Low Oil Pressure

How to keep the Cessna 170 flying and airworthy.

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Re: Low Oil Pressure

Postby Bruce Fenstermacher » Wed Jul 21, 2021 10:12 pm

Contrary to Jim's thought, and my experience with a C-85. I'd just rebuilt it. New bearings and crank all checked out. Within about 10 hours oil pressure started to drop. Had I stopped running the engine, could I have saved the expensive crank. We will never know. There was never any metal found in the screen. By 48hrs I'd had enough experimenting with pressure relief valves and even swapping out the entire accessory case with a different oil pump. I listened to what people had been telling me and pulled the engine apart to find the bearing gone in 48 hours. The crank was beyond use. I have a pretty good theory why my bearings wiped out. Total disassembly and cleaning and inspection a new crank (0-200) and bearings and that engine is still running today.

What puzzles me is that your engine seemed to lose so much pressure so quickly. I wish I could think of any reason that you could justify not pulling the engine down. It's a pain and it is expensive. You might be there. And maybe you will salvage your crank.

I'm still hoping you figure something out other than a tear down. Just don't know what that could be.
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Re: Low Oil Pressure

Postby c170b53 » Wed Jul 21, 2021 11:51 pm

Oh that must have hurt Bruce. I would have thought in your example the metal would have gone somewhere. In this new case, check the screen again and has the oil rapidly changed colour.
If its an old high time engine , I’d exhaust all avenues in the T/S routine before I’d tear it down which is going to be expensive.
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Re: Low Oil Pressure

Postby IP076 » Thu Jul 22, 2021 12:57 am

Thanks for the continued discussion.

As a recap:

I changed the oil, and flew for about 8 hours while noticing this low pressure being a trend. Subsequently I did the following troubleshooting:

1) Changed the oil
2) Removed, checked, cleaned, replaced oil screen - no smoking gun here
3) Removed and replaced with new oil pressure relief valve - found non standard spring and stack of washers - I did inspect the seat area inside and didn't see anything that would restrict the valve from fully seating.
4) Sent off an oil analysis from the original oil change, waiting results

To this point, I did fly it 2 or 3 hours in the process.

My next plan is to swap out the gauge and see if there's a difference there. I toyed with doing this as a T to compare indications directly, but I'm thinking it might be best to go directly to the port on the case, thus eliminating the possibility that there is something restricting flow in the line to the gauge.

I'm hesitant to fly it much more, as it's really unnerving seeing the oil pressure below the green at cruise...and basically on the low stop on the gauge at idle. I'll see what happens with the new gauge and reassess.

In my case it is a fairly high time engine, around 1600 since overhaul. It's also been a substantial calendar time since overhaul, over four decades.
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Re: Low Oil Pressure

Postby Bruce Fenstermacher » Thu Jul 22, 2021 4:39 pm

IP076 wrote:Thanks for the continued discussion.

As a recap:

I changed the oil, and flew for about 8 hours while noticing this low pressure being a trend. Subsequently I did the following troubleshooting:

1) Changed the oil
2) Removed, checked, cleaned, replaced oil screen - no smoking gun here
3) Removed and replaced with new oil pressure relief valve - found non standard spring and stack of washers - I did inspect the seat area inside and didn't see anything that would restrict the valve from fully seating.
4) Sent off an oil analysis from the original oil change, waiting results

To this point, I did fly it 2 or 3 hours in the process.

My next plan is to swap out the gauge and see if there's a difference there. I toyed with doing this as a T to compare indications directly.


I did install a second gauge with a T. The only thing it did for me is more accurately show me the low oil pressure the other one was. I hope you have better results.
I'd be calling the oil analysis place daily for those results.
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Re: Low Oil Pressure

Postby GAHorn » Fri Jul 23, 2021 2:37 pm

IP076 wrote:Thanks for the continued discussion.
.

I'm hesitant to fly it much more, as it's really unnerving seeing the oil pressure below the green at cruise...and basically on the low stop on the gauge at idle. I'll see what happens with the new gauge and reassess.

.


Your previous post led me to believe that you installed the correct/new blue spring and started it up and right-away observed 30-35 psi. You did not indicate at what RPM that 30-35 occurred…and since when I start an engine I always let it idle a few minutes before increasing RPM…I assumed you had that 30-35 psi at idle.

So…. What is your idle pressure? and what is your 2400 pressure… with the NEW spring?
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Re: Low Oil Pressure

Postby IP076 » Sun Jul 25, 2021 1:33 pm

George,

That was at idle to 1000 rpm for 30-35psi, with the new spring.

I’m tied up with work until at least Thursday, If I’m actually free and home that day I’ll be verifying with another gauge and I’ll get you the pressure at the higher RPM.
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Re: Low Oil Pressure

Postby GAHorn » Sun Jul 25, 2021 3:25 pm

OK.. Well, IMO, 30-35 psi at idle is near-perfect. If it were mine, I’d fly it overhead and get it hot to see what pressure it maintains and then land and see what idle produces.
Just hoping the best for you.
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Re: Low Oil Pressure

Postby n2582d » Sun Jul 25, 2021 6:59 pm

IP076 wrote:My next plan is to swap out the gauge and see if there's a difference there. I toyed with doing this as a T to compare indications directly, but I'm thinking it might be best to go directly to the port on the case, thus eliminating the possibility that there is something restricting flow in the line to the gauge.
Jared,
Do you have an 1/8" copper oil pressure line or a hose between the engine and the firewall? If you still have the copper tubing for the oil pressure line it should be replaced with a hose. This has been discussed several times on this forum. Although I think it's unlikely to be the problem, as long as you're checking the line to the gauge, it would be worth checking that the AN fitting which the oil pressure line attaches to in the case is not restricted more than it should be. Also make sure it has an .040" restrictor as discussed here.
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Re: Low Oil Pressure

Postby IP076 » Tue Jul 27, 2021 1:37 am

George,

Sounds reasonable. I'll see what later this week brings for work travel, and if I'm home I'll give it a shot.

Gary,

I still have the 1/8" copper line with -2 AN fittings on the end, I believe. I had not read of the rigid lines being less than favorable, that's something to consider.
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Re: Low Oil Pressure

Postby GAHorn » Tue Jul 27, 2021 4:35 am

IP076 wrote:George,

Sounds reasonable. I'll see what later this week brings for work travel, and if I'm home I'll give it a shot.

Gary,

I still have the 1/8" copper line with -2 AN fittings on the end, I believe. I had not read of the rigid lines being less than favorable, that's something to consider.


You’ve heard of the “Caterpillar Club”…..?? the individuals who have been saved by a parachute?
Well…. our latest Member of the CLE0P Club ….. (Copper Line Equals Zero Pressure) ….is our own former president Jim Wildharber….who landed on a highway after his copper oil pressure line fractured on his RECENTLY OVERHAULED engine….and spewed all his oil overboard in-flight. He saved his own and Lolly’s lives and his airplane…but had to RE-do his engine overhaul.

Chas Lindbergh knew not to trust a copper oil line on the Spirit…. he had it sectioned and pieced-together with rubber line in regular intervals to avoid embrittlement-failure.

Eddie Rickenbacker lost his oil pressure at night when his copper oil pressure line fractured and he survived by pure luck landing in the desert.

Our former historian Cleo Bickford lost his oil pressure due to a fractured copper oil line on his way back from Alaska and wrote about it in an early issue of The 170 News.

It has been cussed and discussed many times here in the forums.

I replaced mine with a rubber line ASAP after I acquired my 170B and immediately replaced the copper line in the 172 I bought recently for a family trainer.

I hope that everyone will strongly consider replacing their copper oil pressure sense line ASAP, with a rubber hose. Cessna quit building airplanes with copper oil sense lines years ago.

Copper becomes brittle with vibration or work-hardening and that pressure sense line is constantly receiving engine vibration. Some folks claim to “anneal” theirs and continue using it… which is NOT appropriate. The FIX is to replace it with aircraft-quality rubber hose. IMO

Precision Hose Technology will make one for you, their PN p/n 193000-2D0183.

The Official Cessna Part Number for the C-150 hose: 359-2D0150 This hose is 15"…. so a PN 359-2D0190 would be 19”…if you can get them to do that.

You can make your own. I made my own by ordering the materials from Spruce. A Mandrel is not needed for 1/8” hose.... you can use the smooth shaft of a 1/8” drill bit. Pressure-test it by plugging one end and pressurizing it with air while submerging it in water.

Custom hoses may also be ordered thru Spruce. Stratoflex makes 3/16” primer fuel lines for Pipers or according to custom orders, with 2D end fittings.

Aeroquip 303 in 1/8” size hose can also be used with oil (despite being described as for air) if you wish to make your own.
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Re: Low Oil Pressure

Postby hilltop170 » Tue Jul 27, 2021 6:47 am

I replaced my copper oil line with a pressure transducer and a digital gauge. Fixed forever!
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Re: Low Oil Pressure

Postby GAHorn » Tue Jul 27, 2021 4:56 pm

I’ve seen two pressure transducers leak over the years in various installations. There’s a diaphragm inside that can eventually fail and they leaked thru the phenolic/base seal. (tho’ it was only messy… didn’t dump the oil overboard.)
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Re: Low Oil Pressure

Postby IP076 » Wed Jul 28, 2021 1:21 pm

George, thanks again for your detailed reply, all very good info.

I’m still stuck working long days, but have tomorrow off, so hope to get to the airplane. I think I’ll take your advice and get it warmed up and see what it looks like. I flew another 170 recently and now that you mentioned it, it seemed to go to about 35-40psi on start and simply stayed there in cruise.

Regarding the copper line. I think I’ll track down those replacements, seems well worth the investment. Do you guys do the primer lines and such too? Going from memory, I think there’s at least three copper lines in mine, two seem to be fuel/primer and one oil pressure.
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Re: Low Oil Pressure

Postby GAHorn » Sun Aug 01, 2021 1:26 pm

It’s true that copper primer lines can also become brittle and fail….but they do not contain fluid under pressure unless the primer plunger is being activated and only while that plunger is being “plunged”…. so failure is extremely rare in that application. Even if that failure should occur the only result is a minor air leak unless the plunger is activated.

I judge that to be so inconsequential that I have not changed it (and if you do change it then you should consider it just like any other rubber fuel line forward of the firewall and assign it a service-life. Engine-overhaul (1800 hrs or 12 years) is what Continental specifies but many airframe mfr’s consider fuel and hyd (pressurized) lines to have 5-year lives.
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Re: Low Oil Pressure

Postby cessna170bdriver » Sun Aug 01, 2021 3:58 pm

GAHorn wrote:I’ve seen two pressure transducers leak over the years in various installations. There’s a diaphragm inside that can eventually fail and they leaked thru the phenolic/base seal. (tho’ it was only messy… didn’t dump the oil overboard.)

Only two (in how many years?); that’s a much better record than the copper line.
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