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Mysterious Oil Pressure...

AlexPeterson

Well Known Member
Today I planned a local flight. The engine was preheated, with its temperatures at about 70-80F before startup. The startup was nominal, oil pressure normal. The taxi to the departure runway is maybe 250 feet. I did the runup, and on bringing the power back down to idle I noticed the oil pressure was low. Ran the rpm back up, pressure 80+. Below is the data. Note the oil temp and oil pressure are both graphed against the left hand vertical axis. One can see the relative divergence of the pressure vs the rpm as the oil warmed up. I've never seen anything like this in 18+ years of flying it. The previous flight was 10 days ago.

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Even though the oil only got to 120F, the idle pressure was way lower than normal. In fact, it was lower than I've seen it even when oil temps are above 200. Obviously, I didn't fly and instead taxied back to the hangar to scratch my head. Seems it could be a leaky relief valve, but I'm not sure. I'm going to go out again tomorrow and let it warm up longer, as well as to verify the sender accuracy. (Confession...I've long been an advocate of putting mechanical oil pressure gauges in as backups. I have one. The way the pressure was tracking so well up and down with RPM it never occurred to me that the indication might be false. So, I never looked at my mechanical gauge, which is sort of out of the way in the upper left corner of the panel. I will look at it tomorrow. :eek:)

Thoughts?
 
My first guess would be something holding the pressure relief valve open. How hard is yours to remove? Those pressures aren't harmful, so I'd probably run the engine first and check the manual gauge before pulling the PRV.
 
My first guess would be something holding the pressure relief valve open. How hard is yours to remove? Those pressures aren't harmful, so I'd probably run the engine first and check the manual gauge before pulling the PRV.

Actually, it's just the opposite. I've seen this before. The oil pressure regulator has a round steel ball over the high pressure oil port held in place by a spring that sets the oil pressure. Sometimes the ball gets stuck over the oil pressure port. Tape the oil pressure regulator with a small hammer and it will probably release when the engine is started again.
 
My first guess would be something holding the pressure relief valve open. How hard is yours to remove? Those pressures aren't harmful, so I'd probably run the engine first and check the manual gauge before pulling the PRV.

+1

It looks like the relief is not fully seating and it looks like that blockage occurred during this data set. It seems to be behaving normally until around 274. After that, it is behaving like the relief is not fully closing. Before, 800 RPM delivers 80 psi. After, 1200 RPM is delivering 60 PSI. 20* increase in oil temp won't do that.

Could be debris or an issue with the spring or cage. While much less likely, you could have thrown a chip off an oil pump gear tooth creating issue on the delivery side vs the regulation circuit. Symptoms would be similar. Same is true of a leak anywhere in the oil delivery system.

Larry
 
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Actually, it's just the opposite. I've seen this before. The oil pressure regulator has a round steel ball over the high pressure oil port held in place by a spring that sets the oil pressure. Sometimes the ball gets stuck over the oil pressure port. Tape the oil pressure regulator with a small hammer and it will probably release when the engine is started again.

I don't think so. A ball stuck in the relief socket will cause VERY high oil pressures at 2000 RPM in a healthy engine. His chart is showing proper behavior, with the relief circuit keeping up with regulation when the RPMs spike. pressure is always stops climbing around 85 PSI. pressure is fluctuating directly with RPM until 85 (initially, 65 later), then it flat lines.
 
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It is definitely worth verifying the readings against the ( known good?) mechanical gauge as you suggest first.

If a discrepancy is proven, it may well be a sender/ wiring issue. I have been tracking a similar problem on an aircraft where the engine had been removed and reinstalled — on first run the oil pressure on GR 4000 went sky high about ten mins into the test flight ( which got my attention!) - subsequent testing showed the reading to be incorrect when checked against a mechanical gauge but did prove it to be directly linked to rpm. Faulty Sender was suspected.

The sender was tested and found to be working fine, so led us to thinking a poor (earth?) connection is causing the problem and possibly allowing rpm signal from the p lead to influence other parameter readings.

Still work in progress on this at this stage but thought it worth mentioning in case it helps.
 
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Alex, the fact that the pressure follows demand (RPM) and drops with temp, and is lower than expected certainly, could indicate either the demand (mass/volume flow) is higher because of the relief stuck open, or a broken spring, or there is "leakage" somewhere that was not there before, like an internal plug is missing.

. . . or Larry has a valid thought with a missing tooth. That can be calculated based on the reduction of mass flow (volume flow) vs rpm from historical data. Might be zebras, but is a valid possibility.

I think the easiest, first diagnostic to check would be mechanical pressure first, then the pressure relief valve for "proper operation" . (+1 for Kyle)

BTW Good find, oil pressure is one of those seldom studied items for me.
 
You may have an indication problem. Check the grounds

Ditto the grounds. Don't rely just on the manifold block for a ground connection. After flying a few years, I started getting funky reading as well once. Installed a dedicated ground and fixed this issue.

Vince
RV-7
 
Ditto the grounds. Don't rely just on the manifold block for a ground connection. After flying a few years, I started getting funky reading as well once. Installed a dedicated ground and fixed this issue.

Vince
RV-7

It's not likely that the pressure would follow engine RPM if this were just an indication problem...

Skylor
 
The one we are sorting did ( but reading high in our case) and turned out, as thought above, to be grounding fault.

The main thing pointing to indication error is just how closely pressure change corresponds to rpm variation and how little lag there is in changes - oil pressure does not tend to change as fast as those indications in the OP?s log.

My money is poor ground or a shielding fault.
 
The one we are sorting did ( but reading high in our case) and turned out, as thought above, to be grounding fault.

The main thing pointing to indication error is just how closely pressure change corresponds to rpm variation and how little lag there is in changes - oil pressure does not tend to change as fast as those indications in the OP’s log.

My money is poor ground or a shielding fault.

without regulation, oil flow and therefore indirectly pressure, is directly related to RPM, as the oil pump is driven 1:1 off the crankshaft. pump output volume is directly proportional to crank RPM. It is the relief circuit that stabilizes the pressure by diverting volume.

Larry
 
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The one we are sorting did ( but reading high in our case) and turned out, as thought above, to be grounding fault.

The main thing pointing to indication error is just how closely pressure change corresponds to rpm variation and how little lag there is in changes - oil pressure does not tend to change as fast as those indications in the OP’s log.

My money is poor ground or a shielding fault.

without regulation, oil flow and therefore indirectly pressure, is directly related to RPM, as the oil pump is driven 1:1 off the crankshaft. pump output is directly proportional to crank RPM. It is the relief circuit that stabilizes the pressure by diverting volume to maintain a set pressure. When the ball is stuck off it's seat, pressure will directly follow RPM until the opening is no longer great enough for the spring to hold the ball there. Then it will flat line, as the ball / spring is able to do it's job.

I would be very surprised to find the symptoms shown on the chart to be the result of an indicator problem. Simply too close of a correlation between pressure and RPM to be an indicator problem.

I have been wrong often, so not suggesting to skip the step of checking instrumentation. Just doesn't seem likely. The sudden nature of it also makes instuments less likely. The chart shows an almost instant change from normal to abnormal behavior.

Larry
 
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First, thanks to all those who replied. Charlie - you still want to put some money on it being an indication?...hahah.

Readings confirmed with the mechanical gauge, and today was a repeat of yesterday. I will pull the relief valve and have a look at it. The observations fit a leaking valve, but it is also hard to imagine it being so stable. I.e., one would think whatever was under the seat would have gone downstream.

I'll report what I learn.

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Do you have a constant speed prop? Looking past the relief to a possible leakage point. Something in the prop/crank cavity area would be my first guess at a sudden occurrence leakage point. Not too many areas that would produce a sudden and significant bleed off of oil volume. Assuming your relief checks out ok.

Larry
 
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Ah well, some you win.... it was wiring in our case (and I did say right t the start, subject to verifiction by mechnical gauge first ....). ;-)

Hope it proves to be simple and not too expensive to fix.
 
Culprit Found!

The little lumpy ball at about the 5:00 position was the cause. Oil pressure normal again. The ball is most likely lead, as it feels metallic when I scratched on it with the corner of a razor blade. It measures .020" in diameter. Amazing that it simply parked there, and I'm quite happy it did so that I could find the smoking gun. Thanks for the thoughts.

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excellent outcome

Thanks for sharing this story - one to file away, a lot to learn here.
 
Normal pressure behavior. Re the debris ball's composition - I didn't dare push much on it as it could have flipped into oblivion. I'll do it when I get a more contained situation.

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Berry interesting . . . . . .

Pretty lucky it did not migrate into the bearings. Knowing what it is and where it originated is awaited.

Weld berry?
Berry from inside the oil cooler?
Hose FOD?
Where are the friends of this Berry?
 
Oil pressure

I did the run up today for first flight after knee replacement. I couldn?t get the oil pressure to stay below 90 PSI until oil temperature got to 154 degrees. What do you think? GRT engine gauges with VDO sending unit, 0-360 A1A
 
Analysis if debris

I put the little .020", roughly spherical blob of stuff in a plastic bag, and studied it under a 40x microscope. It seems to be a tiny blob of cast iron like one might see attached to a freshly cast item. I can see where it was originally attached, as there is what appears to be a fracture point where it broke away. Mildly attracted to a magnet as one would expect. Easily small enough to go through the suction screen, so it could have been wandering around a long time in there.
 
Pretty lucky it did not migrate into the bearings. Knowing what it is and where it originated is awaited.

Weld berry?
Berry from inside the oil cooler?
Hose FOD?
Where are the friends of this Berry?

Great humor! Needed now that MN has fallen... thanks.
 
I put the little .020", roughly spherical blob of stuff in a plastic bag, and studied it under a 40x microscope. It seems to be a tiny blob of cast iron like one might see attached to a freshly cast item. I can see where it was originally attached, as there is what appears to be a fracture point where it broke away. Mildly attracted to a magnet as one would expect. Easily small enough to go through the suction screen, so it could have been wandering around a long time in there.

How did it get past the filter? Even the old screens would only pass 40 microns I thought. Not many cast iron parts between the filter/screen and the relief valve (pretty sure there are none). An interesting mystery. Possibly time to verify that your high pressure relief/bypass valve in the filter housing is not compromised / stuck open. That would allow a free flow of oil to bypass around the filter. The old screens did not have a bypass; only the filter housings.

Larry
 
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How did it get past the filter? Even the old screens would only pass 40 microns I thought. Not many cast iron parts between the filter/screen and the relief valve (pretty sure there are none). An interesting mystery. Possibly time to verify that your high pressure relief/bypass valve in the filter housing is not compromised / stuck open. That would allow a free flow of oil to bypass around the filter. The old screens did not have a bypass; only the filter housings.

Larry

Larry, good question... I believe the only filter bypass route is internal to the filter, but I sure could be wrong.
 
How did it get past the filter? ..... An interesting mystery. Possibly time to verify that your high pressure relief/bypass valve in the filter housing is not compromised / stuck open.

The older AC style filter adapter has a bypass. Most newer adapters don't have one. The adapter's bypass deltaP appears to be in the low teens.

Champion filters bypass in the 8 to 10 psi deltaP range. Company literature seems to suggest they bypass at every cold start.

Sitting here thinking about it, I've examined a lot of filter elements, but I've never really considered bypass valves. Do all the filters have a bypass?

EDIT: Found it in AV-14, the Champion catalog. 48103, 48104, 48110, and 48111 do NOT have an internal bypass. Interesting, given the aftermarket oil filter adapters with no bypass.

Interesting paper here: https://tempestplus.com/wp-content/uploads/2019/01/Alaska-Cold-Starts-0613.pdf

It says the leaf spring in a Tempest filter does not provide a bypass function. The above Lycoming filters use a coil spring, but are the same basic layout.

Alex, what adapter do you have?
 
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Dan, thanks for getting us to critically analyze how the Lyc oil system works. I had assumed (wrongly) that the spring in the back of the filter works as a bypass to unseat the filter element. Once you think about it, it's obvious that it does not.

So, that just adds ammunition to the thought of using the tallest filter that will fit. More filter area = less restriction. I've always used the CH48104 which barely fits in my -6.
 
Dan, I do not know which adaptor I have. EDIT, engine log shows CAM77852. This is the only picture I could readily find, and it doesn't show much:

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Looks like a standard AC type, which as previously noted, does have its own pressure relief.

http://www.vansairforce.com/community/showthread.php?t=45548

Dan, thanks. In these pictures you previously posted, it would see that the relief valve moves towards the engine (this side of the first photo below) if the delta p across the filter is too large. What isn't obvious is the compliance mechanism/spring that must be involved. It seems like a simple cantilever spring or similar. With the oil filter off, should one be able to inspect the part in the second picture and determine if it is not closed?

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Dan, thanks. In these pictures you previously posted, it would see that the relief valve moves towards the engine (this side of the first photo below) if the delta p across the filter is too large. What isn't obvious is the compliance mechanism/spring that must be involved. It seems like a simple cantilever spring or similar. With the oil filter off, should one be able to inspect the part in the second picture and determine if it is not closed?

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That metal flap is spring steel, I believe, allowing it to bend or yield under the designed pressure exposed to it's surface area. It would seem unlikely that you will be able to determine if there is an issue without removing the housing. The most likely issue is a piece of debris caught between the flapper and the seat, similar to what you found in the relief assembly. The lack of light on the backside and the presence of oil covering the parts makes it seem unlikely that you will be able to confirm it is seating properly without removing the assembly.

After considering Dan's comment / speculation that oil is likely going through the bypass for some time on most every startup, it is more likely that the debris just snuck through in one of those events vs a compromised bypass. I didn't know that to be the case when I suspected the bypass.

Larry
 
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Larry, I agree re it looking like a simple flapper from spring steel. I've got an oil change coming in the next couple weeks, and I will try to have a look in there. I'll squirt some acetone in there to clear out the oil. If it is propped open meaningfully, I should be able to see it.

Thanks again to all for the thoughtful comments, the education never ends!

Postscript: The debris in mine looks almost identical to this one I found in this Kitplanes article.
 
It is not spring steel. The cover you see is stamped from mild steel. The dimple in the center is to locate a coil spring which holds down a bakelite disc which holds the port closed. This is identical to what Pontiac used in their V8's.

148.jpg
 
It is not spring steel. The cover you see is stamped from mild steel. The dimple in the center is to locate a coil spring which holds down a bakelite disc which holds the port closed. This is identical to what Pontiac used in their V8's.

148.jpg

That's interesting. The four that I have seen look just like Dan's pic. No bakelite disc. pic also shows no visible step for the disc to seat against, like the one you show.
 
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I did an oil change today, and peeked into the bore which has the pressure relief. I did not see any gap in there indicating some crud, but to be fair, I'm not sure I could have seen something unless it had been larger than maybe .020".

The oil pressure has been completely behaved since I removed the little ball from the relief valve.
 
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