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  #1  
Old 10-07-2020, 06:52 PM
SPX SPX is offline
 
Join Date: Dec 2018
Location: San Diego
Posts: 511
Default Oil pressure fluctuations when at cruise power

About a month ago, the oil & fuel pressure sensors in my IO-360 powered RV-9A were changed from VDO to Kavlico sensors. A short period of time after the change (it wasn't right away, I'd say approx 8 hours later) the oil pressure began to fluctuate at cruise power, as shown in the video.

Today, Bruce and I swapped the wires on the sensors, such that the fuel pressure sensor is now using the former oil pressure sensor, and the oil pressure sensor is now using the former fuel pressure sensor. We made the necessary changes in the AFS avionics so the gauges read correctly.

Upon flying the airplane this afternoon, the same problem still exists. At takeoff power, the oil pressure is solid around 88 PSI. At idle power, as shown in the video, the oil pressure is solid around 55 PSI. It is only when I am at cruise power that the fluctuations shown in the video occur.

Initially I thought it was perhaps a bad sensor, but today's experiment has removed that possibility. Bruce feels good about the connecting wires not being the problem.

So, it leaves me wondering.. if it isn't the wires, what is it? I was wondering if the vibration at cruise power is creating some sort of intermittent connection? Another clue is the oil pressure at cruise, even at the peak fluctuation PSI, is about 7 psi lower than I am used to seeing.

Suggestions? Anyone seen this sort of issue before?

https://youtu.be/jg_kKmKm4tQ
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  #2  
Old 10-07-2020, 08:38 PM
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airguy airguy is offline
 
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Sensor grounding is still a possibility.
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  #3  
Old 10-07-2020, 09:00 PM
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I agree with Greg, ground issue. May be tough to find.
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  #4  
Old 10-07-2020, 09:12 PM
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bruceh bruceh is offline
 
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I don't think it is a ground issue.

The Kavlico senders have a dedicated ground wire, along with a +5V reference voltage wire. The return wires are what gives the pressure indication.

Maybe something mechanical?
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  #5  
Old 10-08-2020, 07:42 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by airguy View Post
Sensor grounding is still a possibility.
Not likely with a transducer (kavlico). The power and ground feed a power supply and the sense lead is given a variable voltage from a solid state chip. Intemittent ground issues will create on/off cycles and NOT, non-zero intermittent readings/fluctuatons.

Larry
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Last edited by lr172 : 10-08-2020 at 07:49 AM.
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  #6  
Old 10-08-2020, 07:45 AM
lr172 lr172 is offline
 
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I doubt it is the sensor. It seems very odd that at both idle and 2700 you get a steady reading, yet at 2400 you get oscillations. I struggle to see how a mechanical issue in the oil delivery system could behave that way. I would be looking for something in the EMS that could be related.

Larry
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Old 10-08-2020, 08:34 AM
SPX SPX is offline
 
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Quote:
Originally Posted by lr172 View Post
I doubt it is the sensor. It seems very odd that at both idle and 2700 you get a steady reading, yet at 2400 you get oscillations. I struggle to see how a mechanical issue in the oil delivery system could behave that way. I would be looking for something in the EMS that could be related.

Larry
The airplane is a fixed pitch prop, so even more strange .. 2400 RPM is about the takeoff RPM, and in the video linked above, is also approximately the RPM that I am running in cruise. On the takeoff roll, and and immediately after takeoff, the oil pressure is stable (warm and cold). It's only inflight at that RPM where the oscillations occur.
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Old 10-08-2020, 11:19 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by SPX View Post
The airplane is a fixed pitch prop, so even more strange .. 2400 RPM is about the takeoff RPM, and in the video linked above, is also approximately the RPM that I am running in cruise. On the takeoff roll, and and immediately after takeoff, the oil pressure is stable (warm and cold). It's only inflight at that RPM where the oscillations occur.
As far as the engine's oil delivery system is concerned, RPM is RPM. No difference in T/O or cruise. I would be looking for something electrical that is only occuring in cruise. I am assuming that you have observed these differences with the oil at the same temp. If not, that could be clue.

Larry
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  #9  
Old 10-08-2020, 12:02 PM
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AlexPeterson AlexPeterson is offline
 
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A couple years ago, after installing a G3X system in my plane, I chased down a problem I was having with cyclical variations in MAP and fuel pressure indications, within certain rpm ranges.

Both of those have restrictors, and both have a high speed (engine rpm or a multiple thereof) pulsatile condition that they are trying to measure. I speculated that the sample rate of the monitor, together with whatever methods the monitor uses for averaging/smoothing, was developing a beat frequency which presented itself in the display. I further reduced the diameter of the restrictor, and was able to eliminate it.

However, the oil pressure is not cyclic in this manner. But, perhaps the hose between the engine and the sensor (presumably on the firewall or engine mount) could be finding a resonance and creating this high frequency pulse, which, when combined with the sampling rate of the engine monitor, might show this variation. Would be simple to strap the hose to something to change its characteristics and look for a response.

Just a theory.
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  #10  
Old 10-08-2020, 02:05 PM
TS Flightlines TS Flightlines is offline
 
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Josh----before you go pulling your hair out looking for electrical issues, install a temporary known good mechanical gauge and check it versus the EFIS readings. If the mechanical portion of the system is working properly the gauge should should be steady. Yes, tee it in the hose after any restrictors so you can read the same pressures. Go fly and try to duplicate the erratic readings. If you see the EFIS acting funny, check the mechanical gauge. Even a 5-7 psi fluxuation should be visible even with a dampened gauge-IF its mechanical. If the gauge is steady and the EFIS is rocking, most probably an electrical issue---ground most likely.

If the Gauge bounces around, then I'd be looking at possibly the oil pump relief spring, adjuster, oil levels, etc.

Just thoughts.
Tom
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