Oil Pressure Issue

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digginfool
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Oil Pressure Issue

Post by digginfool »

Have a question about oil pressure gauges. I have two oil pressure gauges; an electric one that reads on the console and a mechanical gauge on the engine itself. The mechanical gauge has been on the engine (SV540) since I had it built, probably close to 300 hours ago. The electric gauge is new; I just added it last Fall. Recently, the two gauges have not been reading the same. After the engine is good and hot, the mechanical gauge will read near zero at idle while the electric gauge reads about 30 - 35 psi. At cruise, the mechanical gauge reads around 25 - 30 psi while the electric gauge reads 50 - 60 psi. The engine makes no noises that would indicate no/low oil pressure (lifters clattering, rod knock, etc) while idling. Runs smooth as silk at cruise. Is it possible the mechanical gauge has gone bad? I did discuss this briefly with Tom Schmidt and while he thinks it's a good possibility there are clearance issues (i.e. bearings worn out), it could also be the check ball where the gauge is attached to the engine. Not trying to second guess Tom in any way but was wondering what the possibility is that it could be the gauge itself. I know it's rare but they can go bad. I also know that the electric gauges are prone to error as well. Anyone have any experience with mechanical gauges going bad?
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unforgiven11B
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Re: Oil Pressure Issue

Post by unforgiven11B »

Not an aircraft engine guy however, I would pick up a new mechanical gauge and rule it out first. Or get one that is a known working and swap them and go from there. As even new ones fail outta the box had one on a Cadillac.
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Re: Oil Pressure Issue

Post by Slidin Gator »

Diggin,

I agree with unforgiven, I would try another mechanical first, gauges definitely fail, that seems more likely vs. bearings wearing out in 300 hours.

Where do you have the two gauges plugged in? Mechanical on the oil screen and electric elsewhere? This is aiming towards the check valve comment, which is kind of confusing. Are we talking about the ball and spring on the relief valve?

Do you have a Vernathane or ball and spring for the oil cooler bypass circuit?

Image
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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digginfool
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Re: Oil Pressure Issue

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Slidin Gator wrote:
Fri Feb 12, 2021 5:15 pm
Diggin,

I agree with unforgiven, I would try another mechanical first, gauges definitely fail, that seems more likely vs. bearings wearing out in 300 hours.

Where do you have the two gauges plugged in? Mechanical on the oil screen and electric elsewhere? This is aiming towards the check valve comment, which is kind of confusing. Are we talking about the ball and spring on the relief valve?

Do you have a Vernathane or ball and spring for the oil cooler bypass circuit?

Image
The ball and spring is on the relief valve. Tom said it's possible the ball is worn or an issue with the spring. The mechanical gauge, looking from the front of the engine (i.e. opposite the prop, which would normally be considered the front), the mechanical gauge is on the right hand side port, the electrical sending unit is on the left hand side port.
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Re: Oil Pressure Issue

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digginfool wrote:
Fri Feb 12, 2021 6:51 pm
The ball and spring is on the relief valve. Tom said it's possible the ball is worn or an issue with the spring.
IF the oil pressure is ACTUALLY LOW, yes, this is the most likely culprit and is easy enough to check visually, here is a thread:
viewtopic.php?f=47&t=71481&p=688064&hil ... ef#p688064
digginfool wrote:
Fri Feb 12, 2021 6:51 pm
The mechanical gauge, looking from the front of the engine (i.e. opposite the prop, which would normally be considered the front), the mechanical gauge is on the right hand side port, the electrical sending unit is on the left hand side port.
Some pictures would be really helpful here. But the thing is, if oil pressure is low, both gauges should show it. So one of them is lying.

Just remember, the start of all of this was when you put the second gauge on her, who does that. :banghead: Weren't you wining in another post about not having room for both Oil temp and CHT gauges, but you got room for a second oil pressure gauge? :scratch: :stirpot:
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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digginfool
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Re: Oil Pressure Issue

Post by digginfool »

I added the electric oil pressure gauge since I decided that information was more important than CHT. Besides, I had to look down and behind me to see the mechanical gauge that was mounted on the engine. I had one spot on my console and it is now occupied by the electric oil pressure gauge. JS
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digginfool
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Re: Oil Pressure Issue

Post by digginfool »

BTW, nice informative post. I don't have a spare mechanical gauge and hate the idea of buying an extra but I guess that's about the only way I will know for certain.
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Re: Oil Pressure Issue

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digginfool wrote:
Fri Feb 12, 2021 8:47 pm
Besides, I had to look down and behind me to see the mechanical gauge that was mounted on the engine.
This has me doubting the mechanical even more. Plugged directly in it’s going to see all the hydraulic and mechanical vibration. Remote mounting isolates it from both. Turning around to look at gauge, I’d read it 30 psi off easy.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
Dirtman
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Re: Oil Pressure Issue

Post by Dirtman »

Diggin I have had mechanical gauges go bad before although its not happened very often. I know that at one time you had a construction co. of some sort. Check with your mechanic and see if he has a test gauge. I always have a Good quality gauge in my tool box for reference.
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Re: Oil Pressure Issue

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Dirtman wrote:
Fri Feb 12, 2021 11:31 pm
Diggin I have had mechanical gauges go bad before although its not happened very often. I know that at one time you had a construction co. of some sort. Check with your mechanic and see if he has a test gauge. I always have a Good quality gauge in my tool box for reference.
I sold my building, closed the company, fired everyone and retired. :toothy7: But I did buy most of the mechanic's tools since he didn't want to mechanic anymore. I'll have to look and see what's in there. I keep a set of fuel pressure, A/C and compression gauges but never ran across the need for oil pressure test gauges.
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Re: Oil Pressure Issue

Post by SWAMPHUNTER45 »

I can attest that I have had to run up to the parts store before to purchase a oil pressure gauge and plumb it in to verify a reading. Probably for a airboat addict having one in the bin of odd spare parts it’s $25 well spent.
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Re: Oil Pressure Issue

Post by SWAMPHUNTER45 »

Are you taking the readings off the same point on the engine ?

With Cadillac pressure will fade from front to back if clearances open up.

If you two gauges are off one T fitting I would be inclined to think one of the gauges may be defective.
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Re: Oil Pressure Issue

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SWAMPHUNTER45 wrote:
Sat Feb 13, 2021 8:52 am
Are you taking the readings off the same point on the engine ?

With Cadillac pressure will fade from front to back if clearances open up.

If you two gauges are off one T fitting I would be inclined to think one of the gauges may be defective.
The gauges are being fed from the two ports on the accessory housing. The mechanical is attached on the left side (right side as you look from the rear of the engine) and the electrical sending unit is attached to the opposite side. The ports are just above each mag.
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Re: Oil Pressure Issue

Post by Slidin Gator »

Diggin,

There is a reason they came up with terms like Port and Stbd, no ambiguity. Assuming your left (or right :scratch: ) designations mean that the mechanical gauge is connected to the Port side pressure port and the electrical is connected to the Stbd side port, I can definitely say you have a bad gauge and you need a 3rd one as a tie breaker. Any 100 PSI or greater gauge with a 1/8" pipe thread will screw right in, $10 hardware store version works fine to provide a check.

Referring back to to the oil system diagram. The Port side port is located upstream of the oil cooler and oil cooler bypass valve, the first place in line after the pump (Left side of oil cooler on diagram). The Stbd side is located on the oil cooler return port, just upstream of the filter screen. Pressure at the Stbd side port will always be =< (equal to or less than) the Port side connection. The difference would be greatest when engine is cold and the difference should increase vs. RPM.

Image

Port side oil pressure connection, upstream of oil cooler.
Image

Stbd side on oil cooler return
Image
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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Re: Oil Pressure Issue

Post by SWAMPHUNTER45 »

Great info !
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Re: Oil Pressure Issue

Post by fl cracker »

When is the last time you pulled the suction screen?
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Re: Oil Pressure Issue

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SWAMPHUNTER45 wrote:
Sun Feb 14, 2021 7:34 am
Great info !
Swamp, I wish it was, but it is wrong :violent1:

I did some head scratching on this one :scratch: The Stbd side pressure port I showed is correct as the oil cooler return. But the Port side port is actually downstream of that, just prior to the main oil pressure relief valve. The oil filter is between the two, so higher pressure on the Stbd vs. Port pressure port would indicate a clogged oil filter. A bad gauge still remains my call, but a new oil filter might sort things too.

This thread has a lot of good info:
https://vansairforce.net/community/show ... hp?t=45548

Here is my markup of the diagram given in the VansAirforce post:
Image




An older picture of mine showing the inside of the 540 accessory case. Note the oil pressure port running up towards stbd side to enter the cooler bypass passages.
Image


When the Vernathane plunger is pulled back, oil bypasses into the inside of the filter screen. As the oil heats up, the plunger closes the bypass.
Image



From there, oil runs back through the bulge in the accessory case towards Port.
Image



Note the port that routes oil from the accessory case into the crankcase. This port is on the PORT side of the engine, right at the pressure relief valve in the crankcase. My prior thread on pressure relief adjustment applies from here:

viewtopic.php?f=47&t=71481&p=688064&hil ... ef#p688064
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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Re: Oil Pressure Issue

Post by Slidin Gator »

fl cracker wrote:
Sun Feb 14, 2021 8:14 pm
When is the last time you pulled the suction screen?
fl cracker,

That is always a good question to ask, many folks don't realize that there is a suction screen on these engines. Page 2 of my old rebuild thread covers the suction/pickup portion of a SV540 oil circuit.

viewtopic.php?f=85&t=72194&start=25

Image
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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digginfool
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Re: Oil Pressure Issue

Post by digginfool »

If the suction screen was clogged, wouldn't that cause low oil pressure everywhere, not just one port (assuming both gauges are operating correctly)?
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Re: Oil Pressure Issue

Post by Slidin Gator »

digginfool wrote:
Wed Feb 17, 2021 8:36 am
If the suction screen was clogged, wouldn't that cause low oil pressure everywhere, not just one port (assuming both gauges are operating correctly)?
Yes, that is correct.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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Re: Oil Pressure Issue

Post by Slidin Gator »

Diggin, This quote may be useful for your troubleshooting.
The oil pressure in most Lycoming engines is taken off the top rear accessory case. The oil pressure fitting has a reduced orifice in the outlet to the gauge. This helps prevent catastrophic oil loss if the oil pressure line or gauge begins to leak. Carbon or dirt can sometimes clog the orifice and cause an abnormally low oil pressure reading.
The full article has lot's of good information on the oil system.
https://www.cessnaflyer.org/maintenance ... ssure.html
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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Re: Oil Pressure Issue

Post by kwanjangnihm »

Slidin Gator wrote:
Thu Feb 18, 2021 10:12 am
Diggin your orifice is plugged
:lol: :lol: :lol:
He'll cut your throat, baby, stick you in the back, drive off in your Cadillac.
He's more trouble than you think, he'll kill your sugar, leave you in the drink.
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Re: Oil Pressure Issue

Post by Slidin Gator »

kwanjangnihm wrote:
Thu Feb 18, 2021 2:22 pm
Slidin Gator wrote:
Thu Feb 18, 2021 10:12 am
Diggin your orifice is plugged
:lol: :lol: :lol:
I'm pretty sure this is how mis-information on the internet starts. :dontknow:

:lol: :lol: :lol:
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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Re: Oil Pressure Issue

Post by John Fenner »

Port side is pre pressure relief valve, starboard side is of the cooled oil return, pressure will be lower when hot, as stated above, it depends on the cooler oil regulating system.
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