply to monitor temps and listen with a fine tuned ear for unhealthy noises.
Generally routine maintenance with specified oils and filters is the sure way to go.
Now some engines/pumps will also have bypass relief valves to protect against very high pressures from thick oil on cold days. (had a jeep that would blow off the filters on cold days as the relief valve mechanism hit the frame and caused the valve to be jammed)
Also some filter 'cans' have built in check valves to prevent the engine from draining down dry.
Cold oil is thicker therefor pressures were higher.
As oil warms up it gets thinner and pressures usually drop some.
Loose bearings will show better pressures with thicker oils and pressure will drop off as engine gets warmer.
Now worn oil pump can also cause lower pressures with warm oil and dirty or clogged filters can also cause oil to warm up faster.
Lots of things to consider!
Best to not over think things and simply to monitor temps and listen with a fine tuned ear for unhealthy noises.
Generally routine maintenance with specified oils and filters is the sure way to go.
Now some engines/pumps will also have bypass relief valves to protect against very high pressures from thick oil on cold days. (had a jeep that would blow off the filters on cold days as the relief valve mechanism hit the frame and caused the valve to be jammed)
Also some filter 'cans' have built in check valves to prevent the engine from draining down dry.
