oldcutlass

The line is always going to have air in it when you turn the engine off. The oil pressure readings your getting with the mechanical gauge are normal for an older engine. Glad your moving forward.

eazy

And it stays in the line when I run it again?

MDchanic

+1.

I would go so far as to say that those numbers are essentially within spec for any non-new stock Olds engine.

Don't worry about them.



Okay, as a professional plumbing, heating, and air conditioning mechanic, you tell us:
How does a mechanical (Bourdon) gauge work?
Where does the air come from? Where did it go?
Think about it.
You can figure this out without us telling you.

- Eric

69 Ragtop

X3. Air in the line is OK. Get an 1/8" copper line and you won't see it!

MDchanic

Very true.

I know that Eazy can figure this out on his, own, too, so let's wait to see what he says before we say anything more...

- Eric

White_Knuckles

Sorry wiki-leak here:

I've seen gaps in every semi-transparent gauge line I've ever seen. Strange but true.

eazy

I'll have to admit I didn't quite know exactly what the term Bourdon meant, and neither does spellcheck. I had to hit the books because your right MD, I should know this.

I read about Bourdon Tube gauges on page 11 of my refrigeration book. I am still slightly confused by the fact that my refrigeration gauges give me a reading of the temperature/pressure of the liquid refrigerant without the system on. Why does this oil psi gauge not act the same.

I am also questioning the accuracy of the gauge with air in the line. I can purge my refrigeration manifolds and equipment. Sometimes I have to to get an accurate reading.

MDchanic

Okay, now, at least, we're getting somewhere.

This one is easy: Both the A/C gauges and the oil pressure gauges give accurate readings of the pressures in their respective systems when the systems are turned off - in the case of the engine, when the engine is off, the oil pressure is Zero (or, more accurately, is 14.7psi, which is to say it's atmospheric pressure, which the gauge shows as Zero because it is designed to show you the pressure above atmospheric pressure, ie: psig ["psi gauge"], rather than absolute psi, which has an absolute vacuum as Zero).

The oiling system of the engine is an open system - it is open to the air (look at the air above the oil in the oil pan, and how that's connected through the drainback holes to the valve covers, and how they're connected to the carburetor, and that is open to the air).
There is no pressure in the oil galleries of the engine when the engine is turned off, so the gauge reads Zero.

The A/C system is a closed system - the refrigerant shouldn't get out, and air shouldn't get it. The refrigerant that is trapped in the system reaches a phase equilibrium at whatever temperature the engine is at, which involves having a certain vapor pressure, which you can read on your A/C gauges.



So now we come to a basic fact of physics that is vital to understanding A/C operation (and automotive operation, in general, as well): The compressibility of gasses and the non-compressibility of liquids.

As you know, liquids, whether liquid-phase refrigerant, or brake fluid, cannot be compressed - if you increase the pressure on them, that increase in pressure will be transmitted directly across the liquid, but the liquid will retain its same volume.
On the other hand, gasses can be compressed - if you increase the pressure on them, their volume will decrease (they will get smaller).

This is most easily understood by noting the difference in the feel of the brake pedal when you do and do not have air in the brake system: Air is compressible, but brake fluid is not. If there is no air, the force of your foot on the pedal is transmitted directly to the calipers, and the pedal feels hard.
If there is air in the system, the air bubbles compress before the brake fluid can begin to transmit the force, and the pedal feels "soft."

So, you have an open system (the oiling system), in which a pump sucks oil up out of a sump, pumps it through the engine, and it returns to the sump by dripping back down. When you turn off the engine, a lot of the oil that was in the various galleries and passages slowly leaks back down to the sump, through intentionally engineered spaces (the bearing end-gaps, the clearances between the oil pump gears, etc.), and is replaced by air as it does so.
When the engine is started again, oil is pumped back up through the system, and that air is forced back out into the atmosphere.

So, back to the Bourdon gauge...
A Bourdon gauge uses a closed chamber with known elastic qualities to measure pressures: as the pressure increases, the chamber changes shape (bends), and that change is shape causes the indicator needle to move across the dial.

The key here is the fact that the gauge is a closed entity. There is no venting or opening from the place where the gauge attaches to the engine, all the way back to the gauge itself. The gauge and the line to the engine are a big dead-end.

So, engine is off, oil passages contain some air, engine is started, oil is pumped up into passages, forcing air out, oil is pumped through gauge port, with some air ahead of it, after the port come the lines and the gauge, which constitute a dead end, air is pushed into this dead end, and has nowhere to go, so it remains there.

Now, air is in the line from the engine to the gauge. Pressure increases as the oil pump pumps faster, and as it pumps oil into the line. As pressure increases, the air in the line is compressed, until the air is at the same pressure as that of the oil pump, at which point, it transmits its pressure forward to the rest of the column of oil, and to the gauge itself.

When you turn the car off, the reverse happens: Oil pressure drops, air bubbles re-expand, air becomes visible.

This has no effect on the accuracy of the oil pressure gauge, as the pressure that is ultimately reached is the same, but it does affect the speed at which the gauge responds to changes in pressure. Since being able to observe changes in oil pressure within milliseconds of when they occur is not important to us, this is of no significance.

Make sense?

- Eric

eazy

Yeah it makes a lot more sense now. Thanks for the lesson. So we are now at the conclusion that my oil psi is accurate for the age of my engine. Now I have to backtrack and dial in this carb and get some new plugs and wires.

I do still think I need a new oil pan though:20140803_121459_zpsd537cb04.jpg

MDchanic

You're welcome. It's all pretty simple once you get the whole picture.


I would say so.


Also, I'd save my time and money and skip the oil pan - they all look like that.

- Eric

eazy

They all look like someone beat them with a 20lb hammer? I'll take your word for it. I am still cutting out a little bit though. Might just be because I am running a little lean. Would you still recommend I change the oil pump and pickup screen? I fear that it is clogged with nylon as was mentioned previously.

This is really getting fun now. All of my friends ask me is: "When you gonna get that thing running again?", "Does it still run?", "Whats wrong with it?", "Whats it leaking?"

"Nothing is wrong with it," I say with a smile. "It starts and runs fine, it's just an old 350 that has a little drip."

MDchanic

If you think there are teeth in the pickup screen, then it is reasonable to drop the pan, and if you drop the pan, then you MAW replace it, IF the replacement isn't expensive, but the new pan will function no better than the old one, and darned near every one I've seen in the wild does look like someone took a 20# hammer to it.

I would say though, that teeth in the screen do not usually obstruct flow, and that you can bang all those dents out with a 20# hammer if they really bother you.

- Eric

76olds

I would agree with Eric (MDchanic) at least in my case with a 76 olds 350, The chain went at about 85,000 miles with no nylon left on it. Just put a new chain in and drove it to about 120,000 before I had to have it refreshed due to bending a push rod while winterizing. The screen was full of orange bits bit it ran ok till the rod bent. My oil pan also looked the same at one time as well.

Eric

69 Ragtop