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Oil Pressure Fluctuating

Hey guys. I have an issue with my O-320-D2A on my recently rehabilitated RV4

Quick rundown - i'm a young A&P for a living operating a maintenance shop in Massachusetts.

Aircraft flown 450hrs as O-320-E2A powered. Engine overhauled in 1998 by Mattituck, at that time converted to -D2A by upgrading pistons, bringing engine to 160hp. Flown 50hrs until 2001 at which point aircraft was parked in a hangar.

Fast forward 18 years and I come into possession of the plane (thats another story.

I removed all 4 cylinders due to rust on cylinder walls. Tim Hess at Unlimited Aero Engines (Mike Goulian / Kirby chambliss hot rod engine builder from the early days for RBAR). Bottom end, tappets, lifters, ect inspected and all really clean. All four cylinders honed, new rings installed, and piston pins changed to the newer bronze, magnetos inspected, carburetor replaced, hoses, tires, battery, filters, ect ect ect all replaced.

On first flight after all of the engine work pressure was indicating on the low side. 35psi during hot idle, 60ish in cruise. (not fluctuating though). The engine has the short tower non-adjustable oil pressure relief valve. After first flight I added a couple of washers to the releif valve spring and this brought pressure to within limits - top of green arc on startup and in the middle during cruise. During the next two flights the oil pressure was about 70-80 in cruise, with a fluctuation continuously happening at high power settings going up and down from about 40 to 80psi.

At this time i removed the westach electric type gauge and sent to the factory for repair, along with the sender. He checked the calibration of the gauge and returned with a new sensor. I installed the sensor, gauge, and checked the integrity of the wiring, but the fluctuation continued.

The last few flights the fluctuation worsened, but i realized something. At ALL power settings below ~2400, the pressure is rock solid at the top of the green arc. increasing power above 2400 immediately causes the pressure to start fluctuating again. Retarding throttle to below 2400, and the gauge springs back to the top of the arc immediately. This can be done over and over with almost exact results.

My thoughts were a weak oil pressure releif valve spring. Oil pump output (flow) is directly linear to RPM. My thought was that at about 2300-2400 the oil pump output was just enough to overcome the releif valve spring and cause the low/fluctuating pressure.

I replaced the ball and spring and flew tonight, and nothing changed. When i replaced the ball I gave it a couple of light whacks to sort of "seat" it into the case.

I added another washer to the maximum amount of 3 washers, but then it got dark. Another test flight in the morning.

Whats your thoughts y'all???
 
It's a puzzle for sure. If the increased volume of oil pump output at higher RPM were beginning to push the ball off the seat, you would expect it to reach an equilibrium and the pressure to stabilize at whatever the spring tension mandates.

What if, at higher RPM (higher oil pump RPM), the pump were cavitating somehow? Some sort of debris blocking the pump pick-up at higher rate of flow? Might be worth a look in the pick-up screen and borescope up the passage as far as you can.

Ed Holyoke
 
I would think if there was an actual oil pick up problem you would likely see oil temps fluctuating also.. we’re there any other secondary indications this is a mechanical issue? I would shoot the wires again and make sure they are checked pin to pin and pin to ground. Gently massage the harness while you shoot them and look for nominal indications on your meter... should be open to ground and continuity end to end.sometimes you can get some dirt and grime in those connectors also, I’d probably pick up some contact cleaner and clean them real well. Good luck!
 
Hey guys. I have an issue with my O-320-D2A on my recently rehabilitated RV4

Quick rundown - i'm a young A&P for a living operating a maintenance shop in Massachusetts.

Aircraft flown 450hrs as O-320-E2A powered. Engine overhauled in 1998 by Mattituck, at that time converted to -D2A by upgrading pistons, bringing engine to 160hp. Flown 50hrs until 2001 at which point aircraft was parked in a hangar.

Fast forward 18 years and I come into possession of the plane (thats another story.

I removed all 4 cylinders due to rust on cylinder walls. Tim Hess at Unlimited Aero Engines (Mike Goulian / Kirby chambliss hot rod engine builder from the early days for RBAR). Bottom end, tappets, lifters, ect inspected and all really clean. All four cylinders honed, new rings installed, and piston pins changed to the newer bronze, magnetos inspected, carburetor replaced, hoses, tires, battery, filters, ect ect ect all replaced.

On first flight after all of the engine work pressure was indicating on the low side. 35psi during hot idle, 60ish in cruise. (not fluctuating though). The engine has the short tower non-adjustable oil pressure relief valve. After first flight I added a couple of washers to the releif valve spring and this brought pressure to within limits - top of green arc on startup and in the middle during cruise. During the next two flights the oil pressure was about 70-80 in cruise, with a fluctuation continuously happening at high power settings going up and down from about 40 to 80psi.

At this time i removed the westach electric type gauge and sent to the factory for repair, along with the sender. He checked the calibration of the gauge and returned with a new sensor. I installed the sensor, gauge, and checked the integrity of the wiring, but the fluctuation continued.

The last few flights the fluctuation worsened, but i realized something. At ALL power settings below ~2400, the pressure is rock solid at the top of the green arc. increasing power above 2400 immediately causes the pressure to start fluctuating again. Retarding throttle to below 2400, and the gauge springs back to the top of the arc immediately. This can be done over and over with almost exact results.

My thoughts were a weak oil pressure releif valve spring. Oil pump output (flow) is directly linear to RPM. My thought was that at about 2300-2400 the oil pump output was just enough to overcome the releif valve spring and cause the low/fluctuating pressure.

I replaced the ball and spring and flew tonight, and nothing changed. When i replaced the ball I gave it a couple of light whacks to sort of "seat" it into the case.

I added another washer to the maximum amount of 3 washers, but then it got dark. Another test flight in the morning.

Whats your thoughts y'all???

given the non-adjustable relief, I am assuming you have a cage for the ball. You sure that the cage is not bent or have a nick, giving the ball challenges getting back on the seat, assuming it is popping off for the first time at 2400? Symptoms describe a ball that can seal properly and can come off the seat, but is having difficulty getting back on the seat. The fact that it can get to 80 PSI, would indicate that the spring is strong enough the issue is something not allowing the ball to create a smooth rhythm or oscillation to maintain it. The pressure is dropping too much as the ball is not able to easily get back on the seat.

If you don't have a cage, too much spring pressure could be forcing the ball off center and it is struggling to seat. This would explain why you didn't see the problem initially. There are several different spring sizes. Sure you got the right one?

Hard to envision this as a instrument issue, given the repeatability of the symptoms, as described.

Larry
Larry
 
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Where/how is the sender mounted?

It might be worth installing a mechanical gauge to rule out some sort of rpm/vibration feedback/resonance into the pressure gauge's mechanical/electrical system.
 
Fluctuation

Where/how is the sender mounted?

It might be worth installing a mechanical gauge to rule out some sort of rpm/vibration feedback/resonance into the pressure gauge's mechanical/electrical system.

This is classic symptom of electric sender losing ground or ittermittent ground.
 
Agree that a mechanical gage should be plumbed it to see if it agrees with the electric. THEN start troublehooting the engine.
 
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