Oil Pressure Guidance

How to keep the Cessna 170 flying and airworthy.

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GAHorn
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Re: Oil Pressure Guidance

Post by GAHorn »

Cold or congealed oil in the sense line ... and, perhaps even in the gauge itself... will lead to slow indication/response times such as described.

The correct fix is: Drain and clear out the existing line or replace it with new,... drain/clear out the gauge of old oil (if any)... using solvent, clean the line completely, then re-fill it.... the sense line should be filled with kerosene, not air, not oil, not MMO, not any thing but kerosene.

(In a perfect world, there is no actual exchange of fluid thru that line. The line merely is filled with incompressible fluid (kerosene) and it passes along to the gauge the pressures existing at the engine fitting. The line may become filled with oil if the guage or it's fitting leaks ...or if someone once left it full of air instead of kerosene.)
'53 B-model N146YS SN:25713
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Bruce Fenstermacher
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Re: Oil Pressure Guidance

Post by Bruce Fenstermacher »

George your answer sounds definitive. I suppose you have a reference for the use of only kerosene. It won't be the overhaul manual which simply says connect the line under engine installation.
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GAHorn
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Re: Oil Pressure Guidance

Post by GAHorn »

Perhaps I shouldn't have been so adamant about "nothing but" kero, ... I'm fairly sure there's a MIL specified instrument oil that is permissible, but I've read several authoritative sources recommending kerosene. I'm not at my personal library (I'm presently out of town) but I have two instrument overhaul references which I know recommend kerosene, and recall a Cessna service instruction recommending that procedure. I'll try to relocate that document and post it.

Addendum: A quick google search returned : From the War Dept, Technical Manual 1-413, "Aircraft Instruments", dated Feb 1942: "35. Maintenance.—a. When sluggish operation of the oil pressure gage as evidenced by lag in indication is noticed during engine operation in cold weather, the line leading to the gage is disconnected, thoroughly drained, and refilled with aircraft instrument oil. This procedure is performed as often as necessary. ..."

From website http://www.slideshare.net/Mahnil/aircra ... entsystems "25. Bourdon Tube Pressure Gage • Used for higher pressures • Oil pressure – Restrictor on input line to limit leaks – Kerosene to improve movement • "
'53 B-model N146YS SN:25713
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An originality nut (mostly) for the right reasons. ;)
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juredd1
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Re: Oil Pressure Guidance

Post by juredd1 »

Bruce Fenstermacher wrote:Not sure if you understood what i was saying in my last post. What I'm saying is your engine oil pressure at start is likely over the green arc but your gauge system is not able to show that for any of the reasons mentioned previously in this thread. Engine oil does pressure does not rise as the oil warms.
I hear you loud and clear this time Bruce. I was just taking the "oil pressure" term as meaning oil pressure reading when I understand now that is not what was meant. Actual oil pressure being produced and the oil pressure being displayed on the oil pressure gauge are two different things.

Thanks for your patience.

Thanks for you additional input George.

I will see what I can do with that line to the gauge at some point in the near future but will likely wait until the temps are at least above 0.

Justin
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Bruce Fenstermacher
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Re: Oil Pressure Guidance

Post by Bruce Fenstermacher »

What did we do without the internet?

For one we had more questions asked and answered in the 170 News. At least we had one opinion of the answer and it only took six months to get it circulated to the membership if we were lucky.

Several random thoughts which popped into my head after reading "not air, not oil, not MMO, not any thing but kerosene". I'm pretty sure somewhere in these archives is a statement that MMO is kero, red dye and perfume. Of course kero is an oil. And despite best effort I'll bet you can't get all the air out of the instrument. The War Department says to use "aircraft instrument oil" and we have discussed what might be found in a "whisky compass" before to be, whisky, Stoddard solvent (another MMO ingredient) or maybe Kero.

I conclude that you will need some whisky while trying to bleed the air from your gauge and the MMO with it's perfume and die might be an improvement over the clear kero. At least you will smell of perfume and whisky and you will be able to see any spillage with the red dye. BTW anyone tried to buy clear kero lately. Hard or near impossible to do today around here as it all has blue dye so truckers won't run it in their trucks. Of course there is this mystery clear kero like substance called "Lamp oil" sold in small quantities at the hardware store. Probably isn't straight kero or at least the bottle doesn't admit directly to it. Been working fine as an instrument oil in my whisky compass on my desk for the last 10 years though.

And a final thought George. Is your library as large as that in the pictures of the Texas Christmas party in Richard's library. Now that is a library.

But I digress.

George it is nice to put a official source to this practice. Will be interesting to see who posts the Cessna SB first, you or Gary. :wink:
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GAHorn
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Re: Oil Pressure Guidance

Post by GAHorn »

:lol: Yeah.... my writing style sometimes gets me into trouble because it's similar to my speaking style... in an effort to steer folks to the simple answer, I sometimes speak (and write) in an adamant tone-of-voice. But, of course, the internet doesn't carry voice-inflection so no one could tell I was being "jocular" when I made that admonition against MMO for this purpose.

Anyone who has used MMO and spilt any of it will recall it dries to a sticky, red, mess. (The slowest red I know of.) One would not wish that stuff in your oil press. sense line because it would likely make things worse, not better than old, thick engine oil.

Air in the line will cause a sensitive gauge (not the cheap ones we typically find in 170s, ... but real quality instruments) ...the air will compress and fluctuate with each minor fluctuation or pulse of an oil pump or engine anomaly....causing the gauge to "flutter" as well. Air will much more easily escape the line at any fitting not perfectly-sealed and allow more and more engine oil to enter as time progresses. The air also, due to it's ability to compress, will allow a tiny amount of oil to enter the line each time the engine fires up. After many such starts, sufficient engine oil remains in the line to congeal and cause the difficulty.

A liquid, such as a thin instrument oil or kerosene/solvent will keep the line clean longer, and will not compress, thereby transmitting the pressure to the gauge. Since the inst. oil/kero will not compress it will not allow as much, if any, engine oil to enter the line.... and even if engine oil should enter ...it will be only a tiny amount and will become diluted (thinned) by the kero, thereby avoiding the congealing problem for a more protracted period of operation.

This is the sort of explanation I should have made in my first response to this discussion instead of the "conversational" one that failed to give any clue as to my real meaning, warning against using inappropriate materials in that line.

Sorry. I'll work on myself.

(NO! Ol'Gar! THAT is not what I meant!) :evil:
'53 B-model N146YS SN:25713
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T. C. Downey
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Re: Oil Pressure Guidance

Post by T. C. Downey »

gahorn wrote: Air in the line will cause a sensitive gauge (not the cheap ones we typically find in 170s, ... but real quality instruments) ...the air will compress and fluctuate with each minor fluctuation or pulse of an oil pump or engine anomaly....causing the gauge to "flutter" as well. Air will much more easily escape the line at any fitting not perfectly-sealed and allow more and more engine oil to enter as time progresses. The air also, due to it's ability to compress, will allow a tiny amount of oil to enter the line each time the engine fires up. After many such starts, sufficient engine oil remains in the line to congeal and cause the difficulty.
Actualy GA I think you got the theory backwards, a semi solid media, such as engine oil will transmit impulses faster than a compressible media, such as air.
think about hitting a chisel with a hammer versus a spring.
All one needs to do to clear a oil pressure transmitter line is to disconnect both ends and blow it out with WD 40 or any other light distillate oil. connect both ends and start the engine, then loosen the gauge end and allow the engine to provide the oil pressure to evacuate the air from the line.
And remember we have a oriface in the fitting that connects the line to the engine that needs to be clear too. That oriface is what dampens any osolations in pressure to the gauge and limits how much oil can be lost due to gauge or line failure.

Bottom line ? you want the gauge line full of oil, so you will not be waiting for that oriface to fill the line, compress the air before you see a reading on the gauge.

The theory of oil congealing in a confined space simply won't happen, other wise your MMO wouldn't pour out of the can.
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GAHorn
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Re: Oil Pressure Guidance

Post by GAHorn »

Air will be more susceptible to allowing the engine oil to enter the line as previously described. Oil, especially engine oil, will be too viscous in cold wx and be slow to respond/transmit pressure to the gauge....this is the problem.

But I like your use of WD40. :lol:

I agree the line should be filled with oil...but with LIGHT oil which will not congeal. The kerosene is an old timers favorite.
'53 B-model N146YS SN:25713
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n2582d
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Re: Oil Pressure Guidance

Post by n2582d »

Oil Pressure.jpg
From the '69-''76 C-172 Service Manual pg. 11A-6. This is also shown on pg.12-34 of the 100 Series 1962 and prior Service Manual.
Last edited by n2582d on Mon Jan 06, 2014 11:04 pm, edited 1 time in total.
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Bruce Fenstermacher
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Re: Oil Pressure Guidance

Post by Bruce Fenstermacher »

I had a feeling Gary would beat George to posting something official albeit for a 172, it might be as close as we get. George, got anything with 170 on it? :lol:
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T. C. Downey
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Re: Oil Pressure Guidance

Post by T. C. Downey »

gahorn wrote: I agree the line should be filled with oil...but with LIGHT oil which will not congeal. The kerosene is an old timers favorite.
I've operated in -50 temps with engine oil in the transmitter lines of several different aircraft and never had any problems with getting a reading, slow to indicate maybe, but no indication, no. They will be slow to indicate simply because of the oriface restriction on thick oil, and the time that it takes to compress the air in the line.

Kinda like air in your brake line, you gottta push the pedal a lot farther to get a response.

If I were flying in upper mid west tonight I might worry about it. If you are operating in these temps the least of your worries is the transmitter line.
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ghostflyer
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Re: Oil Pressure Guidance

Post by ghostflyer »

If the tube is clogged up with gunky oil the best solution is to replace the copper oil tube all together . This due to the age hardening of the copper tube . Then we take some WHISKY hek hek urr and pour it into ahhh the tube , better taste it first make sure it's the write one. MMMmmmm , another taste it could be the wrong one ? This is the best one . Not too much .we can't waste it. Gee I love this job.
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GAHorn
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Re: Oil Pressure Guidance

Post by GAHorn »

n2582d wrote:
Oil Pressure.jpg
From the '69-''76 C-172 Service Manual pg. 11A-6. This is also shown on pg.12-34 of the 100 Series 1962 and prior Service Manual.
I knew I read it in an official Cessna service instruction document... I just didn't have access while out and about. Thanks, Gary for confirming that, while I'm crazy, ... I'm not as crazy as some might have you believe. :wink:

(Bruce, the instructions are also included in the 100 Series Service Manual, 1962 and Prior, ... a document which includes the C-170, as you already know.) :wink:
'53 B-model N146YS SN:25713
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An originality nut (mostly) for the right reasons. ;)
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juredd1
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Re: Oil Pressure Guidance

Post by juredd1 »

Now that I know what to fill it with offically :? Is taking things apart to fill it as simple as it might sound? Just disconnect from both ends and perfrom the action described? Just trying to get a general idea and any issues I might encounter.

Justin
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n2582d
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Re: Oil Pressure Guidance

Post by n2582d »

ghostflyer wrote:If the tube is clogged up with gunky oil the best solution is to replace the copper oil tube all together . This due to the age hardening of the copper tube. ... .
Back in 1970 the FAA issued AD 70-10-06 to replace the metal oil pressure line between the engine and the firewall on some C-172's with Lycoming engines. Interesting that they didn't include earlier O-300 powered C-172's. Probably because they don't shake as much! I've attached a pdf file of the Cessna Service Bulletins that correspond to that AD. I believe it was Sacramento Sky Ranch that suggested replacing the copper line with 3003-O aluminum but I can't find that reference now that they are out of business. Neal Wright has a good article on replacing the copper line with flexible hose on the C-120/140 website.
Oil Pressure Hose Service Letter.pdf
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Last edited by n2582d on Thu Sep 07, 2023 3:45 am, edited 3 times in total.
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