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miroc
8th May 2011, 20:26
I have seen high oil pressure on my last flight. It was near 90 psi all the time (Cessna 172S). Before it was rather in 50-70 psi region.

The oil is AeroShell 15W50, changed in October and flown about 15 hours since.

I was told the lubrication system could be blocked on some place. That would be bad.

Any ideas?

stiknruda
8th May 2011, 20:38
Miroc - what engine is in the c172?

miroc
8th May 2011, 20:56
It is a Lycoming IO-360-L2A

Miroc

stiknruda
9th May 2011, 06:34
The 360 oil system is really quite simple. An engine driven oil pump circulates lubricant through a series of galleries and oil ways/ports. The oil finds its way back to the sump via gravity and the pump lifts it and pressurises it again.

To prevent too high a pressure, a pressure release valve (prv) is fitted. The prv is sited on the rear of the right hand crankcase above and behind no 3 cyl. It comprises a stainless steel ballbearing and a spring. The ball is forced by the spring against an oil port. When the oil pressure is sufficiently high the ball if forced off its seat and oil is allowed to bypass this restriction thus making sure that the pressure never gets too high.

My first suspicion would be that either the prv has been adjusted to improve pressure or something is preventing the ball from becoming unseated. It's usually a yellow coloured housing with from memory a 7/8" hex as part of the casting. It should also be wire-locked. Not sure how much maintenance you are allowed to do in yr country but your engineer will be able to inspect it in a a few minutes.

Too high an oil pressure is just as bad as too low an oil pressure. 90 is at the top end of the Lycoming scale.

Hope that helps.

A and C
9th May 2011, 07:32
It would be very unusual for a Mechanical problem to result in high oil px when the engine is hot, the oil you are using is top quality and has a very stable viscocity at high temps but is unlikely to be the cause of the high px indication.

I think that you should look at the indication system, if it is one of the electricly driven gauges it is likely to be the problem.

As stiknruda says you can adjust the PRV but this should only be done if you have a calibrated gauge rigged into the oil system DO NOT trust the cockpit indications as they may be the problem.

miroc
9th May 2011, 07:43
stiknruda
My first suspicion would be that either the prv has been adjusted to improve pressure or something is preventing the ball from becoming unseated.

For me it looks like the release valve is working as it should, that's the reason why the pressure stays at 90 psi for every rpm.

If the passage of oil is partially blocked, the pump is pressing the oil through smaller throughput causing higher pressure. This forces the oil through the release valve.

My concern is that part of the engine could not be lubricated...

Is there a way how to look after this?

IO540
9th May 2011, 07:48
I agree regarding the indicators being possibly duff.

I had this issue for a year or two after I got my plane from new. The oil pressure indicator uses a transducer which has a vent hole and it gets moisture in there which corrodes the resistor bridge.

Socata released an SB for it, which mistakenly asks for the connector to be sleeved, so they never sussed that there is a hole in the side of the connector base :)

Incidentally, what exactly is the problem if the oil pressure is too high? Could it crack the crankcase if it was say 1.5x too high? That would suprise me. What could happen is that an "excessive" oil pressure could cause more engine heat to be absorbed by the oil, which would reduce the CHTs but would need a bigger oil cooler to maintain the oil temp to a reasonable level.

miroc
9th May 2011, 07:58
Incidentally, what exactly is the problem if the oil pressure is too high? Could it crack the crankcase if it was say 1.5x too high?

The oil cooler as the probably weakest part of lubrication system will blow. Or the hoses.

The indicator can be the the problem. Need to check this. I flew ca. 4 hours with the high indication. If the engine would not be properly lubricated...

stiknruda
9th May 2011, 08:12
Too high a pressure: as Miroc points out, the oil cooler would be susceptible to damage/bursting but there is a phenomena were the main journals can suffer from a form of cavitation if the oil pressure is too high, this ruins the bearing shells.

IO540
9th May 2011, 08:17
Interesting...

The hoses should be OK because all hoses in common use are 1500psi working pressure. Most people use Teflon hoses nowadays which have a practically unlimited life, too.

The oil cooler I can well believe...

But cavitation of the bearings? Are you sure this is not caused by bits of the camshaft carried by the oil? ;)

stickandrudderman
9th May 2011, 19:19
High oil pressure will cause oil not to be contained at the weakest point in the lubrication system.
That could be a seal, a gasket, the oil cooler, a sensor or anywhere else.
A simple analogy:
Fill a bicycle pump with water, put your finger over the end and push the plunger. Without too much effort the pressure created will overcome your finger's ability to contain it and presto: a leak!
Now consider the same pump with several different holes in it and several different people with their fingers covering the holes. Whoever allows the fluid out first is the weakest!
If the sensor is an electronic one, a simple test is to power up the instruments without starting the engine. If your oil pressure goes up you have an instrument/sensor problem. If it only goes up once you start the engine then you probably have a stuck PRV .

BackPacker
9th May 2011, 20:34
Just out of curiosity. Is the Pressure Relief Valve a safety valve, only to be activated in case of a too-high oil pressure, to protect other components from rupturing/breaking? Or is it a regulator valve to keep the oil at a certain pressure during normal operations?

stiknruda
9th May 2011, 20:51
BP - it's a safety valve - it only operates when oil at acertain pressure dislodges the ball from its seat. The spring that holds the ball in place can be tensioned by screwing the adjuster in and out - in (clockwise) to increase the tension and "jam" the ball harder against its seat.

If it is a Westach guage, the output voltage from the transducer of 0.5V should give 0 psi. And 4.5v gives full scale deflection.

So by disconnecting the transducer from the electrical plug and putting 0.5v to the output the needle should lift off the stop and read 0psi. Up the voltahge to 4.5 and the needle should read fsd.

You'll now know if your guage functions.You can test the transducer in a similar way with amultimeter and an airline.

Don't ask how I know this!!

Allegedly oil cavitation on bearing shells leaves strange witness marks on the bearing face - a bit like corrugations.

BackPacker
9th May 2011, 21:03
BP - it's a safety valve - it only operates when oil at acertain pressure dislodges the ball from its seat.

I thought so. But in that case, why would anyone want to mess with the setting of that valve once it's been calibrated by the factory? (Except to re-calibrate it to the same value?)

You said "either the prv has been adjusted to improve pressure" but I can't place that, unless you suggest that the prv previously had been set to open at 50-70 psi (which is apparently well within the green arc) and has now been reset to the 90 psi setting (which is apparently at the top of the green arc).

But that would suggest that it's a regulator valve, not a safety valve.

Zulu Alpha
9th May 2011, 21:18
I'm not sure its a pressure relief valve. I think it regulates the pressure in normal operation.

I saw an engine that had come back from overhaul where the ball valve had been jammed. The oil cooler had bulged out massively and then failed, so normal operation of the engine had taken the oil pressure way high. This would indicate that the ball valve works as a regulator.

I would be very suspicious of the gauge. I had one suddenly change on me once for no reason. It was a Westach. These require the readout to be adjusted for the particular oil pressure sender unit. This has to be done at the factory I was told, so you can't just change the sender or gauge as they come as a matched pair.

You didn't say which gauge it is.

stickandrudderman
9th May 2011, 22:17
A PRV's sole function is to prevent over-pressurisation.
The engine itself, that is its' tolerances, clearances and wear, is the regulator which is why a worn engine will give lower oil pressure for a given oil viscosity.

IO540
10th May 2011, 06:56
I think there is some ambiguous terminology here.

The Lyco engine has a gear pump for the oil. This means the volume flow rate is proportional to the RPM.

This is no good for lubing the engine, which needs a relatively constant supply of oil over the operating rev range, and a constant volume feed would just give you a massively high oil pressure at anywhere near operating revs.

So one has the pressure regulator valve which is a spring loaded thingy which controls the pressure at that point in the oil system.

At any RPM significantly above idle, this valve is partially open and is regulating the pressure.

The actual oil pressure varies around the engine and will be according to how knackered the engine is. A worn out engine will offer less resistance to oil flow (through the various bearing surfaces).

Also, the point where the pressure gauge is fed from varies according to the engine.

gasax
10th May 2011, 07:29
IO's got it. The valve is a pressure regulator, not a safety valve.

If the valve has been messed about with (typically screwed down to camoflage engine wear!!!) You can usually spot this with a cold engine. The oil pressure will be roughly proportional to the engine speed - until you reach a point at which the valve lifts.

I've been through this process with a friend's aircraft - where after a fair bit of work being carried out the oil pressure was very high. In that case even when hot the oil pressure was proportional to engine speed. A couple fo minutes with the manual showed that the engine had aquired a number of spacers below the cap......

miroc
10th May 2011, 08:06
The PRV is not a regulator, it is a pressure limiting device in this particular case. The engine does not need to strictly regulate the pressure.

The oil pump must have enough capacity to press the oil through the pipes at idle RPM to lubricate all the places. That's the low limit.

At high RPM the pumped volume will cause pressure to be still below the top of green arc, if the oil has normal viscosity. That means at RPM on top of green the pressure will be in green (always was in my engine). If the oil is cold, the resistance in pipes is high, pressure would rise to the top of green arc and that's the point where the limiting device the PRV will kick in and cut of the excess volume to keep the pressure at the set value. Until this moment it is completely closed. Technically it is a D-type regulator (D=derivating, other types are P=proportional and I=integrating).

If during the normal operation let's say 2350 RPM and warm engine the oil pressure was at ca. 60 psi before and now it is 90 psi, that is a signal something is wrong. It can be the gauge. Or it can be that the resistance in system is too high - something is blocked - some parts of the engine are not properly lubricated.


I will check the gauges. It is a G1000, wish me good luck.

If the gauge is OK, the problem is bigger. Did you hear in the past about blocked pipes?

PAPI-74
10th May 2011, 08:49
Single Grade Ashless Dispersant Oils - No general rule exists for every engine type when choosing the correct grade of oil; however, based on the average ambient outside temperature at engine start-up the most common grades are SAE 40, SAE 50 and SAE 60, which equates to AeroShell Oil W80, W100 and W120 respectively.

Traditionally, the choice is associated with climatic zones - AeroShell W80 for cold climate regions (-17 - 21°C), AeroShell Oil W100 for temperate regions (15 - 30°C) and AeroShell Oil W120 for warmer climates (above 26°C).

It maybe that you have gone from a SAE40 (W80) to the multi-grade. What was the previous oil?
Is the CHT & Oil Temp normal?

miroc
10th May 2011, 09:41
It maybe that you have gone from a SAE40 (W80) to the multi-grade. What was the previous oil?
Is the CHT & Oil Temp normal?

No, the temperatures in my region are roughly the same as in UK, previous oil was AeroShell W100 with Lyco additive (prescribed). With this the oil pressures were rather low if idle and warm 45-48psi. At higher RPM between 50 and 70 psi.

The CHT are between 260 and 350 F. I have never seen more than 375 F.
Oil temps are up to 170 F, never seen more.

Miroc

A and C
10th May 2011, 14:47
Quote:-If the sensor is an electronic one, a simple test is to power up the instruments without starting the engine. If your oil pressure goes up you have an instrument/sensor problem. If it only goes up once you start the engine then you probably have a stuck PRV .

This statment is untrue in modern indication systems, these systems have an expected operation range so for instance zero PSI might register as 5 volts at the G1000 and 90 psi as 25 volts this way if there is a problem and the PX sender is telling the G1000 a voltage that is above or below the range the system troubleshooting part of the set up program will tell you that the indication is not plausable and so point to a px sender or wiring problem

IO540
10th May 2011, 15:37
If the gauge shows a nonzero pressure reading with the engine stopped then I would say it is definitely an instrument problem :)

Fortunately, a load of G1000 installation manuals have found their way out into the wild ;)

miroc
10th May 2011, 16:01
Is it possible that the high pressure is caused by dirty/blocked oil filter? This filter is the first resistance in the lubrication system after the pump. Have you ever seen that?

BTW that would be the cheapest solution, to change the oil and filter. :E

I can not positively recall if the reading was zero with engine standing still. But probably was as it would catch my attention if the gauge before the engine start would be in unusual position. Will check next time.

The instalation manuals are worth reading too...

If everything fails and the anomaly can not be diagnosed, what would you do? Overhaul?

Zulu Alpha
10th May 2011, 16:09
Might be a blocked filter. Filters have a pressure relief valve and maybe this is set to 90psi.

New oil and filter is a fairly cheap option.

If the filter is blocked then you do have a big problem, but cutting the old one open will allow you to check this.

I do still think the valve is a regulator rather than a pressure relief. ie it almost always has some oil flowing and the amount that flows controls the pressure. Otherwise the oil pressure would change a lot between idle and full rpm.

Also, I would suspect the oil pressure sender rather than the G1000. It might still read correctly at zero pressure but not be correct at high pressure. The only way is to test it or swap it.

john ball
10th May 2011, 16:16
The oil pressure when hot in my 0-360 engined RV6 is normally 75 to 80 lbs at normal cruise RPM of 2300 to 2400.

What I have fould is that switching from W80 and W100 to Shell 15/50 multigrade has reduced my oil consumption from 1qt every 7-8 hours to 12 -14 hours.

Of course we are all assuming that the gauges are accurate ?

Regards
JOHN