Should oil in pressure line for gauge go all the way to the gauge?

You could put a direct reading gauge on the engine with a light and camera focused on it and a video screen with a big suction cup on it to stick on the windshield. Or maybe an air gauge attached to the oil line to measure the air pressure and compare it to the oil pressure...... with two big suction cups to hold them, and of course a vacuum gauge to measure the suction on the suction cups so you will know when they are about to fall off, which would require another two gauges and more suction cups unless they have a remote sensor.....

Too complicated and expensive. Just lead an oil hose to a covered bucket kept on the passenger seat. Another hose on the side of the bucket near the bottom goes back to the engine. Then you stick a meat thermometer through the cover, and a dipstick. After a little testing to get the return hose sized right and the temperature/bucket level calibrated right, it just a question of checking the dipstick. Could put a sight glass on it if you like fancy. Anyway, you won't need a TV on the windshield then.

--Vic

"Heron McKeister" wrote in news:Ph3%m.5533$ snipped-for-privacy@newsfe16.iad:

Firstly, I'd like to thank all for their friendly and good-natured replies to my posts.

Secondly, here's the explanation: Way, way back in the early '80s, I installed a capillary-type Auto Meter oil pressure gauge in my '75 Corolla. There was air in the line after I hooked it up. The gauge barely read anythng at all. I decided that the problem was probably the air, so I made sure I bled out as much as I could before reconnecting the line. The gauge then displayed a more normal reading.

If the air didn't matter, then I obviously inadvertently did something else that fixed the low reading, but mistakenly attibuted the fix to the removal of the air.

"hls" wrote in news:JOmdneSwssLqQqHWnZ2dnUVZ snipped-for-privacy@giganews.com:

So I've been believing a misconception these last 28 years. You, at least, was civil in pointing this out to me.

Unless there is a blockage somewhere in the line when the pump is pumping 45lbs/square inch or whatever it is at the pump outlet that would be 45 lbs/sq inch at the gauge. Air or no air. Any air in the line would be compressed to 45Lbs and that's what the gauge would read. OK that's it, this is the correct answer, I should know I made a strong D- in physics. Thread closed.

It would have been more useful if it was done with more conventional oil - He said he was doing the test with cold 20w50 oil. With 5w30 oil I would guess the difference between air in the tube and no air would be much less noticeable. The size of the tubing and thickness of the oil will determine how fast the gauge responds. Also, after the air is bled from the tube there shouldn't be any way the oil can drain back out of the tube. If it does that probably means a tiny leak that is too small for oil to leak out but big enough for air to leak in.

The electric gauge/sensor will have some dampening also to protect the gauge from damage. Depending on how it is set up it could also have some small delay.

-jim

Would you use a Fram filter on that hose? I've heard they are just as good as a genuine Toyota Filter.

FatterDumber& Happier Moe wrote in news:4b3d1394$0$5343$ snipped-for-privacy@news.suddenlink.net:

That makes perfect sense, now that I've actually given this some thought.

Makes me wonder what I did wrong those 28 years ago.

small amount of debris in the line that plugged the little hole on the back of the gauge, that got blown out when you bled the line?

I dunno, that's the best I can come up with on short notice

nate

I must be slipping, Teg.

The oil will flow up the line until the pressure of the oil and the pressure of the air will equilibrate.. But there is a BUT...If the small amount of air in that line is heated substantially, it will expand and/or the pressure will increase, more or less as predicted by the universal gas law. This pressure can add to the pressure provided by the oil system, and you may see an increase. In going from ambient of about 100F to 250C, this would not provide a pressure error of more than 20-25%, or - if you were looking for a true oil pressure of 30 psi - about 6-7 psi. A lot of cheapo gauges are not much better than that.

And the electrical components and systems created by the "Lords of Darkness", Lucas

"hls" wrote in news:JP6dnYiPY98RtqDWnZ2dnUVZ snipped-for-privacy@giganews.com:

Well, I never took physics, so this is all somewhat Greek-ish to me.

If the air in the line were to expand due to heat, wouldn't it simply push back on the column of oil being pushed up from the engine?

I can understand that pressure would rise with heat-induced gas expansion in a sealed system, but the oil pressure gauge isn't a sealed system, is it?

Any increased pressure in the line from heating of the air trapped in it would simply become an opposing force to the oil pressure from the pump and would dissipate back into the oiling system. Pressure can not exceed what the regulated pressure is to begin with.

This whole thing is a non-issue.

I must most strenuously disagree. Absent any valving or a significant flow rate, the pressure in a hydraulic system is essential the same everywhere.

Yeah, you could do that. Maybe there's an app for that ...

The oil would only drain to the extent that air could enter the line, otherwise the vacuum and surface tension would hold the oil in place. If the system was sealed properly (a very big IF in this case), then the oil ought not drain from the hose leading to the guage. Small hose, thick oil, sealed end on the hose. The oil should remain in place.

And the answer is yes. The unusual pressure due to the air would push backwards against the oil pressure. Here is where it becomes dicey.. The amount of air in the line is small. So even if it expands, its expansion is limited by its volume, or weight, as a part of the system.

The differences are usually much smaller than the case I related to you.

That is what I had hoped to get across, but on this thread if you make a small approximation, someone will come to take your hide.

In general, true. However I recall hearing for the first time, from my first semester thermodynamics professor (Dr. Izawa), that some liquids can be relatively compressible, namely both ethane and carbon dioxide.