Oil pump location and oil pressure

[XB-VIA]

[quote="nippaero"]Just a quick shot of the oil pump.


Dear sir, I would like to ask you some questions, I have a cessna 170b model 1954, I repair it from scratch as a project here in Mexico, but now I find that every time I start the engine, the oil pressure rises to 95, The engine low pressure, at 15 or 10 psi, and check the compressions of the cylinders and these are at 74-76 all cylinders, but when accelerating the engine I get no more than 2200 rpm, you could send me or upload photos of where The oil pump is located, because I do not have the slightest idea and as a mechanic told me that the oil pump needs to be replaced and I can not find it in any manual or part number, I apologize for the inconvenience sir would greatly appreciate your help.

[Bruce Fenstermacher]

The oil pump is inside the accessory housing on the back of the engine. It is the gears items 23 and 24 on this illustration and of course the area (not seen) that they turn in inside the accessory case.

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But it does not sound like you need a new oil pump. Yes you might need some work and far worse.

First when you start your engine the oil pressure should never go to 95lb. The reason is there is a pressure release that should release excess pressure at 60lb. Yes when operating properly you may see a bit over 60lbs but no where near 95.

When you say the engine oil pressure is low at 10 to 15, when is this. Idle. Engine hot? Engine cold?

Minimum oil pressure at idle on the Continental C-145/0-300 is 10lbs. However if your oil pressure is actually getting that low at idle, it is a really good indications something else is wrong. Could be the oil pump, could be the bypass not closing properly but more likely it is main bearings.

I have more to say later. I'm out of time at the moment.

[Bruce Fenstermacher]

More thoughts.

As your oil pressure goes to 95 initially I have a feeling someone who does not understand the oil system on this Continental has been making incorrect adjustments to the system. Unlike a Lycoming, the Continental only has a by-pass. This by-pass is not a pressure regulator. On a Lycoming, which looks to have the same system as a Continental, it is allowed to adjust pressure by inserting washers under the spring thus making the pressure on the regulator plunger greater. On a Lycoming oil pressure reacting the engine is a function of how much pressure is allowed past the regulator. Adjusting the Continetal by-pass is not allowed and has no effect on low oil pressure. When a Continental is at operating temperature and pressure is below 60 lbs the relief valve should be fully closed. Low oil pressure in a Continental is a function of how much pressure leaks by, for the most part, the main crankshaft bearings.

So I'd bet your oil pump is OK, It is your main bearings that are worn. And that means engine overhaul time.

But there is one thing you want to look at first and that is the oil by-pass. This consists of #12, 13, 14 and 15 on the illustration. You want to remove the acorn nut #15. Remove any washers you find in there. Remove the sprint #13 and the plunger #12. Inspect the plunger end where it sits in the seat inside the accessory case. Inspect the seat in the accessory case. What you are looking for is any roughness, pitting or wear that would not allow the plunger to reasonably seal against the seat. If any question replace the plunger. If the seat you might be able to dress that a bit. If there is any question the spring could be weak, replace that. Reassemble and see what happens.

If you found washers and you removed them and did nothing else I'll be your oil pressure goes no higher than 60lb but your engine oil pressure will still be low.

BTW if you engine oil pressure is above 10lb at idle and above 30 in cruise, you are within limits. However oil pressure that low at idle and the subsequent low pressure around 30 in cruise is an indication its near to overhaul time.

[GAHorn]

It's doubtful his main bearings are sufficiently at fault if when the engine is hot it still maintains 10 psi, which is allowable at idle speeds.
I think you are giving him great advice on the relief valve, presuming he has a C-145 or O-300 engine. (Some other TCM engines do have oil pressure regulators which are adjustable, but I doubt he has one of those installed in a 170. It is Mexico, however, and I've seen a few airplanes down there that have some very surprising undocumented alterations.)

[Bruce Fenstermacher]

According to Continental x30015 Operators Manual for Aircraft Engines 0-300 and "C" series, min oil at idle is 10 lbs.

My comments regarding bearing wear are from experience and observations I have with 3 Continental engines, A C-145, a 0-300 and a C-85. All have basically the same oil pump, oil passages and oil pressure relief. The C-145 and 0-300 did not have bad oil pressure per say but it was not strong either. The engines where torn down for metal in the screen and the bearings where found to be worn. I would not have torn then down because of oil pressure. The first C-145 was worn so bad the crank was through the babbitt and into the brass bearing shell. The crank needed to be turned 10 under. The one I currently have in pieces the babbitt left was extremely thin. Though the bears. which had 1780 hours on them, might have lasted several hundred hours more. I am just as happy the cam wiped out and we pulled the engine a part before the crank was damaged. My real education on small Continental engine oil pressure, the relationship of oil temperature, oil viscosity and what exact the pressure relief valve does was on the C-85. The tuition was a $3000 crank and a new set of bearings.

This fellows description of high cold oil pressure yet low idle pressure is exactly what I saw as my crank was destroying itself and I fiddled with the pressure relief to increase oil pressure as the bearings failed. Why do I suspect someone put washers in his pressure relief but his bearings are likely the problem, cause that is exactly what I did. I could not believe the bearings and crank I'd just replaced less the 25 hours earlier where bad, even though that is what everyone told me. I fooled with every other possibility including swapping in and out 3 accessory cases and oil pump gears.

As I advised if I was him I'd exhaust all other possibilities. But if someone put washers in the relief to jack up the low idle pressure, I believe the main bearings can be called into question.