2002 MXT-7 180-A, 1,200 hours.

Headed out for an aerial recon of Mt. St. Helens with the girlfriend to show her the north side of the mountain from the air. We climbed it two weeks ago and thought she'd love the perspective.

Did normal preflight. 6.5 quarts of oil; just where it should be.

I monitor the engine instruments obsessively, and it turns out Saturday that was a good idea.

Half way there, 35 minutes into the flight at 8,000'...

The oil pressure usually stays 3/4 of the way to the top of the green. Next scan I look to see its just barely in the green, the left side of the needle touching the yellow. It's never been there while running before. The needle occasionally jumps into the green, but spends most of its time borderline yellow.

I reduce power and head for home, cognizant of airports and places to land along the way. The engine continues to run perfectly well and the oil temp is exactly where it should be. I have two oil temp gauges. The OEM one and also a probe for the JPI engine monitor. They agree, all is well. All 4 CHT's are normal.

We land uneventfully back home at RLD. No oil on the engine and no oil on the belly. The correct amount is on the dipstick.

I think this is a faulty sending unit or instrument. I don't picture it being the engine but the plane is grounded. I gave the keys to my AI and he is backed up may be some time before he gets to it. What do you all think is going on?

FWIW the annual was completed 10 hours ago. My lowest compression was 78, the filter looked fine and there were no (known) squalks.

Thanks in advance,

Bill

First, a simple manual gauge (sourced from local auto parts store for ground run) plumbed into the system and held in hand will tell the tale on the engine. If engine is good then you must determine gauge or sending. Stewart Warner or Rochester? Who has a chart for resistance versus pressure?

Thank you for the information. I'll post data when my A/P has a chance to get to it. I was hoping to go to Johnson Creek next week. Hopefully something minor and will be fixed.

UPDATE:

Perhaps this will help the next guy.

I had a little time to see the situation before my mechanic got to the plane. I found the electronic oil pressure sending unit on the back left side of the engine. The connector plug was not on all the way. I removed it and looked at the pins. They were clean. I replaced it making sure it clicked into place. Ground check revealed normal gauge operation. Did a full preflight and test flew it.

At cruise RPM the gauge reads steady at the top of the green and at idle the bottom of the green or 55 PSI. All is normal. Thank you Lord.

Thanks Doc, that’s a useful report. Maintenance-induced failure, I guess - someone dislodged the connection while attending to mags or filter?

I suspect it was maintenance induced but only showed up a few flights after. I will take it on a local flight after work as another test because tomorrow I plan to fly it and my girlfriend across the Cascades and do some back country play at Tieton and Ranger.