Don't think I've read anything quite like this. Was flying yesterday and looked at the oil pressure again after flying for 10 minutes or so and it was down at the bottom of the green.This really got my attention so went to the airport pronto because it is always at the top of the green and when taxiing in it is usually at the bottom of the green or just under. When landing at very reduced power the pressure went close to the bottom. Thought maybe a big leak but no. Needed to change oil( Phillips 20/50) and I have a Capehart filter on my integral oil pump. The screen looked normal and the magnets had very little iron. Went flying again today and paid very close attention to what was going on. Start up idle pressure at top of the green, warm up and takeoff---above the green. Pull power back and it settles at the top of the green after a couple of minutes. Then towards five minutes it starts to drop fairly fast and steady to bottom of the green but stayed there for 20 minutes. When landing and pulling the power off again it goes close to nothing again. I have a mechanical gauge. The oil pump is off a later model O470U so is adjustable. Never had to mess with it before. Does this sound like a spring/bypass oil pump problem or is it a crack in the case opening after the engine warms up. Oil temp is fine----below half way in the green.
Thanks for any ideas or experiences

Is this a mechanical or electrical gauge? If it’s an electrical gauge the sending unit is failing. I hope it's that simple.

172heavy wrote:Is this a mechanical or electrical gauge? If it’s an electrical gauge the sending unit is failing. I hope it's that simple.

Sorry I missed the part about it being a mechanical gauge.

Marty
I experienced low oil pressure after those same conditions you describe but it was defiantly caused by oil temp. My rebuild shop after spending $25k thought a used vernatherm out of another plane was what my motor needed. In flight I would see the oil temp go up and the pressure go down and vice versa randomly. After changing it (only $400) problem solved. Check you temp gauge, if you can take it out put the probe in water and bring it to a boil check it with another thermometer or temp gauge you can trust.

I changed my gauge to an EI digital and see more precise numbers I believe. I tried readjusting the regulator on the oil pump and all that does it regulate the oil press when it's cold, once it heats up you will see the same trends. Take the Vernatherm out and look at the far end that's in the engine if it has an uneven ring like depression on the end it is not seating right, maybe do the same test while you test the oil temp gauge and put it in the water and watch it. It operates like a thermostat in a cooling system, I forget which way it should get longer or shorter as the temp rises, it's function is to divert more oil through your cooler as it gets hotter, I believe it gets longer as it gets hotter (like those young kids do ).

My 2 cents

I've used Aeroshell 50/100 for years and recently in last year or so changed to Phillips 20/50 . After the change I noticed the same-especially at low rpm when hot> not a problem. Unless you can find engine is blowing or leaking oil at a great rate (lower oil pressure ?) -bigger oil cooler helps keep oil cooler -try to stay under 200 f. with shallow climbs .Oil temp come down in cruise after a while .Number 1 issue with any 470-520 Continental engine is cooling -get and keep CHT's down and you'll have less wear and problems -especially with cylinders . Keep baffles good and tight so air flows BETWEEN and around Cylinders and out the BOTTOM of the cowl (cowl flaps?) . I stay between 260-320 on cowl flap airplanes and under 250 in no cowl flap airplanes. There's a STC for Fixed Cowl flaps that will LOWER your CHT 40-70 degrees -NOW and save your cylinders . Cracks in engine case are really rare and mostly in 520's on top side of cylinders between 2-4 .

I am having the exact same symptoms on my 0-470-J11.

The oil seems to be running about 20 degrees warmer than usual. At normal cruise power, 78°F ambient air, the oil temperature leveled off at about 205°F.

I have good pressure when the oil is cool, and there is a predictable drop in pressure to the bottom of the green the moment the oil temperature begins to rise.

DEGJR

After thinking about this, I lean towards what Mike Skup over on Supercub.org said. He thinks a piece of debris might be holding a bypass valve open a little. I am looking at my overhaul manual with an exploded view of a pump. There are two bypass valves. One must be set with a strong spring at something quite a bit higher than green on the gauge and the other is the one that has an adjustment screw. On takeoff they are probably both open dumping oil and when the oil starts to warm the non adjustable closes and the one with the adjustment screw takes over maintaining whatever oil pressure it is set for. Then when landing at idle they are probably both closed and the oil pressure is what it is. Since my oil temp didn't change and is running normal/cool, and this came on all of a sudden, I can imagine that the main bypass is not closing all the way and a chip made it become the oil pressure adjustment. Then with warm oil and idle rpm the oil pump is still bypassing instead of being completely closed. Might run to Omaha to the engine shop to look at a pump and see what I am getting into if I take the bypass valves and springs apart. Looks like it can be done but is a tight fit with the firewall.

Hi 180marty, most likely the vernatherm valve bypassing and causing excessive oil temp, it needs to close and seat properly to route oil through the cooler. Are you getting high oil temp as well? See if you can borrow a valve to try first, it's real easy to change and a good starting point

I would pull the pressure relief valve. I had very similar symptoms. I pulled the relief valve and found a piece of detritus. All in all the fix took about an hour --huge relief, easy to check.

Chet

The problem seems to have cleared up. Another guy on Supercub.org suggested a piece of gasket or similar was holding the bypass open. After 9 flights pressure is back to top of the green at cruise and when landing and 800 rpm is still in the bottom of the green. Whatever it was must have worked it's way through a little at a time since the pressure would increase a little every flight.

One other item, besides a piece of crud, you changed oils. There is a detergent effect from one oil to the other and a bit of potential reaction. On some brands & types (particularly with turbine) you need to fill the engine with your new oil run it to temp, drain and fill it again. Most people do not follow this procedure (perhaps because it is expensive?) to clean out any potential mixing (or at least dilute it so much it is not a problem). So what you may have seen is reacted oil not exactly clogging, but slowing the passage of oil through your cooler, which retained a whole bunch of your old Aeroshell.

There are also a bunch of places on your crank and around your engine that are there to collect heavier components of the blow by of the cylinders. Ever remove a prop and look in the crank cavity? There is always a bunch of gooey grey gunk there. This is actually centrifuged out of the oil to collect there (it is mostly lead). There are a few more places around the engine for this collection and it is one of the reasons for overhaul (among many) to clean them all out. So to the point, some oils have a greater detergent effect than others. So the Phillips may be more reactive than Aeroshell (I never checked) and cleaned out some of this crud to let it circulate a bit, causing less efficiency in the vernatherm and/or your pressure regulator spring/ball assembly.

I've heard burning ethanolized gas causes oil pressure fluctuations.

I've heard burning ethanolized gas causes oil pressure fluctuations

Can you go into more detail on how exactly that happens? Actually, the same oil has been used always----Phillips 20/50. If it is gasket, I was thinking maybe something from the oil pump since it goes well past the top of the green every takeoff when the oil is still not warmed all the way. Seems there could be a lot of stress in there until the oil warms and thins.

180 M

Seems to me that the oil starts out thin with a viscosity of 20 for easier cold starts and thickens up as it warms up.

Course I bin rong b4

Trim

How does the oil screen / oil filter look when you inspect it? I had a similar oil pressure situation years ago with an O-200, turned out the sprague in the key-type starter clutch was coming apart. The resulting debris was going through the oil system and some of it was keeping the pressure relief valve from seating.

Trimtab wrote:180 M

Seems to me that the oil starts out thin with a viscosity of 20 for easier cold starts and thickens up as it warms up.

Course I bin rong b4

Trim

Common misconception....

All oil, always gets thicker as it gets colder... Multi vis, just gets less thicker than straight weights. The smaller number is the weight it will behave at when cold, but it will still get thicker. 20W50 still gets stiffer, it just gets as stiff as a straight 20 instead os as stiff as a straight 50...

And even that is an over simplification, but should give you the idea.

Take care, Rob