Oil pressure crept up during maintenance?

[swatson999]

OK, this is weird...I finished a trip for Thanksgiving stuff, and after shutdown, drained the oil and started preparations for the annual inspection, fiberglass finish work (prepping for paint), etc. Let it drain for a day or three, then replaced the oil filter and filled it up. Haven't started it yet, as I've been doing the gazillion things for annual condition inspection, but....here's the weird part.

I had the system on several times, once for about 2-1/2 hours while I changed to Princeton fuel convertors and calibrated the system, etc. At one point, I noticed that my Hobbs meter on the SV had increased! It had added about 3 hours or so since I drained the oil after the last trip.

I looked at the data files in Savvy Analysis, and it looks like at some point the oil pressure went above 15 psi (and it's staying there, apparently, at least until I start the engine). It can be seen in several files, over a period of couple of weeks, going up a little to around 12, then up some more, etc., until finally over 15.

What could explain this? I never saw it before, because I always started the engine for leak checks immediately after filling it with oil.

I'm quite sure the sensors are okay...this must be some strange phenomenon

 

[Dynon]

Which oil pressure sensor are you using? If the legacy VDO-style - the "silver bell" style - know that one of the reason that we moved away from them was that they sometimes developed stickiness that causes incorrect readings over time. If it's the newer kavlico sensor, then we'd suspect the sensor a lot less. Butt all that said, if everything else on the display looks OK, and the wiring to the pressure sensor hasn't changed, it's most likely either the sensor itself or the sensor seeing some real pressure somehow.

 

[swatson999]

Yep, it's the legacy style sender. I'll see what it does after I do a post-maintenance run-up this weekend.

 

[swatson999]

Just to follow up...I'm still seeing oil pressures in the 12-14 psi range about half the time with the engine cold and shut down prior to engine start.  And during flight, the pressure is reading higher than it has for 160+ hours, going above 90 and bouncing around (3-4 psi changes over a second or two or three, up and down, etc.).

ETA: It doesn't start the flight that way...it's okay and in the normal cruise range of mid-80s for about 30 minutes, then starts going wonky (with no change in MAP or RPM).  The plots on Savvy Analysis show jittery readings after that.

Given that oil pressure is unlikely to be changing that fast, or sitting in the engine pressurized to 14 psi overnight, I'm betting the sending unit is bad, so I'm replacing it tomorrow with another VDO (they're cheap).  If that one is okay, I'll keep it until either it or the fuel pressure VDO fails, and then replace them both.

If the *new* one reads high in flight, though...any thoughts?  I can't imagine the engine suddenly within the last 15 hours or so decided to somehow make the oil pressure "jumpy" and occasionally higher than "normal" as delivered from Lycoming.

ETA: Looks good...mid-80s and steady throughout 40 min of flight. So musta been bad VDO sensor. Lesson learned: if the pressures start to increase *slooooowly* over several weeks/months, replace the sender with a better one

 

[dlloyd]

Steve,
What you describe, I could have written. Very similar with both oil and fuel pressures. Got tired of replacing the cheap senders and bought two Kavlico sensors. Both have indicated exactly as expected for the last 50 hours.

 

[swatson999]

4 hours of flight this weekend and oil pressure was rock steady.

So what is causing these transducers to fail? They've been around for ages, and seem to work fine in other applications (auto?).

What is causing so many failures in aircraft, and is there something that could be done to prevent them? Heat? Vibration?

 

[Garrett]

I think engine vibration is at least partially to blame. I remote mounted the oil pressure sensor next to the fuel pressure sensor on the firewall. See page 7-35 Rev.P in the SkyView install manual where they acknolegde this problem. I have heard of this solution frequently in the aviation community. My oil or fuel presure seemed to always go out over mountains or open ocean. Have not had a problem in the last 100 hours since the move.

 

[dynonsupport]

Vibration kills them, as well as sitting in one pressure region all the time while vibrating. Auto applications are constantly changing pressure, which helps and spreads out the wear.

This is why we switched to all solid state, no-moving parts sensors a few years ago once they became affordable. Plus, all car manufacturers went solid state forever ago too!

 

[Garrett]

Thanks for filling in the details Dynon. I have always wondered about the difference between aviation and automotive in regards to the sensors.

 

[swatson999]

Yeah, mine is mounted on the firewall, which shouldn't have much vibration, but certainly there is *some* (as in every airframe).

I'll no doubt end up changing them both to the Kavlicos at some point (hopefully it can wait until next year's annual condition inspection).