Oil pressure gauge advice

[QDMQDMQDM]

Here's a question and if anyone comes up with an answer which consists entirely of a sanctimonious 'You should take your aircraft to be scrutinised by a licenced engineer' they can go and boil their heads. ;) I'm just asking a question, OK?!

More or less since I've owned my aircraft (65 hours of flying time), the oil pressure gauge has not been entirely consistent. (It's a 150HP Lycoming O-320 A2B on a PA 18.) Sometimes I'll start up and do the run-up with the oil pressure near the top of the green. Then, it might move to the upper orange for the take-off and 10 minutes afterwards (at 2250rpm) until the oil warms up and it comes down to the upper green. Why is this? Why will it sometimes start and run in the green and sometimes in the orange? Sometimes it will start int eh green and remain in the green. Sometimes not. It's never very consistent, until a while into the flight. Is this lack of consistency due to:

a) Cold engine, cold oil?
b) Sticking gauge?
c) Malfunctioning oil pressure relief valve?
d) Something else?

All through, I should say, the engine has run sweetly and smoothly and has only ever started first time. It's a beautiful, low-time engine, running 20/50 Multigrade and uses very little of it.

QDM

[Keef]

I'd guess it's one or more or all of those things. Maybe also various fluids condensed into the oil that then clear out later.

Ours does the same - except that it never goes out of the green (gets very close) so we don't worry about it.

I've not watched it carefully and consistently enough to identify any trends or causes: as far as I know, it could be totally random.

Engine likewise sweet as could be, and now over 2,100 hours into a 2,000 hour TBO.

[Circuit Basher]

I am not a licensed a/c eng, but......

if I saw that sort of symptom on a car of mine, I'd be strongly suspicious of solid contaminants (of the gungy / metallic variety) either around the seat of the pressure relief valve or possibly in the pipework to the gauge. It could also be a faulty gauge (over reading is unusual as a result of the formerly mentioned problems - they're more likely to cause under reading). If it's on a Private C of A, I'd check the pressure relief switch, then flush the gauge pipework.

Alternately.....
go talk to your friendly neighbourhood licensed a/c eng. ;) ;) The reasoning for this is that one of your piston rings may be breaking up, causing the crankcase to become pressurised by the combustion chamber pressures.

Personally, for the sake of an hour of the guys time to have a look and assess the problem and give you a judgement as to whether it is potentially hazardous to continue flying, I'd get me cheque book out. I pay insurance companies for similar reasoning!!!

[LowNSlow]

QDMx3, I assume that upper orange is higher than the green? (Sorry if it sounds daft but I'm used to gauges calibrated in psi rather than colours.)

If I'm right in the above assumption, I'd take your aircraft to .......

Seriously, I would assume that if the engine has low hours the clearances between bearings etc are still tight/close. A low ambient temperature at start up will cause the thicker oil to flow less quickly through the engine causing a higher than "normal" pressure reading.
Check the OAT before startup and see if lower temps correlate to higher oil pressure indications.

To check for a sticking gauge try the time honoured method of tapping the glass! However if it was a sticking gauge it would remain at the higher reading rather than falling back to the lower reading as the engine warms; ie you would have had to have had a higher reading to get the gauge to stick in the orange in the first place.

It could be the oil pressure relief valve sticking closed until the engine warms up and it frees itself. If this is the case it might be worth pulling the valve and checking it cos if it REALLY sticks you'll have blown gaskets/seals and there might be a loud bang followed by an expensive silence after your oil has jumped ship. This scenario demonstrates the fact that the propellor is only there to keep the pilot cool. You'll be sweating if it stops especially if your windscreen is covered with the contents of your oil sump! A person I know had 90% of the contents of the sump disembark VERY quickly after the feed pipe to the oil pressure gauge came off. Fortunately there had been a pre-existing oil pressure fluctuation problem so she was watching the oil pressure gauge like a hawk and spotted it the instant the pressure fell rapidly off the clock. Fortuantely a successful landing ensued with no further damage.
Before starting a panic remember that it is far more likely for the oil pressure relief valve to stick open rather than closed resulting in the inability to maintain oil pressure when the engine and the oil are hot

Lastly it could be your oil filter is getting choked and if the ambient temp (again) is low, the cold oil is having a bit of trouble getting through it.

[Windy Militant]

QDM,
please don't take LowNSlows advice It could turn nasty! Never ever tap the glass, tap the panel next to it or the bezel of the gauge but never the glass. Having to pay for a new glass or even gauge is bad enough, but the embarressment of explaining to your friends and the nice people in casualty how you did it......
;)

[nonradio]

3Q: even us 65HP boys have the same: slightly higher when cool and lower when hotter (especially on hot days!) I use single grade 50 wt oil, not a multigrade, so I can't comment on the effect that might have. I would say the steady state behaviour ie warmed up,stabilised cruise figures are the best guide. Does it have the same pressure at idle after start as when idling after warmed up?
Wish mine used v. little oil though.
LnS: ditto WF re tapping of glass, tut, tut. Also, it sounds like your chum's oil gauge line didn't have the required restricted 'ole - or else the fitting at the crankcase blew out!

[LowNSlow]

Point taken re: tapping the glass chaps. I did mean tap gently though not rapping it with the knuckles, honest :D :D It would indeed be highly embarrassing to pay a visit to Casualty with gauge glass sticking out of ones bleeding digits/knuckles......

The fitting did indeed come off the crankcase nonradio and what a mess oil makes of an airframe when it's on the outside of the engine rather than the inside! :eek:

nonradio, agree about the steady state indications giving the best guide but instantaneous high pressure at start up could still blow seals etc if it gets into the red and the seals are a tad tired.

My old 65hp Cub was wonderfully oiltight. My Ingenier friend did a great overhaul on it. The Cirrus in my Auster is another story although it is a lot better than some I've seen.

[Don D Cake]

QDM3

Try a post on the Piper Owners Society forums

Piper Owner Society (http://www.piperowner.org/new/forum/forums.htm)

[QDMQDMQDM]

Thanks, everybody. Very helpful. I'm inwardly digesting and I'm going to keep a diary of what it actually does from now on.

QDM

[englishal]

go talk to your friendly neighbourhood licensed a/c eng. The reasoning for this is that one of your piston rings may be breaking up, causing the crankcase to become pressurised by the combustion chamber pressures

If this is the case the oil will be very black, in a short time......PROBABLY just a guage thing and nothing to worry about. Still better to be high than low eh?

(Saying that though, all the core plugs blew out of my car engine, spewing oil everywhere, and catching fire. Well, it was an old engine, and to ensure I made it round the M25 I added about 2 litres of extra oil....possibly not a good idea;))

Cherio
EA:D

[LowNSlow]

englishal probably not the core plugs old chap (LNS slips pedants hat on) the core plugs are in the water cooling cirucit in the engine block. With all that excess oil you probably blew every oil seal in the engine as well as having it coming out of the engine breather! (hat off). I'll bet having an engine fire on the M25 was fun, not :eek: At least you wouldn't have been going very fast at the time :D :D

[QDMQDMQDM]

If this is the case the oil will be very black, in a short time......PROBABLY just a guage thing and nothing to worry about. Still better to be high than low eh?

I change oil and filter every 25 hrs and it is barely discoloured.

QDM

[jason_beall]

What you are reporting sounds normal to me. The colder the oil, the more viscous and the harder time it has gettign past the oil pressure relief valve. It thins as it warms. Check the POH or engine manual...as long as it's within limits it's probably ok. My Lycoming engine manual notes max startup press, min idle press., min flight, etc.

My guess is the times you start it up and it only comes into the green is when the engine/oil is warm? Probably goes to high end on cold mornings?

-Jason

[A and C]

It would seem to me that this engine requires the oil PX gauge checking , this should be done ON THE GROUND by rigging a calibrated direct reading gauge into the oil system.

This will enable the gauge in the cockpit to be checked and the oil system PRV to be adjusted if required.

Once the engine is hot a compression check should be carried out to check for the remote posability of a broken piston ring.

This should not cost you more than two hours of an engineers time and could save you a lot of money .