Jeff,
The only thing that I can think of that would cause an “actual”
fluctuation of oil pressure of that magnitude and cycle is periodically drawing
air into the oil pump pickup tube. That does not explain the significant
difference between high and low readings. My “most likely,” and
perhaps easiest to test, cause is an electrical issue. Bad lead or ground in the
wiring, defective sensor itself, or even instability in the display unit. I say
easiest to test because plumbing in a direct-reading mechanical gauge will
isolate and confirm or eliminate electrical/sensor as the problem right off the
bat, and if that’s the actual cause and you don’t get to that area
to investigate right off the bat, you can chase this bunny rabbit for a long
time… In my experience, electrical problems like this have a bad habit of
working OK for a while, leading you to a false hope that some other change you’ve
made has fixed the problem…then it comes back…
Sorry to be negative; hope this helps.
Bob
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf Of Jeffrey Liegner, MD
Sent: Thursday, May 24, 2007 1:13 PM
To: lml@lancaironline.net
Subject: [LML] LIVP Oil Pressure Question (TSIO550E)...Fluctuations
Noted
OK, here's an oil pressure question.
I've been running my new (fast build) TSIO550E engine for
some 45 hours now. I recently was flying with an instructor who noticed
the OP fluctuating for no apparent reason, all other parameters static and in
level cruise. Being the new kid, I had seen this but assumed the
fluctuations were not out of bounds. It had caught my attention, but TCM
was not too concerned, and so I've been looking at it without knowing what to
do.
The graph below depicts the oil pressure (black saw tooth
line) during level cruise condition.
The oil and filter has been changed (a bit earlier than the
planned 50 hrs), oil sent for analysis (pending).
The oil pressure transducer did not have a grounding wire to
the sensor, so I rigged one, but this had no effect during a recent long
flight.
Paradoxically, if you pull the throttle, reducing MAP, the
fluctuations and the overall OP increase briefly (but notably).
Of course, if you close the oil door and raise oil temp, the
overall oil pressure lowers, but continues to bounce around.
Some have suggested the next step would be opening and
cleaning the oil pressure relief valve. One idea was to vent any trapped
air in the hose exiting the oil pump fitting and going up to the sensor
(mounted on the fire wall), thinking the air was causing some compressive
fluctuations.
Others have mentioned the oil cooler vernatherm.
It does not seem to be related to radio COMMs, as I have
deliberately turned off all radios (during this 17 min plot) and observed no
difference.
TCM's first and only thought is a sensor (electical)
problem. They have asked me to check for same fluctuations on the ground,
static at 1700 RPM. If observed, then hook a mechanical guage and
watch. If not seen on the steam guage, ignore it or change sensor.
If not observed on the ground, then they had no further contributions (yes, I
have his name, date and time of the warrentee call).
Any incredible insights? Shared for my own benefit and
perhaps to help others.
This is a 17 min plot of RPM (scaled down by 1/30th), oil
temperature (scaled to 1/3rd), plus MAP, Fuel Flow and Oil Pressure (seen as
the black saw-tooth plot). Flight was level at 15,500'
RPM (red) is constant except for a brief RPM increase to
2600 to see if this had an effect. Oil temp (brown) fairly
constant. Fuel flow (blue) from ROP to LOP. MAP (yellow) initially
at 36" then lowered to 32", again held constant.
You'll notice the oil pressure fluctuates from low 60's to
as high as (briefly) 92 psi while engine parameters remain static, while also
fluctuating within each moment as much as 14 psi.
PS This was data pulled of the Chelton in a
complicated dance of data crunching, in case you wanted to know.