Bourdon Tube Oil Pressure

[JGoodish]

My Cherokee (with Lycoming engine) has a Bourdon tube type oil pressure gauge in the panel. In the process of trying to determine the cause of a slight drop in oil pressure, I disassembled and cleaned out the lines from the accessory case on the engine to the gauge itself. In the Piper maintenance manual, the instructions for reinstallation mention only the physical connections. There is no mention of a requirement to prime the lines with oil nor bleed the system between the engine and the gauge.

Is priming the lines a requirement, or will they fill themselves? And if the latter, is the system self-bleeding, or will I have to bleed it (I assume at the gauge itself). Anyone familiar with Piper's system in the Cherokee?


Thanks,
JKG

 

[Tom-D]

My Cherokee (with Lycoming engine) has a Bourdon tube type oil pressure gauge in the panel. In the process of trying to determine the cause of a slight drop in oil pressure, I disassembled and cleaned out the lines from the accessory case on the engine to the gauge itself. In the Piper maintenance manual, the instructions for reinstallation mention only the physical connections. There is no mention of a requirement to prime the lines with oil nor bleed the system between the engine and the gauge.

Is priming the lines a requirement, or will they fill themselves? And if the latter, is the system self-bleeding, or will I have to bleed it (I assume at the gauge itself). Anyone familiar with Piper's system in the Cherokee?


Thanks,
JKG

Bleed it,, by loosening the line on the back of the gauge while running the engine at idle.

 

[Dan Thomas]

Air in the line only makes the gauge slow to respond, especially in cold weather. The air will compress and the gauge will eventually read properly when the pressures balance.

Some mechanics fill the line with 5606 fluid, which doesn't stiffen so much in the cold.

How "slight" is this pressure drop?

Dan

 

[N801BH]

Air in the line only makes the gauge slow to respond, especially in cold weather. The air will compress and the gauge will eventually read properly when the pressures balance.....


Dan

Very true....

 

[mondtster]

Ideally you would probably bleed it, but I'd guess that many mechanics don't.

 

[Tom-D]

The snubber built into the gauge port can't work with out oil in it. it is there to protect the gauge internal gears and tube.

Bleed it and see how much better it works

 

[JGoodish]

Air in the line only makes the gauge slow to respond, especially in cold weather. The air will compress and the gauge will eventually read properly when the pressures balance.

Some mechanics fill the line with 5606 fluid, which doesn't stiffen so much in the cold.

How "slight" is this pressure drop?

Dan

Maybe 15-20psi, so that full throttle gets barely into the green (above 60psi) with oil temps at around 120F (so well below normal cruise oil temps). Range of indicator motion from full power to 1000RPM appears to reflect an approximate 10psi swing, which is a smaller range of motion than we're used to observing. We've owned the aircraft for 15 years, and have always experienced fairly consistent behavior from the gauge. What changed from normal indication one day to abnormal indication the next? Well, the ambient temperature dropped significantly, but that's happened every year around this time.

I pulled the relief valve and cleaned it, but didn't find noticeable corrosion pitting or debris or anything else unusual. The cleaning (and reassembling with the same number of washers) didn't have any effect on the behavior of the panel gauge.

This past weekend, my mechanic and I put a recently calibrated, oil stabilized auxiliary pressure gauge on the system and verified that the engine is producing pressures from 50 at idle to just over 80 at full throttle, which are values that are more consistent with the prior behavior of the panel indicator. So, we're thinking that the gauge or line to the gauge might be the problem, and therefore the decision to clean out the line before we look elsewhere. I'd say a mix of clean and rather dark motor oil came out of the line.

The line to the gauge is clean and the system is presently closed to prevent contamination. It sounds like I just need to decide whether I want to fill the line with something specific and then bleed, or just bleed it with the engine running. We're obviously still not sure where the problem is, so this is all part of the troubleshooting effort to isolate it.


JKG

 

[Tom-D]

It sounds like I just need to decide whether I want to fill the line with something specific and then bleed, or just bleed it with the engine running. We're obviously still not sure where the problem is, so this is all part of the troubleshooting effort to isolate it.JKG

Decide how much contamination you want to put in your oil

 

[Henning]

Just crack the nut on the back of the gauge with the engine running and have an oil sorb rag ready to wrap around it to contain the mess.

 

[bnt83]

The snubber built into the gauge port can't work with out oil in it. it is there to protect the gauge internal gears and tube.

Bleed it and see how much better it works

Guess I'm not following you. I have plumbed a differential cylinder compression tester (just using it as a regulator) with a long enough hose to sit in the cockpit and watch both the tester and the oil pressure gauge read essentially exactly the same across the whole scale.

 

[JGoodish]

Decide how much contamination you want to put in your oil

Well, I imagine that since it's a pressurized stub, not much of whatever is in the line would circulate back out, but it does seem like just bleeding it at idle makes the most sense for a number of reasons.

Thanks to all for your responses.


JKG

 

[Tom-D]

Guess I'm not following you. I have plumbed a differential cylinder compression tester (just using it as a regulator) with a long enough hose to sit in the cockpit and watch both the tester and the oil pressure gauge read essentially exactly the same across the whole scale.

A snubber does not effect the reading, it simply dampens out surges in the line and saves the gordon tube. it will not work with out oil in it.

the indicator system was designed to be filled with engine oil. and has operated well for many years that way, why mess with success?

 

[JGoodish]

A snubber does not effect the reading, it simply dampens out surges in the line and saves the gordon tube. it will not work with out oil in it.

the indicator system was designed to be filled with engine oil. and has operated well for many years that way, why mess with success?

It is interesting (though not shocking) that Piper omitted mention of this in the maintenance manual for the airplane.

If cleaning and bleeding the line doesn't solve my particular problem, I suppose I'll then have to look at replacing the gauge. Not sure how long those type of gauges last.


JKG

 

[Tom-D]

It is interesting (though not shocking) that Piper omitted mention of this in the maintenance manual for the airplane.

If cleaning and bleeding the line doesn't solve my particular problem, I suppose I'll then have to look at replacing the gauge. Not sure how long those type of gauges last.
JKG

Check the fitting that is the connection between the engine and the gauge line, it should have an oriface opening that some times gets debris in the oriface.

you can tap in a direct reading gauge to the engine at that fitting to see if the readings are the same.

And remember there is no requirement for calibration of any gauges in Part 91 aviation.

 

[bnt83]

A snubber does not effect the reading, it simply dampens out surges in the line and saves the gordon tube. it will not work with out oil in it.

the indicator system was designed to be filled with engine oil. and has operated well for many years that way, why mess with success?

Indeed. I checked this 40 year old gauge to a calibrated compression tester and it was very accurate.

 

[bnt83]

There could be some sort of blockage like a piece of carbon. I've heard of an old O-300 powered 172 "losing oil" pressure shortly after departure and it turned out to be a large piece of carbon that got caught in the fitting at the engine for the oil pressure line.

 

[Tom-D]

There could be some sort of blockage like a piece of carbon. I've heard of an old O-300 powered 172 "losing oil" pressure shortly after departure and it turned out to be a large piece of carbon that got caught in the fitting at the engine for the oil pressure line.

After start up there is no flow thru the tube to the gauge, so why would it loose oil pressure if the engine didn't?

A chunk of carbon in the oriface will stop the oil pressure from ever coming up, but it will not stop it from going down.

 

[bnt83]

After start up there is no flow thru the tube to the gauge, so why would it loose oil pressure if the engine didn't?

A chunk of carbon in the oriface will stop the oil pressure from ever coming up, but it will not stop it from going down.

Don't ask me. It was the story from the pilot who stated himself is paranoid and watches oil pressure a lot.

It could have had a leak somewhere down stram of the clog....

 

[Dan Thomas]

A chunk of carbon in the oriface will stop the oil pressure from ever coming up, but it will not stop it from going down.

That there is probably the problem. That engine fitting has a tiny hole that would easily plug and act that way.

Dan

 

[Tom-D]

That there is probably the problem. That engine fitting has a tiny hole that would easily plug and act that way.

Dan

I remove them and soak them over night in Hoppe's #9 wash them in soap and water and blow them out before reinstalling. ( its a Number #80hole)

 

[JGoodish]

Check the fitting that is the connection between the engine and the gauge line, it should have an oriface opening that some times gets debris in the oriface.

you can tap in a direct reading gauge to the engine at that fitting to see if the readings are the same.

And remember there is no requirement for calibration of any gauges in Part 91 aviation.

Already tapped a gauge just past the oil temperature sensor, and it indicated expected pressure readings. So if there was something blocking the system, it seems that it would have been downstream of that point, which is what I cleaned out. Now I just have to get back out there to bleed it and see what the panel gauge reads.


JKG