M20J oil pressure anomaly

[marcusku]

I noticed early on after getting the airplane that the oil pressure read lower at various times.  At first I suspected the gauge but then noticed that it only happened when the oil level was lower.  As long as it was over 6 qts the gauge showed the middle of the green while in cruise at 2400 rpm.  When the level gets closer to 5.5 qts it shows at the line between yellow and green and also will flutter a bit at times.  I added an oil pressure sensor to the JPI to get some actual numbers and found if I have over 6 qts, I get a reasonably consistent 78 psi.  When I get closer to 5.5 qts it goes down to 65-70 psi and also the pressure will jump around 2 psi or so.  With the pressure being less and jumping around a bit I've been nervous to run it any lower to see what happens.

It doesn't make any sense to me why pressure would change at all with the level of oil in the sump.  To me at a given rpm & oil temp, the pressure should always be the same unless the pump was drawing in some air.  In theory it should get the same pressure even if you only had 2-3 quarts--as long as the pump wasn't taking in any air?  Anyone else observe this or have some ideas?

[Yetti]

yep.  When mine start pulsing, it's time to add a quart.

[carusoam]

Sounds like you are describing air entering the oil line...

Since Yetti described what happens when oil gets added...

It sounds like we have proof that there is a particular oil level that allows air into the line... filling above that level, keeps the air from entering...

Now the question is... has the oil pick-up gotten altered somewhere along the line?  An IO360 should have a minimum oil level of around two quarts...?

Expect that oilP and OilT need to stay in the green zones on the gauges...

I would expect that the lower the oil level goes, the oil pressure may follow it as well as even more air may be involved...

Our oil pumps are gear pumps and are pretty good at delivering oil and air through them... so the pump won’t get disturbed by the air... but it would be good to not have air in there when the oil is what you want...

Anyone have a picture of what the oil pick-up looks like in a Lycoming engine?

If it has a weighted rubber hose.... expect something may have gone wrong with the hose....

PP conjecture only, it has been a while since I have seen an oil pick-up in a Mooney...

Best regards,

-a-

[jaylw314]

IIRC, by regulation (can't remember where), aircraft engines MUST have a sump capacity twice that of the minimum safe oil level, ANY engine with an 8 quart sump must be able to operate with at most 4 quarts.  On top of that, the IO-360 TCDS specifies that of 8 quarts, 6 quarts is usable so the minimum usable oil level should be 2 quarts.  Not that I plan to do that any time soon, but if your oil pump is sucking air at 6 quarts, I wonder what the odds are that the problem is simply too little oil...

[marcusku]

If indeed I'm sucking some air already at 6 quarts, it seems unlikely that it would develop any pressure at 4 quarts much less 2.  I did see in the engine owner's manual that the minimum oil was 2 quarts.

[1964-M20E]

I'm just speculating here but there maybe several things going on.

 

1. There maybe some foaming happening therefore introducing some air into the system even though the pickup is fully submerged.
2. The oil maybe taking longer to return to the sump and the actual oil level in the sump is lower than you would expect.
3. Any pump is only good for a certain amount of head pressure i.e. output pressure.  The height of the suction also affects the output pressure.  At lower oil levels more suction is required.  The height difference between 6 qts and 5qts is minimal.  I'm not sure if that can account for the changes.
4. A whirlpool could be forming at the suction allowing air into the system.  I think this  highly unlikely but a consideration.

[ArtVandelay]

...so the minimum usable oil level should be 2 quarts.

To be clear, I don’t think that means you only have 2 quarts, that’s the level in the sump, that doesn’t count the oil in the lines, oil cooler, etc...I would guess there is another quart distributed throughout the system when the motor is running.
A interesting test would be to change attitudes (climb,descend ) to see if it makes a difference...I assume the oil pickup is in the rear of the tank and when in a descent the level at the pickup would drop.

[jaylw314]

57 minutes ago, teejayevans said:

To be clear, I don’t think that means you only have 2 quarts, that’s the level in the sump, that doesn’t count the oil in the lines, oil cooler, etc...I would guess there is another quart distributed throughout the system when the motor is running.
A interesting test would be to change attitudes (climb,descend ) to see if it makes a difference...I assume the oil pickup is in the rear of the tank and when in a descent the level at the pickup would drop.

I assumed that when TCDS specifies 2 quarts as the minimum usable, that that means there are 2 quarts in the sump when you would normally check it, e.g. after allowing oil time to drain back into the sump.  IIRC, the sump is a good deal more narrow than a typical auto engine sump, so it should be less vulnerable to sucking air if you have more than the bare minimum...

[DonMuncy]

11 hours ago, jaylw314 said:

IIRC, by regulation (can't remember where), aircraft engines MUST have a sump capacity twice that of the minimum safe oil level, ANY engine with an 8 quart sump must be able to operate with at most 4 quarts.  

I read somewhere that the regulations required that the sump capacity must great enough that with the maximum oil consumption, and normal cruise range, the oil level would not go below minimum oil level before running out of fuel. Even if that were true, I don't know what they would say about long range tanks.

[INA201]

I had this same exact issue.  The oil pressure would drop when I was 5 1/2 quarts or below.  I went as far as thinking that carbon may have built up around the screen so I had it pulled and inspected.  The screen was clear.  60 PSI is the minimum for the J and I typically see between 65-70 PSI under normal conditions.  The pressure would run as low as 59 with less than 5 1/2 quarts.  I bought a new spring and ball for the relief valve but haven't installed them yet.

[marcusku]

You wouldn't think the relief valve would care how much oil is in the sump but maybe?  Alternatively, is it possible return oil is getting restricted somewhere and not draining back to the sump as quickly as it should?  That would mean there are a couple quarts of oil sitting somewhere in the engine where it isn't supposed to.

[RLCarter]

Engine oil removes heat, lower oil levels can't dissipate as much heat. As the oil temp goes up oil pressure goes down.... 

[N201MKTurbo]

The oil pump is mounted to the accessory case. The inlet passage is cast into the accessory case and the sump. The passage passes through the sump gasket at the back of the sump. If that gasket seal is not perfect, it could be sucking air in at the gasket. I’m not sure if the oil level in the sump is above that gasket at 6 quarts or not, you can see if it is by taking the dipstick out and holding it next to the dipstick tube and seeing where the 6 quart line is.

[INA201]

1 hour ago, marcusku said:

You wouldn't think the relief valve would care how much oil is in the sump but maybe?  Alternatively, is it possible return oil is getting restricted somewhere and not draining back to the sump as quickly as it should?  That would mean there are a couple quarts of oil sitting somewhere in the engine where it isn't supposed to.

I was planning to try and  raise the normal operating pressure to 75 or so by making these changes. This would be separate from the low oil pressure with low oil capacity anomaly.

[marcusku]

2 minutes ago, INA201 said:

I was planning to try and  raise the normal operating pressure to 75 or so by making these changes. This would be separate from the low oil pressure with low oil capacity anomaly.

Makes sense.  Where's a good spot to obtain model specific service manuals?  Does the factory manual cover the whole aircraft including the engine or are there separate various manuals?  I realize there's not a lot I can touch but I like to know how things are put together.

[orionflt]

something else to consider is the possibility that the return lines and journals back to the sump could be narrowed and restricted not allowing the oil to return to the sump fast enough. 

A couple of questions: how many hours on the engine? and how long since it was overhauled? also how often do you change your oil and how often did the previous owner?

you may just need to flush your engine oil system.   

Brian

[marcusku]

14 minutes ago, orionflt said:

something else to consider is the possibility that the return lines and journals back to the sump could be narrowed and restricted not allowing the oil to return to the sump fast enough. 

A couple of questions: how many hours on the engine? and how long since it was overhauled? also how often do you change your oil and how often did the previous owner?

you may just need to flush your engine oil system.   

Brian

I wondered if oil not getting to the sump fast enough could be a possibility too.

The engine has about 1200 hrs SMOH, total time ~3200 hrs?  It varies with season, time when it works our well but I change every 30-50 hrs-more often in winter.  I put on 140 hrs or so the past year so it's been changed at least 3 times in the past year.

[carusoam]

Oil pressure indication takes three things...

1) oil flow generation, oil flow is generated by the gear pump... which lives in the accessory case... (old AD for aluminum gear wear)

2) oil flow restriction, oil flow is restricted by all the channels of the engine, and the big one at the end, an adjustable pressure regulator.

3) the oil pressure gauge, which includes its sensor and location...

4) oil pressure takes flow of oil, that gets constrained, then measuring it properly...

 

Things that can effect the oil pressure...

1) changes in oil viscosity...

• OilT
• oil age
• oil type
• exhaust bubbles
• air bubbles...
• (bubbles to the extent of causing foamy looking oil)

2) engine wear... (one way trip, not restorable by adding oil)

3) oil pressure regulator... (adjustable spring and valve)

 

One oddity of this discussion is the OilP challenge is related to the oil level, without it getting very low... ask @M20Doc?

Best regards,

-a-

[INA201]

5 hours ago, marcusku said:

Makes sense.  Where's a good spot to obtain model specific service manuals?  Does the factory manual cover the whole aircraft including the engine or are there separate various manuals?  I realize there's not a lot I can touch but I like to know how things are put together.

I have a 1978J and this is what I ordered. I researched out the parts but not 100% sure they are correct as I haven’t installed them yet.  The type of pressure regulator I have is the long one as opposed to short one(confusing I know)that sits on the top rear of the engine and is safety wired in place.  Mine allows for washers to be placed in it to help with raising the pressure.  Adding washers alone will help but I wanted to start with a fresh spring and ball then go from there.  

 

[nels]

I was told once and remember, as it’s true, raising the oil pressure comes at the cost of lowering oil volume to the moving parts. Pressure regulator just reduces flow. The pressure is usually measured at the oil pump side of the pressure regulator. The amount of oil at the other side of the regulator is what’s important.

[marcusku]

On ‎12‎/‎14‎/‎2018 at 1:33 PM, INA201 said:

I have a 1978J and this is what I ordered. I researched out the parts but not 100% sure they are correct as I haven’t installed them yet.  The type of pressure regulator I have is the long one as opposed to short one(confusing I know)that sits on the top rear of the engine and is safety wired in place.  Mine allows for washers to be placed in it to help with raising the pressure.  Adding washers alone will help but I wanted to start with a fresh spring and ball then go from there.  

 

Makes sense to start with that in your case.  In my case, I have good pressure when the oil is high so I wouldn't think it would change anything for me.

[marcusku]

On ‎12‎/‎14‎/‎2018 at 6:53 AM, N201MKTurbo said:

The oil pump is mounted to the accessory case. The inlet passage is cast into the accessory case and the sump. The passage passes through the sump gasket at the back of the sump. If that gasket seal is not perfect, it could be sucking air in at the gasket. I’m not sure if the oil level in the sump is above that gasket at 6 quarts or not, you can see if it is by taking the dipstick out and holding it next to the dipstick tube and seeing where the 6 quart line is.

This document also points to this as a possible problem.  http://www.rvplane.com/pdf/XP360_OverhaulManual.pdf 

For that to make sense with my symptoms the leak would have to be on the inside of the engine only though.  If it were leaking on the outside I would think it would draw some air all the time and the level wouldn't matter.

Looks like there are only two joints between the sump and the pump, as described above and between the pump and the accessory case.

[jetdriven]

On 12/14/2018 at 5:48 PM, nels said:

I was told once and remember, as it’s true, raising the oil pressure comes at the cost of lowering oil volume to the moving parts. Pressure regulator just reduces flow. The pressure is usually measured at the oil pump side of the pressure regulator. The amount of oil at the other side of the regulator is what’s important.

Old wives tale.  The regulator simply dumps excess oil above a certain pressure.

[INA201]

1 hour ago, marcusku said:

Makes sense to start with that in your case.  In my case, I have good pressure when the oil is high so I wouldn't think it would change anything for me.

Correct, but I seem to have your issue as well.  Just wanted to post the other option.

9 minutes ago, jetdriven said:

Old wives tale.  The regulator simply dumps excess oil above a certain pressure.

Is it true that there are no components between the oil pump and the pressure regulator other than the oil pressure sensor? Why do folks say things like,”the bearing tolerances may have shifted contributing to lower oil pressure.” 

[N201MKTurbo]

3 minutes ago, INA201 said:

Correct, but I seem to have your issue as well.  Just wanted to post the other option.

Is it true that there are no components between the oil pump and the pressure regulator other than the oil pressure sensor? Why do folks say things like,”the bearing tolerances may have shifted contributing to lower oil pressure.” 

The oil pump is a positive displacement pump. It will pump a volume of oil according to RPM. The oil goes from the pump to the two main oil passages, the prop governor and to the gear shafts in the accessory case. The pressure regulator just dumps excess pressure back into the sump. If there is a leak somewhere caused by a spun bearing or a gear shaft is loose or something else, the regulator will completely close and the oil pressure will vary with RPM at some pressure below the regulated pressure.