Low Oil Pressure

[plbarrett2004]

Hello all.

I'm flying my phase 1 period in my RV-8 behind an overhauled angle valve IO-360 A3B6D. Since the early hours of phase 1, the oil pressure has been on the low side of normal. (40's to 50's). I have made a couple adjustments along the way, each time seeing an initial rise in pressure, and then it settling back in to a lower pressure (albeit it is better than when I started).

The first few flights, the oil pressure was averaging 58 psi. Then after several more flights, the oil pressure settled down to around 48 psi. A bit low for my liking. So I pulled the oil pressure relief valve. I have the non-adjustable tall tower. Inside I found the 68668 2.04" Purple spring that is to my understanding normally for the short towers. This had 3 washers underneath. Over the next couple flights I increased this to 7 washers. This brought the PSI back up close to 60, but not quite. At this point, I decided a stiffer spring was in order. I installed the 61084 2.18" No-Color spring. This is the most common. I've done a couple iterations with this spring, including adding washers more than once. I believe I'm up to 7 washers. On today's flight, the average was about 55 psi.

It seems the washers make only a marginal difference. My biggest gain was going up in spring tension, so I'm contemplating now moving up to the LW-11716 2.12" White spring.

My question for the group is - am I missing something? Could something else be going on? Could it be a pressure sender issue? My instinct tells me the sender is ok, and that the next spring up will likely be the final solution. But perhaps I'm overlooking something.

The engine has been performing flawlessly so far. It's at 34 hours now, has great compression, low CHTs, good looking plugs, and great oil analysis. By every metric break-in has proceeded well.

Attached is today's oil pressure readings.

 

[hgerhardt]

Certainly check the oil pressure via a mechanical gauge to verify what the electric sender says.

I have exactly this engine in my RV and it typically runs 75 psi at 2300 rpm with oil at 180-190F with 20W-50. You say it's been freshly overhauled...what are the bearing clearances compared to what the table of limits calls out? On the high side of acceptable? That could contribute to low pressure when hot.

One other thing: that engine is a Dual-Mag which has a an oil hole in the bushing which supports the stub shaft of the mag gear. If you removed that mag and installed a cover, that oil hole is a significant internal leak. I fixed that on mine by machining a piece of round stock to the same dia as the mag gear stub shaft and bolted that to the cover plate so that the feed hole is plugged like a mag would do.

 

[FMF15E]

Same exact engine

I flew the same engine with the mags removed in an RV-8. About 25 hrs into flight I had a low oil pressure indication and landed immediately. I checked the bypass and cleaned the ball and spring. No problem since, but I have actually removed washers to lower the pressure to get a lower cruise oil pressure.

 

[plbarrett2004]

I will verify the sensor pressure with a mechanical pressure gauge, and I will also take a look at the logs for the bearing clearances - thanks!

I do have a magneto installed - a single mag that has been converted for operation in the dual mag accessory space.

 

[lr172]

My question for the group is - am I missing something? Could something else be going on? Could it be a pressure sender issue? My instinct tells me the sender is ok, and that the next spring up will likely be the final solution. But perhaps I'm overlooking something.

The engine has been performing flawlessly so far. It's at 34 hours now, has great compression, low CHTs, good looking plugs, and great oil analysis. By every metric break-in has proceeded well.

Attached is today's oil pressure readings.

Doesn't sound right to me. If you can only get 55 PSI with the stronger spring and 7 washers, I am guessing that something is not right. If the system capacity was where it should be, the stiffer spring and extra washers would have made more of a difference. FYI, on most lyc's 40-50 is not the low end of normal, that is BELOW the normal range, at least for conditions other than idle. Many reasons for this and not enough data here to speculate. Step one is to confirm the OP is accurate.
'
Is this a wide deck or narrow deck?

 

[plbarrett2004]

It's a wide deck engine. And I'm tracking 25psi minimum for idle operations, and 55-95psi for normal operations, and 115 maximum for startup taxi and takeoff. Oil levels are normal. Oil temps, CHTs, and everything else look good. But yeah, it makes me uncomfortable. I've attached charts for a couple other flights. Interestingly I got a slightly higher oil pressure when running lower MP and lower RPM on the last flight yesterday.

 

[turbo]

you could have a mechanical OP gauge also. mine is rock solid at 77 psi all the time. good luck.

 

[jrs14855]

Oil Pressure

It is very likely that you are getting a false reading on the gage. Look up the Marvel/Scott info on Lycoming oil pressure. The Cessna 172's built since the restart of single engine production take the oil pressure from the front of the right gallery. So 80 # on those aircraft is 70-72 at the rear of the gallery.
My personal minimum is 80# for non aerobatic and a bit more for aerobatic.
The higher pressure iv very beneficial for valve lubrication.
A note on the electric gages; the Vans style gages are voltage sensitive so if you lose the alternator expect to see a progressive drop in op as the voltage slowly drops.

 

[plbarrett2004]

Thanks for the input. Currently reading oil pressure with the G3X system - GEA24 to Kavlico pressure transducer.

I'm going to hook up a mechanical gauge tonight and verify sensor readings.

Found this from Marvel/Scott and gave it a read. Good info - thanks.

http://acversailles.free.fr/documentation/08~Documentation_Generale_M_Suire/Moteur/Soupapes/Augmentation_de_la_pression_d%27huile_sur_Cessna_172.pdf

 

[fl-mike]

what sump are you running?

 

[plbarrett2004]

Hi Mike. Horizontal intake and stock sump.

So tonight, I hooked up a mechanical pressure gauge and fired it up. I got identical readings from both gauges.

That done, I installed the LW-11716 2.12" White spring with 4 washers. The relief valve appears absolutely pristine in terms of there being no build up, and the mating surfaces are unharmed and clean. Initial run-up showed a higher psi than I had seen previously during start up - hitting 105 as a max with relatively cool oil. At this point, I decided to take it flying. The good news is, it essentially stayed above 55. (Attached graph from the flight) But not by much, and not as much as you would anticipate with the new spring, nor was the operating pressure as high as I would like to see. I would say it was an average of 58 to 60.

Its operating in the green. I looked through the paperwork and couldn't find notation of the tolerances. Bought the engine second hand and the shop that did the overhaul isn't around anymore. Not much i can do to find where the bearings were with regards to tolerances.

My options as I see it are:

1. Call it good for now. Not what I want to do.
2. Add more washers and see what I get.
3. Investigate some other cause - but what?

 

[fl-mike]

How many quarts are you running?
The reason I asked about the sump is that my Titan IO horz cold sump needs more oil than I was used to in the old plane with a standard vertical and I was sucking air.
The clue was that pulling g brought the pressure up (as the oil was pulled down to the sump pickup).
The G3X data recorder was great at connecting the dots....eventually!

 

[plbarrett2004]

Hi Mike. I'm running about 5 quarts. Give or take a little - I've got to re-calibrate the dip stick...

I did go back through earlier flights last night and noticed that the readings seemed to be more consistent earlier on - that is to say, less fluctuation. So, perhaps that is another symptom...

 

[lr172]

Hi Mike. Horizontal intake and stock sump.

So tonight, I hooked up a mechanical pressure gauge and fired it up. I got identical readings from both gauges.

That done, I installed the LW-11716 2.12" White spring with 4 washers. The relief valve appears absolutely pristine in terms of there being no build up, and the mating surfaces are unharmed and clean. Initial run-up showed a higher psi than I had seen previously during start up - hitting 105 as a max with relatively cool oil. At this point, I decided to take it flying. The good news is, it essentially stayed above 55. (Attached graph from the flight) But not by much, and not as much as you would anticipate with the new spring, nor was the operating pressure as high as I would like to see. I would say it was an average of 58 to 60.

Its operating in the green. I looked through the paperwork and couldn't find notation of the tolerances. Bought the engine second hand and the shop that did the overhaul isn't around anymore. Not much i can do to find where the bearings were with regards to tolerances.

My options as I see it are:

1. Call it good for now. Not what I want to do.
2. Add more washers and see what I get.
3. Investigate some other cause - but what?

You have gone through three higher strength springs and all sorts of washers and still the OP is lower than most see. To me, that is a sign of weakness in the oil system. That could be an under performing pump or sucking air on the input side of it OR it could be too little restriction on the output side. Numerous issues can cause that, like loose bearings or a leak in the system, like an internal plug left out, or various other issues that not limiting the flow of oil.

Hard to provide guidance. You are producing enough pressure to meet the minimums, but hard to say whether you should be happy with that. I wouldn't, but honestly unsure if I would tear the engine apart to address it. I would consider straight 40 or 50 wt oil, as that should bring up the hot oil pressure some. But you will have to play with the springs to get the cold oil pressures below the max. I would definately plug the oil feed at the unused mag, as recommended in another post of yous.

 

[mahlon_r]

Hi Mike. I'm running about 5 quarts. Give or take a little - I've got to re-calibrate the dip stick...

I did go back through earlier flights last night and noticed that the readings seemed to be more consistent earlier on - that is to say, less fluctuation. So, perhaps that is another symptom...

Try adding two more quarts. With it at 5 qts. give or take, you could be cavitating the pump a bit. If that doesn't help you definitely have an issue within the engine. My guess would be a missing 1/8 pipe plug. Are you sensing oil pressure at the normal location on the upper rear of the accessory housing? If not maybe you sense location might be confusing the issue.
I am assuming there is no acro/ inverted oil system hooked up to the engine, If there is bypass it and see if the problem stays with the engine or the acro system.
Good Luck, Mahlon

 

[plbarrett2004]

Thanks to all responses.

I'm pretty unsure what to do next as well. Tearing the engine apart... yikes. Definitely don't want to do that... but wanting to, and should could be two different situations... I'm not inclined to leave this be.

My sensing location is the standard location on the rear accessory case. There is no acro / inverted oil system. There is a mag installed currently. One mag and one EI.

I will try a couple more quarts tonight and see what I get.

 

[jrs14855]

Pressure

If you do not get positive results from the increased oil level I would stop flying the airplane.
One alternative to a complete teardown is to remove the accessory case and check the allen head plugs in the crankcase, hidden inside the acc case. A lot of work but way less than a complete teardown.
That you are not getting normal results from spring and washer changes is very concerning.

 

[plehrke]

Thanks to all responses.

I'm pretty unsure what to do next as well. Tearing the engine apart... yikes. Definitely don't want to do that... but wanting to, and should could be two different situations... I'm not inclined to leave this be.

My sensing location is the standard location on the rear accessory case. There is no acro / inverted oil system. There is a mag installed currently. One mag and one EI.

I will try a couple more quarts tonight and see what I get.

Can you plot your oil pressure vs RPM? Pressure will vary with RPM change. In section 5.3 of the Lycoming Operator’s Manual it states that you need to have greater than 55 psi and less than 95 psi at 2000 RPM with a warmed up engine and oil. Also 25 psi is listed as the idle pressure so to me idle is 600-800 rpm.

This thread has caused me to pay a little more attention to my oil pressure. I have always gotten 62-65 psi any time above 2000 rpm but noticed that during decent with rpm around 1600-1800 I am more like 48-52 psi. If I draw a straight line between the two points that Lycoming lists (25 @ 600-800 and 55 @ 2000) my decent of 1600 rpm the oil pressure should be over 45 psi so mine appears to be fine.

Does anyone know where there is a Lycoming plot of RPM vs oil pressure? Quick Google search turned up nothing for Lycoming, plenty for cars. Is the pressure vs rpm linear? Probably not.

 

[plbarrett2004]

Ok - well it sure seems that adding 2 quarts did the trick. See attached oil pressure graph from the flight. Max of 107psi on take off. Averaged about 81 psi throughout the flight. Thanks to everyone contributing here!

That said, I now have a few questions.

1. I confess I don't really understand this, but i want to learn. If the pickup is low, and i'm told these things can run down to as low as 2 quarts (not that you want to or should), how is it cavitating at 5? Is it because of the amount of oil being drawn out of the sump?
2. What is the next step? It appears that now i might be a little on the high range. Stick with what i've got? Take washers out? Go to the next size spring down (61084 2.18" - No color)?
3. I had a local A&P recommend that its time (35 hours) to move to AD oil. This seems to be a good opportunity to do so. Any reason I shouldn't - such as, perhaps I should nail down pressures before making the change?
4. Do these results mean that cavitation is in fact the issue? Further investigation...?

 

[plbarrett2004]

Chart with RPM from this afternoon.

 

[fl-mike]

Great news!
IMO, Five is just not enough. You have filter volume, the prop hub, everything flying around the engine, and that soda straw sucking down the local sump level around the pickup.
You now know that if you are showing five, you need to add, so that is a good data point.
In my case, once I saw the interior of the sump, it all made sense as there is not a deep well around the pickup and a pretty broad and flat sump.
I would probably turn down the pressure a bit, but others can chime in. You for sure don't want to risk blowing out a crank or other seal.

 

[jrs14855]

Oil

The standard vertical sump requires a minimum of between 2 and 3 quarts. The horizontal sump, 360A style requires a lot more.
Some aerobatic users have cut off the sides of the wide sump and welded covers on because of the issues with aerobatics.
The over 100 on first takeoff and then down to 80 is not what I would expect to see in relatively warm weather.
My personal standards I would be very happy with 80 in cruise.

 

[lr172]

Ok - well it sure seems that adding 2 quarts did the trick. See attached oil pressure graph from the flight. Max of 107psi on take off. Averaged about 81 psi throughout the flight. Thanks to everyone contributing here!

That said, I now have a few questions.

1. I confess I don't really understand this, but i want to learn. If the pickup is low, and i'm told these things can run down to as low as 2 quarts (not that you want to or should), how is it cavitating at 5? Is it because of the amount of oil being drawn out of the sump?
2. What is the next step? It appears that now i might be a little on the high range. Stick with what i've got? Take washers out? Go to the next size spring down (61084 2.18" - No color)?
3. I had a local A&P recommend that its time (35 hours) to move to AD oil. This seems to be a good opportunity to do so. Any reason I shouldn't - such as, perhaps I should nail down pressures before making the change?
4. Do these results mean that cavitation is in fact the issue? Further investigation...?

If a normal oil level (5 is far from abnormal) produces low OP and a high oil level produces a high OP, then you have a leak in the oil pickup tube or structure that is at a point between those to level lines in the oil pan. When the oil level drops to a certain point, the leak allows air into the stream going to the pump. It is not cavitation per se, but the fact that air is compressible and oil is not and therefore the pump performance drops significantly when air is present in the oil.

Most modern lyc's have the oil pickup cast into the oil pan. Possible yours was a bad casting with some type of hole in that area of the casting part way up the pan.

FYI, there are numerous different sump designs with different requirements. Some are fine with as little a quart in the pan and others need two or three quarts in the pan to work correctly. Be cautious of generalized advice.

The problem is that you now NEED 6-7 quarts to get good oil pressure and most engines once over 6-7 quarts start blowing it rapidly out of the breather and will need to watch the oil level more closely.

Larry

 

[jrs14855]

Oil

Lycoming o 360 A4 has the deep sump and the minimum oil level is 2 quarts. I cannot find an exact number for 360 angle valve but it is much higher.

 

[fl-mike]

FYI, there are numerous different sump designs with different requirements. Some are fine with as little a quart in the pan and others need two or three quarts in the pan to work correctly. Be cautious of generalized advice.

Larry

yes, and like I said, The Titan design seems to need more in the system due to the pickup design. Not sure what exact sump the OP has.
Here is the Titan. I think it is pretty obvious that that sump will suck air pretty easily if the level is low due to local pull-down. Pulling g made my oil pressure stabilize. Makes sense.
Followed by the Superior, that has a very deep well around the pickup point. Pretty sure that will not be as level sensitive.
And finally, the Lycoming IO sump, that is in-between.
I think those "wings" contribute to the issue vs a deep carb sump that I was used to and could run 5-6 all day long with no issues. As someone noted, some of the acro folks cut those off.
Neither of us had "low" oil pressure per the Lyc range, but the fluctuations were confusing. I was told numerous times that it was fine, but something was "off" and I eventually figured out why.

 

[hgerhardt]

If a normal oil level (5 is far from abnormal) produces low OP and a high oil level produces a high OP, then you have a leak in the oil pickup tube or structure that is at a point between those to level lines in the oil pan. When the oil level drops to a certain point, the leak allows air into the stream going to the pump. It is not cavitation per se, but the fact that air is compressible and oil is not and therefore the pump performance drops significantly when air is present in the oil.

Most modern lyc's have the oil pickup cast into the oil pan. Possible yours was a bad casting with some type of hole in that area of the casting part way up the pan.

FYI, there are numerous different sump designs with different requirements. Some are fine with as little a quart in the pan and others need two or three quarts in the pan to work correctly. Be cautious of generalized advice.

The problem is that you now NEED 6-7 quarts to get good oil pressure and most engines once over 6-7 quarts start blowing it rapidly out of the breather and will need to watch the oil level more closely.

Larry

"Be cautious of generalized advice". Spot-on with that part!

Had plbarrett2004 said he was running 5 qts in his IO-360-A3B6D in his initial post, I would have said that's 'normal' behavior for the horizontal induction sump that engine comes with. My own IO-360-A3B6D behaves the same way, as I found out a while ago when I had the same symptoms. I noticed during a forward slip that the pressure was sagging a bit and then came back while straight and level. Can't run these sumps much below 5 qts or they start sucking air, and it's not due to a casting flaw. It's due to a DESIGN flaw.