low oil pressure

Went flying yesterday and happened to glance at my oil pressure guage. It read 50lbs!! when normally it is around 80+. Immediately turned back toward the airport watching the guage all the time and all of a sudden it is back up to 80. Kept watching and it was wandering all over the place. Went ahead and landed and tried to figure out the problem, guessing it was either the guage or the sender. My hanger neighbor said he has seen that happen when air gets in the sender line. I just changed oil the day before and thought well.....o.k. sure, why not, only in over 800 hrs of oil changes every 50 hrs. it has never happened before. Took the sender line off, took the o.p. restrictor fitting off, and checked wiring to sender, everything looked fine. Put it all back together, added another quart of oil and flew some more. O.p. right on 82 lbs....just like always. Don't know what I did different this time but everthing seems normal now. Anyone had this happen before??

Dave, this is not an uncommon occurance. There are a couple of things that can cause this phantom. One is as your hanger mate mentioned that an air bubble could have been trapped in the line. Another is that a piece of trash, lint etc. made it's way to the sender and or pressure relief valve. Most likely an air bubble occured during the oil change.

Allen B.

yeah, I can relate.

Yep, same here, on a recent trip. I usually see 75psi or so, and it was down to 60-65. I typically see about 5 psi drop with hot oil vs. cold, but we were up to temp with all else looking normal. I understand your concern! Had me looking for go-to-nearest on the chart. It held all the way home and I started the annual. Drained oil, cleaned the suction screen, checked grounding of the pressure sender unit and found nothing amiss. I pulled the old style tower pressure regulator and checked for signs of debris at the relief ball and again, nothing unusual. I added one more washer and pressure came back up to 85, a bit higher than necessary for sure, but may show the relief spring is getting tired. Oil analysis and filter media inspection shows no signs of metal or unusual wear, so all appears to be fine. I haven't flown it yet, but we'll see if that additional washer proves to be too much.

RESISTANCE.....

Resistance in any connection between the sender and the gauge will cause a reading change. This also goes for the ground wire.

If you install vans gauges, and jump the ground wire from gauge to gauge and then to ground, resistance at each connection will add up and make each reading wrong.

Air in the line. Air compressers have gauges, and they have air in the line!
The air in the line will not change the reading, but it may coushion it.

Warren

I'm a bit of a broken record on this... but, it is exactly avoiding situations like this that compelled me to put a small mechanical backup oil pressure gauge in my plane.

False indications on the rest of the engine parameters are not too bothersome, but low oil pressure indication means you really should land right now. Some might say keep flying and see if the oil temp rises, but not me.