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kgwilson

Oil Pressure off the scale

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It was a cracker day today so I decided to go flying. I started up & warmed the (Jab 3300A) engine. All good. Taxyed across the grass to the sealed taxiway & was about to do the runup when I noticed the oil pressure gauge was off the scale. The needle went as far as it could go, probably 800kPa. Top of the scale is 500 kPa & pressure when warm usually sits around 350 which is what it should. I suspect the sender is faulty as the gauge dropped back a bit at very low idle before the engine stalled & when it stopped returned to zero as it should. Has anyone had a similar problem?

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Stuck oil pressure relief valve?

 

Cheers,

 

Jack.

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Posted (edited)

I’ve had a sender pack it in. (Sounds similar).

Edited by reggie

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9 out of 10 times it's the sender. If it's a VDO type, they really don't like the vibration.

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A Relief valve cannot stick. Nev

 

I have had it happen numerous times on automotive engines over the last 50 years, its always a possibility to consider.

 

Cheers,

 

Jack.

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A relief valve definitely can stick, it can be jammed in its travel movement by a tiny piece of metal debris, or a tiny chunk of carbon.

I've even had a relief valve stay open and dump every skerrick of the oil flow, resulting in zero pressure. This was caused by a tiny piece of metal getting jammed in the seat, incredible and impossible as it seems.

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9 out of 10 times it's the sender. If it's a VDO type, they really don't like the vibration.

The gauge is a VDO which came with the engine as new and the sender is the standard Jabiru installation so I assume VDO as well.

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The other possibilities for excessively high indicated pressure (which possibilities are lower than a faulty sender) are a blocked filter, or a blocked oil gallery.

However, both of these are generally due to excessively dirty oil, not a problem if you're anal about oil changes, as I'd expect most aircraft owners are.

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I've never ever had one stick closed. Some can be incorrectly adjusted or assembled and not open at all..

They can end up not closing , due foreign material. If it's a ball, how can it stick? A plunger could stick if it rusted which is possible but unlikely unless it's been stored for years. where the preservative might go hard. Nev.

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I'm not privy to the design of Jab oil relief valves, but many engines use a spool-type or plunger-type relief valve with a needle seat that is an improved design over a simple ball-and-spring.

If the Jab oil pressure relief valve design is a simple ball-and-spring, then yes, there's very little chance of a ball sticking, as compared to a spool design.

However spool or plunger designs can stick due to varnish or gums from combustion, from carbon particles, or from tiny metal particles becoming embedded in the clearance gap between the spool or plunger, and the valve body.

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The Jab oil relief valve is a plunger type and from the drawing in JEM0002-8 maintenance manual the seat looks to be a needle type. I will check it out tomorrow and hopefully all will be revealed. The oil is clean (so clean it is hard to see the level on the dipstick) and as there are only 3 hours to go till the next 25 hourly oil & filter change I may as well do the change now.

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Posted (edited)

Stick CLOSED?? The area of movement is cleaned of build up of sludge etc by the action of it.. New engines may sometimes get some metal slivers from the machining of threads Burrs etc and where new unions may be assembled. especially with carburetters. In a gearbox that kind of thing does happen to the indexing plunger but they do have metal chips formed, sharp and a significant size.. That's why they have a magnetic drain plug.

All I'm suggesting is it's MOST unlikely and practically able to be disregarded as a likely cause.. There's some pretty ordinary relief valve designs where the oil pressure does fluctuate and cause concern. The SENDER is generally the culprit. I still favour a simple bourdon tube gauge with a small capillary copper tube. Nev

Edited by facthunter
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Stick CLOSED?? The area of movement is cleaned of build up of sludge etc by the action of it.. New engines may sometimes get some metal slivers from the machining of threads Burrs etc and where new unions may be assembled. especially with carburetters. In a gearbox that kind of thing does happen to the indexing plunger but they do have metal chips formed, sharp and a significant size.. That's why they have a magnetic drain plug.

All I'm suggesting is it's MOST unlikely and practically able to be disregarded as a likely cause.. There's some pretty ordinary relief valve designs where the oil pressure does fluctuate and cause concern. The SENDER is generally the culprit. I still favour a simple bourdon tube gauge with a small capillary copper tube. Nev

 

High frequency vibration can cause failure in copper lines.

 

Cheers,

 

Jack.

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Posted (edited)

A simple bourdon tube gauge certainly eliminates electrical and sender failures - but copper tubes can fracture or split with movement or vibration, and you can lose all your oil.

While an electrical gauge setup may have a higher chance of failing to work, it's probably a better option, as against the risk of additional oil piping that can fail, and dispose your vital oil into the atmosphere.

Edited by Guest
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Posted (edited)

Yes it can. You have to have it in coiled length or use a flexible section. Using a fine bore means the loss is at a very slow rate. so the first part should be a fine bore pipe. This slows the reading a bit but Nothing's perfect. Oil will start a fire often better than fuel so a lot of care is required with oil lines. I've had senders leak oil too. Nev

Edited by Guest
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For anyone interested I checked the pressure relief valve & it was perfect. A new oil pressure sender has solved the problem.

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Posted (edited)

Incorrect indications are a constant issue in aircraft.. Do you trust them or not? It's possible to have more likelihood of the indicator failing than the system it's monitoring, failing. That's clearly not a good set up when such is the case. Glad you fixed it.. Bit of a relief? Pun intended.

Erroneous indications have happened many times in my career, so it's not just a hypothetical. Puts you in a great quandary. Many fire warning systems are duplicated and frequently tested.. to try to avoid false warnings. Nev

Edited by Guest
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After talking to others around here I agree totally. One bloke replaced 3 electric oil pressure gauges/senders before he went to a capillary type. It cost a lot more apparently but has never failed. It also has a restrictor built in to stop all the oil disappearing if the line fails. I could have thought "Its just a crappy instrument" & flown anyway but that would be just plain dumb. It may be the instrument 99% of the time but even at those odds I am not going to take the chance.

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The oil-pressure sender is screwed directly onto the engine block and so gets all the engine vibration. If you used a length of rubber hose before the sender, the sender would last longer but you will have introduced a place where a failure would have serious consequences. If that hose were to burst then you would lose all the engine oil.

While I personally have not had a sender fail, it is quite common. Maybe I will buy a spare just to have a new one handy.

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The oil-pressure sender is screwed directly onto the engine block and so gets all the engine vibration. If you used a length of rubber hose before the sender, the sender would last longer but you will have introduced a place where a failure would have serious consequences. If that hose were to burst then you would lose all the engine oil.

While I personally have not had a sender fail, it is quite common. Maybe I will buy a spare just to have a new one handy.

 

Add it to your personal aircraft maintenance schedule, replace every 50 hours.

 

Cheers,

 

Jack.

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Some info for those with Rotax engines.

Generally they'll (vdo) last around 400/500 hrs in Rotaxes give or take.

Depending on whether a "bed" or "ring" mount is used. Ring mounts taking alot more of the vibration out.

There is a local aircraft with 700 hrs and the original sender with a ring mount.

A good carby balance will reduce vibration and make the sender last longer.

Original senders came with a brass anti-vibration ring, so it is fairly easy to tell an original unit.

Rotax then moved to a black "Honeywell" unit. This was not much of an improvement.

Mine (bed mounted) lasted 350 hrs...... and Honeywell stopped making them.

The next and current sender is the "Keller" which is expensive but seems alot more reliable.

NOTE: The VDO and honeywell/keller senders are NOT compatible. The "signal" system is different, so requires a different gauge.

The threads are also different. VDO/Honeywell, 1/8 npt. Keller, M10x1.

Some tap m10 straight over the 1/8. Some replace the oil pump housing.

I've (going from honeywell to keller) used an adapter and 90 degree fitting for a better fit and to stop the cable running into the exhaust.

Ultimately, for those with vdo sender units the easiest option is to simply replace it with another vdo unit.

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With Jabirus, failure of the sender has been common for years. Its not to do with the orifice which is already part of the sender. Its caused by the improper location of the sender onto an oil port upstream of the oil cooler where pressure pulsations are high howeveroilsender.JPG.a118bd536a8979afc69ad76ea837cf76.JPG the actual oil pressure to the gallery and mains (where the reading matters) is lower.

 

If you dont want to keep replacing senders every 150 hrs then shift it to the other port available on the front of the crankcase.

 

see where the windings are worn out due to consant movement.

when the wiper fails open the gauge will read full scale

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