Low Oil Pressure

How to keep the Cessna 170 flying and airworthy.

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Re: Low Oil Pressure

Postby Bathman » Sat Jul 10, 2021 7:30 pm

Main bearings gone
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Re: Low Oil Pressure

Postby GAHorn » Sat Jul 10, 2021 7:48 pm

Bathman wrote:Main bearings gone


Unlikely in this case…. since it apparently occurred immediately after the engine was modified…rather than gradually over time.

But you’re correct that worn main bearings are a main concern when low oil pressure exists.
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Re: Low Oil Pressure

Postby daedaluscan » Sun Jul 11, 2021 1:57 pm

GAHorn wrote:I have another point of view relative to Bruce’s opinion. If there’s any oil pressure AT ALL… say 5 psi…then the bearings will be fine. It’s only necessary that oil be present to prevent dry bearings to lubricate and cool them. This is based upon what i’ve been taught at automotive and aircraft engine tech schools.

I also have a real-life experience in this matter.

Example: I was cruising at 11K’ in a C-414 when the left oil pressure suddenly dropped to only 5 psi. It ran there for at least ten minutes while I decided what to do about it…..and it may have run as long as 20 minutes because that’s when I wrote down the cruise parameters I traditionally keep.
I feathered that engine and landed and called Ram Aircraft Engines in Waco, Tx (it was a Ram conversion). Their chief engineer wanted to know if the prop ever indicated any kind of “surge” or RPM excursion…. and it hadn’t and I told him so.

He said that if the RPM didn’t waver then the core engine was unharmed and to search for issues such as oil pressure relief or oil filter by-pass issues.

We found some chunks of “pink plastic” beneath the oil pressure relief valve-seat and inside the spin-on filter…..and cleaned out the relief valve…changed the oil and filter…. ran the engine for about 20 minutes to “flush it”…then changed the oil and filter again.

It turned out that AeroShell issued an AD note that their production facility suffered a pump-failure that released plastic into the oil and the AD was applicable to a certain LOT of oil…which the maintenance facility who maintained the airplane had ignored.

That engine was not hurt by the event…. and the Ram engineer convinced me that as long as RPM variations do not occur and as long as a minimum of 5 psi is maintained that the engine crank and cam and conn-rod bearings are not injured.

In THIS discussion-thread…we are observing about 20 psi oil pressure…. Clearly sufficient to prevent damage, IMO.

YMMV


I must say this really surprises me. I had always assumed significant oil pressure was needed for these bearings to function. My dad used to go nuts on me if I lowered the rpm on our tractors too low while idling because I was sick of the noise. He told me that I was risking lowering the oil pressure too far. He was an apprenticed diesel mechanic, cast white metal bearings, etc.
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Re: Low Oil Pressure

Postby GAHorn » Sun Jul 11, 2021 8:39 pm

If your oil pressure gauge is correctly marked….it’s lower red-line is at 5 psi.

However, the Continental Operators Manual specifies 10 psi minimum at idle.

Here’s a screen-shot of the Continental Operators Manual on Low Oil Pressure troubleshooting:
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D15FDA1E-F419-408C-B2BB-51DDFD62EF6B.png
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Re: Low Oil Pressure

Postby IP076 » Sat Jul 17, 2021 10:55 pm

Can’t seem to find a copy of a maintenance manual for the C145. Anyone got the torque spec, if any, for the oil pressure relief valve cap/acorn nut?
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Re: Low Oil Pressure

Postby n2582d » Sun Jul 18, 2021 6:14 am

190-210 in./lbs. according to Continental Maintenance Manual, Publication M-0, Table B-2.
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Re: Low Oil Pressure

Postby IP076 » Mon Jul 19, 2021 6:09 pm

Thank you Gary for the torque specs.

I removed the existing pressure relief valve. I’ve attached two pics.

The spring is not the same as the replacement. Additionally I’ve found a stack of about 3/8” worth of washers in the cap.

ECECF763-439A-4C40-8AD3-7D165BACB0CB.jpeg


F8E56164-5544-492F-BD18-DC5BD721B3B6.jpeg


Correct me if I’m wrong, but I’m thinking this larger spring and stack of washers may have been a previous attempt to boost the oil pressure. If that’s the case, returning to the specified part is probably not going to make my situation better, and perhaps make it worse. Thoughts?
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Re: Low Oil Pressure

Postby IP076 » Mon Jul 19, 2021 10:02 pm

For what it's worth, I replaced the existing spring and washer pack with the new one. On start up with cold oil, pressure went to 30-35 psi instead of the typical 50-55 psi.

Looks like I'm not the first person with this airplane to discover this problem. Back to the drawing board.

I guess the last item on the troubleshoot list is the gauge itself, though with the change in spring and subsequent change in pressure, I feel as though that's a long shot.

After that I guess there's the theory of the bearings or the oil pump being bad.
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Re: Low Oil Pressure

Postby ghostflyer » Mon Jul 19, 2021 10:50 pm

I had been working at this company for a short time and just finished overhauling an engine and a 20hr oil change was done. On start up for some reason it back fired on start up.This was not good . A normal start was then completed. When we fitted the overhauled engine to the aircraft a new electronic [EI] oil pressure gauge and transducer was fitted . After start up to my horror and heart palpitations ,I watched the oil pressure slowly drop. But oil temp hadn’t had a chance to climb and the engine RPM stayed constantly at 1000rpm. No unusual noises were occurring. Engine shut down . Oil sample checked and nothing in the oil .
Checked oil pressure relief valve all good . The GM of the company came out and was observing the situation very carefully. Top plugs were removed and engine turned over by hand ,all good. I was explaining to him what I had done and all I got was a grunt. As he walked away he mumbled “we have $35,000 invested in that engine “.
Yep, I had all the pressure in the world but it wasn’t oil pressure . This engine was nearly a new engine with all the new parts fitted. I remembered seeing a dash 3 hose and gauge sitting on a bench in the hangar. The gauge went up to 100psi and had being used before on engines due to its oily condition. Fitted the “wet line” and gauge . Started the IGSO-540 and oil pressure was sitting at 75psi. It was a the near new transducer that had failed . [including my heart].
Ps. I didn’t start the overhaul job on that engine ,I came in at about 75% completion. The original 2 guys that had started the job had quit the company.
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Re: Low Oil Pressure

Postby c170b53 » Mon Jul 19, 2021 10:59 pm

D1E84197-2E1B-47BB-A587-17DED98A371C.jpeg
Unlikely to be the pump (housing in center has two holes) although not impossible. The oil pump housing is the aft engine case and you’d have to have significant wear of the housing walls to affect pressure.. Although the result maybe had to stomach, good for you for looking into it.
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Re: Low Oil Pressure

Postby Bruce Fenstermacher » Tue Jul 20, 2021 1:05 am

Lots of folks don't understand the difference between a Continental high pressure bypass valve and an Lycoming oil pressure valve. They put washers and stiffer springs in the Continental bypass and all they do is raise the high oil pressure that oil bypasses. They do not realize that at normal operation the Continental bypass is closed. Or it should be.

A last ditch effort to insure there is no carbon or other junk on the plunger or set, you could lightly lap the plunger to the seat. Now that your an expert at removing the Acorn, spring and plunger.
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Re: Low Oil Pressure

Postby IP076 » Tue Jul 20, 2021 10:11 pm

Thanks everyone for the feedback.

Bruce, couple of questions since I’m definitely learning here.

Regarding the use of the spring/washers in the Continentals, I get what you’re saying as it just dumps more as a bypass. I guess I don’t understand why it results in a higher indicated oil pressure.

Secondly, would you lap the seat dry or use a small bit of compound? I see folks lap valves in installed cylinders with a small bit of compound. Just asking cause it seems to me that it would be easy to guarantee a need for a new engine if you get that wrong.

Thanks again!
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Re: Low Oil Pressure

Postby GAHorn » Wed Jul 21, 2021 10:57 am

While you are saying “typical” is 50-55 indicated…. 30-35 that you are now seeing with the correct press-relief-spring is considered normal by Cont. Have you flown it up to normal operating oil temp yet to see what it indicates?


Here’s screen shots of two pages out of the B-model OM:

09570ED0-08EE-4656-A44C-6CBE10E18661.png

0CD2D77F-B478-445E-8A70-1E7D9FB754D1.png
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Re: Low Oil Pressure

Postby IP076 » Wed Jul 21, 2021 1:37 pm

George,

I have not flown it. My thinking is that if it previously indicated 50-55 on start with cold oil and then indicated about 20 in cruise, that starting at 30-35 is only going to result in a lower pressure in cruise once the oil warms up. Now perhaps since the only change has been the pressure relief valve, maybe the cruise power oil pressure doesn’t change. I guess I don’t see how it could be better in cruise now.

I flew it a few times early in this process, but I’m at the point where I’m a little concerned about causing any more damage or worse.
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Re: Low Oil Pressure

Postby c170b53 » Wed Jul 21, 2021 6:12 pm

As Bruce mentioned, its a high pressure relief valve not a pressure regulator therefore pressure is unregulated and will be dependant on RPM. If for some reason it does not seal once the high pressure event is over then leakage through the valve will affect system pressure regardless of oil temp. So I would go try it, stay in the pattern and practice a few landings and see what you have. If it is worn bearings and there was no metal in the screen / filter then you’re unlikely to do any further significant damage and you’ll further the T/S tree.
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