Severe Jittering Oil Pressure
in 912 / 914 Technical Questions
10 years ago
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Re: Severe Jittering Oil Pressure
by Bill Hertzel » 8 years ago
I don't have a dog in this fight but I find the problem interesting to read.
As a Troubleshooter by heart, I would like to throw out some thoughts.
The recurring theme seems to be that parts are changed and the problem goes away for a time and then returns.
So it appears that replacing the parts themselves do not cure the symptoms but just upsetting the system does, at least for a while.
Consider what may be causing the pressure fluctuations.
The first consideration would be an issue with the pressure valve.
The Oil pump pumps way more oil than the engine needs.
A majority of the oil is returned to the pump as the pressure is relieved.
I would not expect the "Seat" on the valve to be an issue because there should never be any time that it would be closed.
It is more like a floating variable restriction than an actual valve.
Oil is essentially non-compressible.
The oil pump produces a pressure pulse as each "Chamber" contracts.
This will result in 6 surges per camshaft revolution or 3 per crankshaft rev.
At 3600 rpm that is 10,800 pulses per minute or 180 pulses per second.
You would think this high frequency would be easy to dampen out!
But what would do the dampening? The valve?
The valve will need to flutter in time with the pump.
If it gets in resonance with the pulses it will beat the Seat to Death.
Sometimes changing the ball for the mushroom works and sometimes a different spring works.
Sometimes nothing works.
The poor valve will need to flutter 180+ times a second unless something can absorb the pulse elsewhere.
The oil filter comes to mind.
It sets horizontally and will trap a small but significant amount of air in its upper portions.
Every time you open the oil pump for maintenance, some air will enter the system and recharge the Oil filters "Air Shock".
After awhile the air gets cleared from the filter and then the pressure fluctuations return.
This is all speculation.
John, Without touching anything else; remove your oil filter, drain it of oil and put it back on.
Does the Oil Pressure stabilize for a while?
If so, Then What?
I noticed that the 912is folks do not seem to be complaining.
What's the difference? A lot!
The valve just a 6 gram flat-topped cylinder that slides over the spring.
The "Seat" is a flat faced plug with a hole in it.
There is nothing remotely looking like it seats or seals.
It is just a lightweight floating variable restriction.
So where do we go from here?
I don't know!
Maybe lightweight is the answer.
The extra inertia may cause the valve to open too far and then close too far causing the flutter.
A lighter valve could react faster to the pulses.
If you have a Mushroom that is giving you problems, try drilling it out to reduce its mass.
Take all this as just food for thought from a crazy thinker.
EDIT:
I just noticed another difference.
The 912iS Oil Pump has 6 lobes producing the 180 pulses per second as mentioned above.
The ULS Oil Pump has 4 lobes producing only 120 but larger pulses per second at 3600 rpm.
They truely are different animals.OilValve1.jpg (You do not have access to download this file.)OilValve2.jpg (You do not have access to download this file.)OilValve3.jpg (You do not have access to download this file.)
Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.
Thank you said by: RotaxOwner Admin
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Re: Severe Jittering Oil Pressure
by Roger Lee » 8 years ago
Hi Bill,
Where did that cylindrical cap come from?
I have never seen a cylindrical cap like that on the spring? That isn't the mushroom oil pressure regulator part I was talking about.
Rob,
What is this part? Something new or some after market?
Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell
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Re: Severe Jittering Oil Pressure
by Bill Hertzel » 8 years ago
It is likely you have never seen the Flat Cap because the 912iS engine never has Oil pressure problems like the ULS and you have never had the need to disassemble the Oil Pump.
The Flat Cap is a standard part on the 912iS.
It is illustrated as a Flat Cap in the 912iS Parts Catalog.
Section 79-20-00, Page 4, Item #2, PN.856330, "Regulating Piston"
I am just showing changes that Rotax has made to similar engines that appear to solve the Oil Pressure fluctuation problems.
These parts will not fit in a ULS Oil Pump (M12 vs. M22) but they shed some light on the real solution.
The complete 912iS Oil Pump assembly -Might- fit on a ULS engine.
The casting looks the same. The external ports have moved slightly.
The Oil Pressure and Temperature Sensor Port Boss's are not drilled but that would be an easy fix.
The 6 lobe "Pressure Inner and Outer Rotor" -Might- be drop-in replaceable.
There is a problem without a solid solution and the people in the field are starting to get desperate or at least frustrated.
Again, this is all just throwing parts on the table to see what the differences are.
Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.
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Re: Severe Jittering Oil Pressure
by Roger Lee » 8 years ago
Hi Bill,
Thanks for the update. You are right I have never taken one out of a 912iS engine. Interesting.
Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell
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Re: Severe Jittering Oil Pressure
by Ken Ryan » 8 years ago
Bill beat me to the punch: that opening the system to replace parts seems to fix things (rather than actually replacing parts). That seems like it should be a pretty big clue. I have a couple of questions: Do these jittering oil pressure problems occur primarily in the 912ULS, or do they seem to be as frequent in the 80 hp engine and the 914? Could the high compression of the ULS be a factor?
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