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  #1  
Old 03-23-2020, 08:32 PM
AlexPeterson's Avatar
AlexPeterson AlexPeterson is offline
 
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Location: Maple Grove, MN
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Default Mysterious Oil Pressure...

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.



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. )

Thoughts?
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  #2  
Old 03-23-2020, 08:53 PM
Kyle Boatright Kyle Boatright is offline
 
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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.
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  #3  
Old 03-23-2020, 09:07 PM
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Fred.Stucklen Fred.Stucklen is offline
 
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Quote:
Originally Posted by Kyle Boatright View Post
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.
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  #4  
Old 03-23-2020, 09:54 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Kyle Boatright View Post
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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Last edited by lr172 : 03-23-2020 at 10:05 PM.
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  #5  
Old 03-23-2020, 09:59 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Fred.Stucklen View Post
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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Last edited by lr172 : 03-23-2020 at 10:08 PM.
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  #6  
Old 03-24-2020, 01:12 AM
Charliesixtysix Charliesixtysix is offline
 
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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.

Last edited by Charliesixtysix : 03-24-2020 at 01:17 AM.
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  #7  
Old 03-24-2020, 05:54 AM
BillL BillL is offline
 
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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.
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  #8  
Old 03-24-2020, 06:09 AM
birddog486 birddog486 is offline
 
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You may have an indication problem. Check the grounds

Last edited by birddog486 : 03-24-2020 at 06:16 AM.
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  #9  
Old 03-24-2020, 10:47 AM
n567vb n567vb is offline
 
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Quote:
Originally Posted by birddog486 View Post
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
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  #10  
Old 03-24-2020, 11:22 AM
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skylor skylor is offline
 
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Quote:
Originally Posted by n567vb View Post
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...

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