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Engine expert help. Oil pressure

Jrskygod

Well Known Member
On a recent trip I had a warning light come on and observed the oil pressure on my O320-D1A engine declining to about 30 psi. After about 30 butt clenching seconds it went back to around 80 psi my normal range. I have flown the plane now another hour or so with no abnormal indication. An oil change, filter cut and screen inspection showed nothing amiss. The oil change had 52 hours on it so there was a good sampling of any debris in the system. I’m not about to stick my head in the clouds until I get this figured out. This engine has been dead on reliable for over 900 hours and now this. Hopefully someone in VAF land can give me some reliable information to assist in determining the cause of the issue.
 
If the oil pressure relief valve stuck open for a few seconds, this is what you’d see. You might remove it, check the ball and seat, and look for any debris in the spring or housing.
 
Healthy engines rarely lose half their pressure and come right back, at least not at a constant RPM and temp. Very low oil level (like a quart) can do this though, especially with thed nose pointed down, as it will suck air for a bit. I suspect you would have noticed this during the oil change. Most likely reason is an electrical issue (i.e. failing crimp) somewhere in the indicator circuit. The VDO senders register a higher resistance for lower pressures. An intermittent bad connection will increase resistance and therefore read a lower pressure. With these symptoms, the relief apparatus doesn't seem likely (they just don't stick their head out once every 800 hours, though it could be a new failure). Also worth pulling the sump screen for a look. Very large debris or a large quantity of small debris in there could cause this, as could something like a rag floating in the sump (don't laugh, as it happens more often than you would guess) and occasionally partially blocking the two inlet holes, but also seems a low probability.

Larry
 
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Larry - The oil level was at 7 quarts which is where the engine likes it without excessive loss. I was on autopilot in relatively smooth air when the event happened. I have since drained and changed oil, inspected both the screen and filter without any sign of abnormality. I really dont understand how the relief valve could cause this issue. The one time event is really puzzling. I checked the sender connections and they looked like I just installed them. put a bit of silicone grease on them and exercised them on and off a few times just to be sure of a good connection. I guess Ill pull the pressure relief valve to inspect it next.
 
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Larry - The oil level was at 7 quarts which is where the engine likes it without excessive loss. I was on autopilot in relatively smooth air when the event happened. I have since drained and changed oil, inspected both the screen and filter without any sign of abnormality. I really dont understand how the relief valve could cause this issue. The one time event is really puzzling. I checked the sender connections and they looked like I just installed them. put a bit of silicone grease on them and exercised them on and off a few times just to be sure of a good connection. I guess Ill pull the pressure relief valve to inspect it next.

The way that the relief valve can cause a low oil pressure is simple - the “relief valve” is actually the oil pressure regulator - if it sticks open, the pressure in the system drops. It’s badly named, in my opinion…..

Not saying that was your problem, but wanted folks reading this in teh future to understand what it does!

Paul
 
. I really dont understand how the relief valve could cause this issue. The one time event is really puzzling. I checked the sender connections and they looked like I just installed them. put a bit of silicone grease on them and exercised them on and off a few times just to be sure of a good connection. I guess Ill pull the pressure relief valve to inspect it next.

The relief ball can get lodged in an open position and cause issues. However, if cruising along at a steady RPM (oil flow volume and therefore pressure induced with a constant restriction is linear to crankshaft RPM) when noticing a pressure drop greater than 50% would imply that the ball moved further away from the seat or a large opening in the oil system was created, reducing restriction. This just isn't possible without some type of damage to the spring, in which case it wouldn't come back 30 seconds later. Debris generally creates issues when reducing RPM. Reduced RPM creates a lower flow/pressure and the ball tries to move back closer to the seat to keep pressure up, but it can't due to debris or the ball hanging up on something. It is pretty rare to have debris cause pressure loss at high RPMs. More common at low RPMs.

My money is still on a intermittent wiring issue or an intermittent failure in the sender itself. I would replace the sender and re do the wiring before pulling the press relief, though that isn't hard to do. When there is something wrong with the relief, you will have issues more often than once i 900 hours and there is not much to go wrong beyond a spring at failure, at least problems at high RPMs.

Larry
 
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Paul and Larry - Yes my understanding is that the relief valve is the pressure regulator in the system. Although I haven't had time to remove it yet, I'm thinking like Larry that if something happened to the spring or debris got stuck under the ball and seat that the pressure would not have come back up. I've ordered a new pressure sender as the one on the plane is a Kavlico that needs to be replaced anyway. I'm pretty sure its not a sender issue but I'm going to replace it. Further wiring investigation next on the agenda.
 
Still unanswered

Still unanswered is Toolbuilder's question:

Is the warning light "driven" by the indicator, or is it a separate source of truth?

The answer will vastly help in troubleshooting. If they are completely separate systems, then you have an actual oil pressure issue.

If the light is only driven by the indicator, then most likely you have an indication problem.
 
Still unanswered is Toolbuilder's question:

Is the warning light "driven" by the indicator, or is it a separate source of truth?

The answer will vastly help in troubleshooting. If they are completely separate systems, then you have an actual oil pressure issue.

If the light is only driven by the indicator, then most likely you have an indication problem.

He answered it above. He has a Kavlico transducer and that sends a variable voltage. Therefore it must be connected to a device that can convert that variable voltage to a displayable pressure reading. You can't connect a transducer to a warning light.
 
He answered it above. He has a Kavlico transducer and that sends a variable voltage. Therefore it must be connected to a device that can convert that variable voltage to a displayable pressure reading. You can't connect a transducer to a warning light.

Is there a stand-alone oil pressure gage, or is the oil pressure indicated on an EFIS? If on a stand-alone gage, what drives the warning light?

If pressure is indicated on an EFIS, is the low pressure warning light separate from the EFIS? If it's part of the EFIS, then a false low pressure signal from the transducer could turn on the oil low pressure warning.

This still seems more like an indication problem to me. Plus, indication issues are easier and cheaper to troubleshoot.
 
He answered it above. He has a Kavlico transducer and that sends a variable voltage. Therefore it must be connected to a device that can convert that variable voltage to a displayable pressure reading. You can't connect a transducer to a warning light.

No answer was provided. Many of us have an oil pressure switch in the system which turns on the hobbs. I originally also had a low pressure warning light on my panel connected to the normally closed side of this switch. I eliminated it when I upgraded the panel to an efis. It is possible the OP has this switch. Maybe someday we'll get an answer.

I have many times on this forum suggested one should consider a backup mechanical gauge for oil pressure. It is about the only engine parameter that you really, really need to be sure of. Bad indication? Fly home. Low or no oil pressure? You will be landing shortly, might as well do it on your terms.
 
I have an EFIS which drives all sorts of low pressure alarms AND a simple idiot light driven off a separate pressure switch on the oil galley.

If I get an EFIS warning along with the idiot light, then its a pretty strong indication a lack of oil pressure is the culprit.
 
To safety tie or not

I know Lycoming does not safety wire the nut. Is there a need?
 

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The way that the relief valve can cause a low oil pressure is simple - the “relief valve” is actually the oil pressure regulator - if it sticks open, the pressure in the system drops. It’s badly named, in my opinion…..

Not saying that was your problem, but wanted folks reading this in teh future to understand what it does!

Paul

I’m with Paul on this. You had 52 hours on your oil, so you had dirty oil. Something might have gotten stuck in that ball valve, which would give you just what you saw. An obstruction stuck on the ball valve and then being rinsed, or passed through would give you the symptoms you experienced. You now have clean oil, and if an obstruction on the ball valve was the problem it should be OK now. On the side of caution and due diligence, maybe you could fly it again, staying directly over the airport for an hour, varying your power settings and see what you find. If that is successful, then venture out a little, until you prove your hypothesis. Kind of like a mini Phase 1. Pulling the ball valve for inspection probably would show nothing since it’s performing fine now, and is probably therefore clean. I don’t know what else could cause a sudden dramatic oil pressure loss that clears up relatively quickly, except an oil pressure indication malfunction, as others have mentioned on this post. Those things are relatively easy to verify as well.
 
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