Question regarding low oil pressure

[Pritam]

Hey guys,

I'm preparing for my checkride in a C-162 and I noticed that my POH says this "If the low oil pressure indicator (OIL PSI) turns red, and oil temperature indicator (OIL °F) remains normal, it is possible that the oil pressure sending unit or relief valve is malfunctioning".

I spent quite a bit of time reading up on relief valves, but I still don't understand how a malfunctioning relief valve would explain this behavior. As per my understanding, the relief valve is used usually to avoid high oil pressure scenarios where it opens up to divert away some of the excess oil. In the case that it is malfunctioning, I'm assuming it opens up at a lower pressure resulting in not enough oil flowing through the system which shows up as low oil pressure on the indicator. Although, if this is the case the engine should also be heating up due to lack of oil and as a result the oil temperature indicator should go up as well. As a result, I'm confused how the temperature remains normal ?

I'd really appreciate some help here.

 

[Capt. Geoffrey Thorpe]

Low oil isn't no oil.

 

[Pritam]

Low oil isn't no oil.

But even if you're low on oil, shouldn't engine temperatures go up due to insufficient lubrication which in turn would increase the oil temperature ?

 

[LongRoadBob]

I'm not sure, but say the malfunction is that it is opening up not due to high pressure, but at normal pressure. When opened there is then even lower pressure (pressure drops) but not overheating? Relief valves, kind of like fuses, maybe can weaken and open when faulty at much lower pressure?

Just an uneducated guess on my part here. they do say "malfunctioning" and so you cannot treat it as if it were working as designed. Other malfunctions I can think of would all be leaks of some kind bleeding off pressure...but they could happen anywhere, at connections, valves, etc.

Ok, one more, even the mechanism that moves the needle itself?

 

[Pritam]

I'm not sure, but say the malfunction is that it is opening up not due to high pressure, but at normal pressure. When opened there is then even lower pressure (pressure drops) but not overheating?

I guess this is probably where I'm confused. Is it possible to have low pressure but no overheating ? My assumption was that the low pressure implies insufficient oil reaching the engine for lubrication and hence it should overheat.

 

[GlennAB1]

But even if you're low on oil, shouldn't engine temperatures go up due to insufficient lubrication which in turn would increase the oil temperature ?

Low oil quantity could result in a slight temperature increase due to insufficient cooling of the oil because it is being forced to recirculate through the engine more rapidly.

 

[LongRoadBob]

I guess this is probably where I'm confused. Is it possible to have low pressure but no overheating ? My assumption was that the low pressure implies insufficient oil reaching the engine for lubrication and hence it should overheat.

Yeah...I see your point there. I'm not sure, best I wait here with you until someone comes with the answer.
As I said, mine were just guesses. Oil pressure has to be calibrated so the needle is showing correct pressure, and I guess that could be off and indicate low pressure where the actual pressure was still normal.

But if there was in fact low oil pressure, I'm not sure where the breakpoint of how low it would be before significant heating happened. I better just shut up now I think...will check back here because now I'm interested in the answer too. Good question!

 

[Chip Sylverne]

But even if you're low on oil, shouldn't engine temperatures go up due to insufficient lubrication which in turn would increase the oil temperature ?

The oil in the sump acts as a heat sink. Since the pressure drop is due to the relief valve and not loss oil in the sump, it'll still absorb and shed heat. Loss of oil volume in the sump should result in higher oil temps, but in the real world it doesn't always work that way. I lost oil pressure in a Cherokee on final once, shut it down, glided in and found 2 qts in the sump and the belly covered in oil. Oil temp never went up a degree over normal.

 

[Capt. Geoffrey Thorpe]

I guess this is probably where I'm confused. Is it possible to have low pressure but no overheating ? My assumption was that the low pressure implies insufficient oil reaching the engine for lubrication and hence it should overheat.

Lubrication doesn't fail the instant that pressure drops - you can even run for tens of seconds with no oil pressure at all with no measurable damage. If the pressure is just low, it will still be circulating and providing some lubrication. Also, a leaky bypass valve may still be flowing oil through the cooler, etc.

 

[Tom-D]

Pressure is = the resistance to flow. when the relief valve is stuck open, there is no resistance to flow.

 

[texashikergal02]

"I'm preparing for my checkride in a C-162"

I'd hazard a guess you might want to have that checked out before your checkride, cause your DPE will obviously grill you on that.

 

[Dan Thomas]

The relief valve is the pressure regulator of the oil system. The oil pump is designed to pump more oil than the system needs, and the relief valve, which is just a plunger (a steel ball in Lycomings) with a spring behind it that forces it against a seat. The pressure in the system pushes the plunger (or ball) off the seat when it reaches the right pressure, and the oil that gets through the opening goes back to the sump. It's nothing more than that. Imagine a hole in the side of your garden hose, and you have to hold your thumb over it to generate pressure in the hose so the sprinkler will work. If the pressure in the hose gets too high, it will force your thumb away from that hole and let some of the water out, reducing the pressure. If you want more pressure, you have to press harder on the hole. In the engine, if more pressure is desired, the spring behind the plunger or ball is shimmed, or on some Lycomings, it's adjusted with a screw. In the OP's case, the low pressure "due to a malfunctioning relief valve or an indication problem" is caused by a sticking relief valve or broken spring or the oil pressure gauge or sender is defective.

If the spring breaks, pressure falls badly. If the engine wears so that the bearings have much more clearance, so much oil might flow that the pump can't keep up and the pressure falls even though the relief stays closed altogether. If the plunger sticks in its bore, which they can if the system gets sludged up enough over time, it might stick open and let enough oil out that pressure can't build. Lycoming's ball generally doesn't do that.

Even low pressure is sufficient to get oil to where it's needed. Lots of old, small Continentals happily fly around on 25 PSI.

Continental's looks like this (parts 31 thru 34):

And it works something like this, except shims instead of a screw make the pressure adjustment:

Lycoming's looks like this, either with shims or the screw:

The typical pressure system looks like this. Note the pressure relief in the bottom right corner. In a Continental, the relief is at the end of the system, not the beginning. Doesn't make too much difference.

 

[LongRoadBob]

The relief valve is the pressure regulator of the oil system. The oil pump is designed to pump more oil than the system needs, and the relief valve, which is just a plunger (a steel ball in Lycomings) with a spring behind it that forces it against a seat. The pressure in the system pushes the plunger (or ball) off the seat when it reaches the right pressure, and the oil that gets through the opening goes back to the sump. It's nothing more than that. Imagine a hole in the side of your garden hose, and you have to hold your thumb over it to generate pressure in the hose so the sprinkler will work. If the pressure in the hose gets too high, it will force your thumb away from that hole and let some of the water out, reducing the pressure. If you want more pressure, you have to press harder on the hole. In the engine, if more pressure is desired, the spring behind the plunger or ball is shimmed, or on some Lycomings, it's adjusted with a screw. In the OP's case, the low pressure "due to a malfunctioning relief valve or an indication problem" is caused by a sticking relief valve or broken spring or the oil pressure gauge or sender is defective.

If the spring breaks, pressure falls badly. If the engine wears so that the bearings have much more clearance, so much oil might flow that the pump can't keep up and the pressure falls even though the relief stays closed altogether. If the plunger sticks in its bore, which they can if the system gets sludged up enough over time, it might stick open and let enough oil out that pressure can't build. Lycoming's ball generally doesn't do that.

Even low pressure is sufficient to get oil to where it's needed. Lots of old, small Continentals happily fly around on 25 PSI.

Continental's looks like this (parts 31 thru 34):

And it works something like this, except shims instead of a screw make the pressure adjustment:

Lycoming's looks like this, either with shims or the screw:

The typical pressure system looks like this. Note the pressure relief in the bottom right corner. In a Continental, the relief is at the end of the system, not the beginning. Doesn't make too much difference.

Damn...what an excellent turtorial on the issue! I'm not the OP, and was thinking along these lines, but only in general, that a leak or the gauge itself becoming un calibrated, etc. but became interested in the eventual explanation and this is sure it! It is a very complete explanation.

With visual aids even. Thanks so much! It's good to know this too. Others here also came with the correct explanations as to how it would still be able to keep the temp. In the green in some cases, but this is the ultimate answer.

 

[luvflyin]

But even if you're low on oil, shouldn't engine temperatures go up due to insufficient lubrication which in turn would increase the oil temperature ?

I've heard that lower oil quantity can make for lower temperature. Reason being that less oil means all of it will get through the oil cooler more often. Of course it can't be so low that pressure and lubrication suffer. It's probably not that simple but it does have a logic to it.

 

[Timbeck2]

I don't know about you guys but I'm really starting to like this Dan Thomas character.